Spectacular murmurations of wading birds at The Wash estuary near Snettisham

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Landmark report details how human activities can disrupt animal migrations

Animals that migrate face myriad threats to their survival, all of which have a common denominator: human activity. That’s according to a first-of-its-kind report published Monday by the Convention on the Conservation of Migratory Species of Wild Animals (CMS), a United Nations treaty. More than one in five of the species listed by CMS are threatened with extinction.

Habitat destruction, pollution, unsustainable hunting and fishing, and climate change are among the ways people are disrupting routes migratory species traverse each year. To mitigate these threats, the convention calls for more meaningful action and cooperation across borders and between governments, the private sector and other stakeholders.

WATCH: Why large numbers of reptile species face extinction and what that means for our ecosystem

The unique report comes as no surprise to ecologists who work with migratory animals. Global research efforts are endeavoring to track and map exactly where and when various creatures travel each year, information that can be used to better protect them.

A female endangered Mediterranean monk seal visits the shore of Jaffa

An endangered and rare female Mediterranean monk seal visits the shore of Jaffa in Israel, May 15, 2023. Photo by Amir Cohen/Reuters

That work has a long way to go, though, said Emily Cohen, an animal migration ecologist at the University of Maryland Center for Environmental Science. And it’s happening as these species are actively grappling with modern borders, technologies and industries that jeopardize their migrations and their survival.

“It’s really an exciting time to study migratory animals,” she added. “But at the same time, we’re losing them. They’re blinking out. So it’s timely, but also kind of scary. And this report reflected that.”

The risk of extinction is also mounting for animals that aren’t on the CMS list, according to the report, which noted that between 1970 and 2017, there was a 15 percent average drop in monitored populations of migratory species across the globe.

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The convention’s list includes heavily endangered animals and those whose populations require international conservation efforts to address the serious pressures they face, CMS executive secretary Amy Fraenkel said.

“If a species is crossing national borders, it means that one country alone cannot do what it takes to achieve the conservation protections and needs of the species,” Fraenkel said.

Who migrates, and who is under threat?

From whales to wildebeests to warblers, a huge variety of animals cover enormous distances each year in search of certain climatic conditions, or breeding or feeding grounds.

bird

A male Lesser Kestrel is pictured in a grassland in Maharashtra, India, Jan. 10, 2021. Photo via Sumeet Moghe/Wikimedia Commons under a Creative Commons license

Some — like many songbirds — are obligate migrants, meaning they’re hard-wired to make their journeys, which can be spurred by cues like day length, Cohen said.

For other species, migration is a kind of learned cultural practice, said Matthew Kauffman, a wildlife biologist with the U.S. Geological Survey who studies ungulates, or hoofed mammals. He said that young animals absorb a “cultural repository of knowledge” that exists in the population they’re born into by following the leads of other animals.

Mammals, birds, reptiles, insects and fish are among the types of animals that CMS covered in its report.

READ MORE: How climate change is throwing off key timing for wildflowers and trees in spring

One section of the research lists 180 heavily endangered migratory animals that international law bans from being killed or taken into captivity. The report found that more than 80 percent of these species face the threat of extinction. Some groups face starker realities than others; more than two-thirds of reptile species and nearly all fish species listed by CMS are threatened with extinction.

Why migratory species matter

The argument to protect migratory species is not just environmental, but cultural and economic, too. People and ecosystems across the globe rely on these creatures in a wide range of important ways.

The report points out that many groups of people have strong cultural or spiritual ties with migrators. Cohen noted that annual sightings of animals like songbirds that traverse continents on their migration journeys offer an opportunity to be inspired by and connect with those animals.

A man releases a Green turtle into the sea at the Sea Turtle Conservation Center of the Royal Thai Navy, in Sattahip

A man releases a Green turtle into the sea at the Sea Turtle Conservation Center of the Royal Thai Navy, in Sattahip, Chonburi province, east of Bangkok, Aug. 1, 2012. Photo by Chaiwat Subprasom/Reuters

Threatened or dwindling populations of migratory species can have ripple effects across ecosystems. Pollinators play a key role in transporting seeds, for example, while other animals help transfer nutrients across land and sea.

Whales fall into the latter category. When they migrate from polar waters toward the equator, they help move nutrients to parts of the ocean that typically don’t have a lot to spare, said Daniel Costa, director of the Institute of Marine Sciences at the University of California, Santa Cruz. As they travel, whales release urea as waste, a source of nitrogen that’s useful to other members of the marine ecosystem.

READ MORE: These pansies are evolving to rely less on pollinators. Here’s why that may spell trouble

The report also noted that some communities derive economic value from migratory species thanks to the ecotourism industry, or rely on these animals as food sources.

What interventions would help these animals?

Migratory species face multiple challenges, and addressing them requires multifaceted solutions. The loss of habitat and overexploitation from hunting and fishing are two of the most pressing threats, according to the report.

Human activities and structures can obstruct migration routes, making it difficult or even impossible for animals to embark on their journeys. Pollution — including light and noise pollution, plus contaminants like pesticides and plastics — is another major problem, too.

Whale shark swims next to volunteer divers after they removed an abandoned fishing net in a protected area of Ko Losin

A whale shark swims next to volunteer divers after they removed abandoned fishing net that was covering a coral reef in a protected area of Ko Losin, Thailand, June 19, 2021. Photo by Jorge Silva/Reuters

In the report, overexploitation encompasses both intentional and accidental harvesting of species for sale as food, pets or products. Many marine migrators are at particular risk of ending up as bycatch, or animals harvested by people that intended to catch different species.

Recommended interventions involve addressing the damage caused by humans, such as by identifying, protecting, restoring and monitoring key habitats to help decrease disruption to migration routes. The convention also calls for collaborations between stakeholders to combat overfishing and bycatch, in addition to illegal hunting, as solutions to overexploitation.

WATCH: Nature knows no borders. Border security can take a heavy toll on endangered wildlife

Achieving these goals requires a shift in perspective, said Amanda Rodewald, senior director of the Center for Avian Population Studies at the Cornell Lab of Ornithology. She added that healthy environments are crucial to the health of both people and animals.

“There’s growing recognition that many of the same steps we need to take to protect migratory species, as well as biodiversity more generally — those are the same steps we need to take to protect human health and well-being,” Rodewald said.

That means the solutions available to address challenges like climate change — such as bolstering green spaces in urban environments — can have multi-fold benefits across species, including ours, she noted.

Fraenkel also pointed out a more philosophical reason to protect migratory species, noting that animals like dolphins and elephants, for example, have complex social structures and are exceptionally intelligent. Upholding the ways that these creatures connect people and countries, she said, “is a way of thinking about nature.”

“I think that’s very helpful to realize that it is still one planet, and it means that we have to work together across country lines and make sure that we’re all doing what we can to see that nature continues to thrive,” Fraenkel said.

Bella Isaacs-Thomas is a digital reporter on the PBS NewsHour's science desk.

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animal migration essay

The tough decision of which species to save from extinction

Science Nov 26

ENCYCLOPEDIC ENTRY

Migration is the seasonal movement of animals from one habitat to another in search of food, better conditions, or reproductive needs.

Biology, Ecology, Geography

Greater Snow Goose migration

Greater Snow Geese flying over the Saint Francis river. Geese go through a migration every year to escape the harsh northern winters.

Photograph by David Doubilet

Greater Snow Geese flying over the Saint Francis river. Geese go through a migration every year to escape the harsh northern winters.

Migration is a pattern of behavior in which animals travel from one habitat to another in search of food, better conditions, or reproductive needs. There are two important factors that make migration different from other types of animal movement: First, migration happens seasonally , and second, migration involves a return journey. This makes it different from e migration , when animals travel to find a new, permanent place to live. Many animal species migrate, including species of fish, crustaceans, amphibians, reptiles, insects, and mammals. These animals might journey by land, sea, or air to reach their destination, often crossing vast distances and in large numbers.

One of the main reasons animals migrate is to find food. In Tanzania, wildebeests ( Connochaetes taurinus ), zebras ( Equus quagga ), and gazelles ( Eudorcas thomsoni i) migrate in huge herds. They roam the Serengeti looking for fresh grass and water, which are hard to find during the dry season. Humpback whales ( Megaptera novaeangliae) migrate for food as well. In the summer, they travel to feeding grounds near the polar ice, where the water is full of krill and small fish. In the winter, they migrate back to warmer waters to raise their calves.

Other animals migrate because of the climate or seasons . For example, monarch butterflies ( Danaus plexippus ) migrate to avoid cold temperatures in the winter. These butterflies cannot survive freezing temperatures, so they fly from Canada all the way to Mexico, where they gather to keep warm over winter. They make the return journey over many generations , stopping to lay eggs on milkweed plants along the way. The caterpillars eat the milkweed and then finish the journey as butterflies.

Finally, some animals migrate for reproductive reasons: either to find a mate, raise their young, or to spawn . For example, salmon start life in rivers and migrate to the sea to feed and grow. After spending up to seven years in the ocean, they migrate back to the rivers they were born in so that they can spawn . Christmas Island red crabs ( Gecarcoidea natalis ) migrate for similar reasons. They spend most of their life in the forest but migrate to the ocean to reproduce .

Recent improvements in technology have helped us understand migration better, but there is still a lot we do not know. Scientists are not yet entirely sure how animals know where to go and when to leave, especially when they have never made the journey before. Some researchers suggest that these animals use a mix of stimuli, such as sunlight, the Earth’s magnetic field , and chemical cues, to find their way.

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Across the world, migrating animal populations are dwindling. Here's why

Nathan Rott at NPR headquarters in Washington, D.C., September 27, 2018. (photo by Allison Shelley)

Nathan Rott

animal migration essay

Ninety-seven percent of migratory fish species are facing extinction. Whale sharks, the world's largest living fish, are among the endangered. Ullstein Bild/Ullstein Bild hide caption

Ninety-seven percent of migratory fish species are facing extinction. Whale sharks, the world's largest living fish, are among the endangered.

Every year, as the seasons change, billions of animals embark on journeys to find food, to get to better habitats or to breed. They migrate in groups and as individuals, flying, swimming, crawling and walking across international borders and through habitats to survive, transporting seeds and nutrients.

A major new report by the United Nations finds that humans are not only making those journeys more difficult, but have put many migratory species in a perilous state.

Nearly half of the world's already threatened migratory species have declining populations, the first of its kind U.N. report found. More than a fifth of the nearly 1,200 migratory species monitored by the U.N. – whales, sea turtles, apes, songbirds and others – are threatened with extinction.

"These are magnificent species that take unbelievable journeys, in some cases, that are economically beneficial [for humans], as well as the stuff of poetry and song and cultural significance," said Amy Fraenkel, executive secretary of the U.N. Convention on the Conservation of Migratory Species of Wild Animals.

California sea otters nearly went extinct. Now they're rescuing their coastal habitat

Research News

California sea otters nearly went extinct. now they're rescuing their coastal habitat.

The report, compiled by conservation scientists, is the most comprehensive assessment of the world's migratory species ever carried out. It looked at 1,189 different species that are already protected by the Convention on Migratory Species — a 1979 treaty intended to conserve species that move across international borders — to see whether conservation efforts are working.

In some cases, they are. Wildlife crossings are helping animals traverse over roads and fences. Regulations are helping prevent poaching and overconsumption of some threatened fish and mammals. Habitat protections are giving species room to move and prosper.

To reverse population declines though, the report's authors said, those "efforts need to be strengthened and scaled up."

The publication is the latest global report to raise concerns about the planet's non-human inhabitants. A 2019 assessment on the world's biodiversity found that 1 million of the Earth's estimated 8 million species are at risk of extinction, many within decades, because of human activities like overconsumption, deforestation, pollution and development. A 2022 report by the World Wildlife Fund found that wildlife populations have declined by an average of 69% in the last 50 years.

For migratory species, the threats from human activities can be amplified. Protections for species vary from country to country. Enforcement of conservation laws can differ depending on locale.

Hunting and fishing – overexploitation – and habitat loss from human activities were identified as the two greatest threats to migratory species, according to the new report. Invasive species, pollution – including light and sound pollution – and climate change are also having profound impacts, the report found.

A Florida park just saw a record number of manatees gather together in its waters

A Florida park just saw a record number of manatees gather together in its waters

Many species migrate with the change of seasons. Human-caused climate change is altering seasons , lengthening summers, shortening winters and shifting the timing of spring and fall. Scientists have documented animals, like birds in North America , adjusting the timing of their migrations to match those shifts. Not all are keeping pace with the rate of change, leading to what scientists call phenological asynchrony.

World leaders from the 133 countries that have signed on to the Convention for Migratory Species are meeting this week in Uzbekistan to chart a path forward.

The new report, Fraenkel said, should give the parties a sense of urgency, but it should also be a guide for anyone "who wants to keep seeing the birds flying and the whales jumping in water," she said. "Look at this report and find something [you] can do to help these incredible species continue to survive."

  • endangered animals
  • animal conservation
  • overfishing
  • biodiversity
  • climate change

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Animal Migration

animal migration essay

What is Migration?

Most people think of migration as the seasonal movement of a flock of birds between their breeding and non-breeding sites. In fact, bird migration is probably the biological phenomenon that has attracted the most interest among non-scientists, and has one of the longest traditions of scientific investigation in biology (Berthold 2001). However, there are many other forms of animal migration, including journeys between east and west, complex round-trips involving land and ocean, altitudinal journeys up and down mountains, and vertical movements through the water column of oceans and lakes (Hoare 2009). What sets migration apart from other forms of movements is that migration typically involves travelling from one type of habitat to another (Aidley 1981).

Who Migrates?

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The common name of the southern right whale ( Eubalaena australis ) (Figure 3b) refers to the fact that they were considered the ‘right’ whale to hunt. They are slow-moving and have large amounts of blubber, which meant that they floated after being killed. Many tens of thousands were slaughtered until the international whaling ban in 1935. Southern right whales migrate from their Antarctic feeding areas to temperate breeding areas along the costs of Chile and Argentina, southern Africa, and Australia and New Zealand, covering 2,500 km each way. Their migration is fuelled entirely by fat accumulated during their four-month stay in the icy Southern Ocean around Antarctica, where they skim the surface waters for zooplankton. Amazingly, they will not feed until their return a year later (Hoare 2009).

Why Migrate?

Migration is an adaptive response to the seasonal or geographic variation of resources (Gauthreaux 1982). The annual cycle of the seasons produces large differences in the duration and intensity of solar energy received in each hemisphere at any given time. Many migrants take advantage of favorable food and weather conditions offered in certain areas during very limited time periods. For instance, migratory birds like the bar-tailed godwit that breed at high latitudes (e.g., in the arctic tundra) exploit the extraordinary abundance of food during a few weeks in early summer and profit from long days, which allow them to extend foraging time. By leaving these areas after breeding, they avoid the northern winters with short days, low temperatures, and low food availability (Pulido 2007). In other cases, the resources needed in different life stages may be found in different locations. For example, young salmon migrate to sea to take advantage of the great abundance of food and high growth potential available there, but must someday return upstream to the small rocky tributaries required for spawning, a trip sometimes involving thousands of kilometres (Figure 4; Dingle 1996).

Preparing for Migration

Navigation during migration, visible cues, invisible cue, what triggers migration, taking advantage of outside help with migration, human history of animal migration.

The earliest evidence of humans being aware of animal migration dates back to the Stone Age. Some rock art images, portraying animals moving across the African savannah, are as old as 20,000 years. They were produced by nomadic hunter-gatherers, and could have served as a field guide to potential food, or as a visual record of good hunting areas. The philosophers of Ancient Greece were the first to develop a theory of migratory behaviour in animals, even though their conclusions nowadays seem rather fanciful. Aristotle (384–322 BCE) realized that a number of birds were migratory, and his creative explanation for the sudden disappearance of summer migrants, such as warblers and swallows, was that they had morphed into different species present in winter. His idea of transmutation persisted well into the Middle Ages in Europe. According to another theory, which was still widely believed in the 1800s, migratory birds disappeared into the mud at the bottom of ponds and lakes where they would spend the winter. Since then, our understanding of migration has indeed improved.

How Migration Is Studied

Bird banding.

Instrumental in studying migration were banding (also called ringing) studies. Banding dates back to 1899, when Danish teacher Hans Christian Mortensen visited European starling ( Sturnus vulgaris ) nests and gave each nestling an aluminum leg band engraved with a return address and a unique serial number. If anyone encountered one of his banded birds, they could send back information on the time and place where it was found. Since 1899, more than 200 million birds are estimated to have been banded worldwide, of which only a fraction have been recovered. However, even a recovery rate as low as 1 in 300 — the average for small birds — still provides valuable insights into the routes that migrants take. Alternatives to banding include labeling with dye and attaching plastic tags to the neck or back, which is also used for mammals (Hoare 2009).

The rapid development of radar during the Second World War enabled actual migratory journeys to be plotted for the first time. Modern radar is powerful enough to pinpoint the height, speed, and wing beat rate of individual birds and bats. Its aquatic equivalent — sonar — can detect shoals of fish moving underwater.

Tracking Devices

It is possible to study animal movements without directly observing an individual. Fitting animals with radio transmitters and using hand-held or stationary antennas allows following their whereabouts within a range of several kilometers. Alternatively, satellite transmitters beam signals to orbiting satellites, which then relay the data back to computers on the ground. GPS tags use the satellites of the Global Positioning System to record data such as location and time. They can be attached to mammals with a neck collar or to medium-sized birds with a backpack. Even smaller are geolocators, which are miniature light level loggers that can record sunset and sunrise, from which the location of the individual can be reconstructed. Some weigh less than 1 g and can last for many years. However, they need to be retrieved to access the data they store.

Stable Isotopes

It is now possible to measure the amount of stable isotopes such as deuterium (a form of hydrogen), oxygen, carbon, nitrogen, and sulfur in the tissue of migrants. Isotope levels in the plumage of a migratory bird match that of the vegetation of its breeding ground and can therefore serve as an indication of its place of birth (Hobson 2005). The same technique has been used to establish the hatching place of individual monarch butterflies wintering in Mexico.

Animal migration involves travelling from one type of habitat to another, which is often linked to the cycle of the seasons. Preparing for the migratory journey, which poses extreme energetic demands on the animal, usually involves a number of physiological changes, such as storing large amounts of fat and reducing the size of the organs that are not needed during migration. Migrants use a variety of cues to find their way, such as the magnetic field of the earth, changing concentration of minerals in the ocean water or polarized light. The breath-taking nature and raw beauty of animal migration has inspired humans for the past 20,000 years and continues to do so to this day.

Acknowledgments

References and recommended reading.

Aidley, D. J. Animal Migration . Cambridge, UK: Cambridge University Press, 1981.

Berthold, P. Bird Migration: A General Survey . Oxford, UK: Oxford University Press, 2001.

Bowlin, M. S., Bisson, I. A. et al . Grand challenges in migration biology. Integrative & Comparative Biology (2010).

Brower, L. P., Fink, L. S. et al. Fueling the fall migration of the monarch butterfly. Integrative & Comparative Biology 46 , 1123–1142 (2006).

Dingle, H. Migration: The Biology of Life on the Move . New York, NY: Oxford University Press, 1996.

Egevang, C. et al . Tracking of Arctic terns Sterna paradisaea reveals longest animal migration. Proceedings of the National Academy of Sciences 107 , 2078–2081 (2010).

Gauthreaux, Jr., S. A. The ecology and evolution of avian migration systems. Avian Biology 6 , 93–68 (1982).

Gill, R. E. J., Tibbitts, L. T. et al . Extreme endurance flights by landbirds crossing the Pacific Ocean: ecological corridor rather than barrier? Proceedings of the Royal Society B, Biological Sciences 276 , 447–457 (2009).

Hoare, B. Animal Migration. Remarkable Journeys by Air, Land and Sea . London, UK: Natural History Museum, 2009.

Hobson, K. A. Flying fingerprints: Making connections with stable isotopes and trace elements. In Birds of Two Worlds: the Ecology and Evolution of Migratory Birds. eds. Greenberg, R. & Marra, P. P. (Baltimore, MD: Johns Hopkins University Press, 2005): 235–246.

Jenni, L. & Jenni-Eiermann, S. Fuel supply and metabolic constraints in migrating birds. Journal of Avian Biology 29 , 521–528 (1998).

Piersma, T. Phenotypic flexibility during migration: optimization of organ size contingent on the risks and rewards of fueling and flight? Journal of Avian Biology 29 , 511–520 (1998).

Piersma, T. & Gill, R. E. J. Guts don't fly: small digestive organs in obese Bar-tailed Godwits. Auk 115 , 196–203 (1998).

Pulido, F. The genetics and evolution of avian migration. BioScience 57 , 165–174 (2007).

Ramenofsky, M. & Wingfield, J. C. Regulation of migration. BioScience 57 , 135–143 (2007).

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Animal Migration

Investigate several examples of animal migration, including migration by monarch butterflies, pronghorns, and animals of the Florida Wildlife Corridor.

Biology, Ecology, Geography, Physical Geography

Two elk trot across a field toward the trees.

Photograph by Alex Edelman / ZUMA Press, Inc. / Alamy Stock Photo

Many animals journey great distances, or migrate, as part of their life. They do this for various reasons, including to mate and find food. Use this idea set to learn about several migratory species, including monarch butterflies and the challenges faced by these insects. Additionally, introduce students to pronghorn migration through the camera lens of photographer Joe Riis to understand how the Geo-Inquiry Process was used to learn the importance of migration bridges. Lastly, visit the Florida Wildlife Corridor through a conservation photographer’s experience to learn how corridors can facilitate wildlife movement across large areas.

Four monarch butterflies flutter around a set of flowers.

Migrating Monarchs

Trace the migration of monarchs through North America to better understand the challenges these insects face. Read this article to begin. According to the article, how do monarchs use milkweed? Then introduce this map of monarch migration and have students identify specific locations where monarchs breed and feed throughout the year.

Direct students to compare this migration map to other maps as they assess possible issues monarchs may face along their migration route. Students can begin by using MapMaker's Land Cover and Population Density map layers, as well as MapMaker's Streets base map to understand how land is used along migratory routes. As students compare the migratory routes to the MapMaker data, have them create a list of possible issues monarchs may face, especially considering their dependence on milkweed. Example responses may consider the impact of highways, cropland, and dense urban areas. Students can compare their lists to the threats scientists have identified using  this article .

As an extension, have students develop plans to build or improve upon monarch-friendly landscaping on their campus, or if they live in an area outside of monarch habitat, develop recommendations that other schools might use. Encourage students to share their plans with the school’s administration.

Pronghorn antelope walking through a field of tall grass and wildflowers.

Bridges for Migration

Investigate the use of wildlife bridges to help facilitate animal migration. As a class, begin by reading this story about Joe Riis , a National Geographic Photography Fellow. Students can answer the following questions as they read: What is a pronghorn? What did Joe Riis want to find out about pronghorns? How did he obtain information about pronghorns? What was he able to show with the information? What impact did his work have on pronghorn migration? In what way can his work help other migrating animals and humans?

Emphasize the use of wildlife bridges by reading this article to determine where else in the world they are used and what makes them successful. Then explain how Joe Riis used The Geo-Inquiry Process to help him develop a geographic perspective in order to determine the importance of wildlife bridges. List the steps of The Geo-Inquiry Process (Ask, Collect, Visualize, Create, and Act) on the board. Help students identify the steps of the process by specifically connecting them to Joe Riis’ work: Ask— What questions did Joe ask? Collect— What data did he collect? Visualize— What visuals did he create? Create— What did Joe create from his work? Act— What actions occurred following his work?

Tell students they can conduct their own geo-inquiries using these steps. Encourage students to brainstorm a local environmental problem they are interested in learning about and addressing, especially those pertaining to wildlife crossings. Use the teacher resources on this site for information and handouts to help students start a Geo-Inquiry project.

Bridge across a four line highway. The bridge is lined with trees and grass and is designed to allow wildlife to safely cross the road.

Wildlife Corridors

Explore the use of wildlife corridors using Florida as a model. Students can start by using the street and satellite views on Google Maps to predict which areas across Florida are likely natural habitat and home to a variety of wildlife. Students can base their predictions on the land cover or the locations of identified protected areas. Emphasize the unconnected nature of these areas by, for example, pointing out the distance between the Ocala National Forest and Goethe State Forest . Ask students to speculate why having unconnected natural habitat might cause issues for wildlife. Then have students propose paths, or corridors, that could help facilitate wildlife movement between the areas they identified earlier. Students can compare their proposals to the proposed Florida Wildlife Corridor depicted on these maps .

Describe the Florida Wildlife Corridor as a proposed network of connected land and water stretching across the state that is designed to support native wildlife. Students can watch this video of National Geographic Grantee Carlton Ward Jr. , who founded the Corridor project, to learn more. As a class, discuss how Ward Jr.’s work as a conservation photographer might be helpful to successfully implementing the Florida Wildlife Corridor.

To conclude, share this article about the creation of a wildlife corridor in Brazil. Students can use both projects as examples as they evaluate the advantages and difficulties of implementing wildlife corridors.

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ANIMAL MIGRATION AS A MOVING TARGET FOR CONSERVATION: INTRA-SPECIES VARIATION AND RESPONSES TO ENVIRONMENTAL CHANGE, AS ILLUSTRATED IN A SOMETIMES MIGRATORY SONGBIRD

Jonathan w. atwell.

* Ph.D. Candidate in Evolution, Ecology, and Behavior; Department of Biology and Center for the Integrative Study of Animal Behavior (CISAB), Indiana University, Bloomington, Indiana

DAWN M. O’NEAL

** Post-doctoral Research Associate; School of Ecology, University of Georgia, Athens, Georgia

ELLEN D. KETTERSON

*** Professor of Biology; Department of Biology and Center for the Integrative Study of Animal Behavior (CISAB), Indiana University, Bloomington, Indiana

Identifying important “migratory species” and the characteristics of their migrations might sound like a simple starting point for efforts to conserve and protect animal migrations. However, migrations are dynamic phenomena that vary over space and time, and migratory behaviors can vary substantially among closely related species, subspecies, races, or populations, and even among individual animals within a single population. The migratory behaviors of populations or individuals can also change rapidly—or be lost entirely—in response to habitat alteration or climate change. These complexities present both challenges and opportunities for initiatives to conserve animal migrations. In this Article, we discuss the concepts of intra-species variation in migration and the sensitivity of migrations to environmental change, and we consider the implications of these topics for legal, policy, management, and research agendas.

I. Introduction

Animal migrations are dynamic phenomena that vary over space and time, even among closely related species, populations, and individuals. For example, in many animals there is substantial geographic variation in the migratory tendencies of different subspecies, races, or populations—birds that breed in the north may migrate long distances south to spend the winter, whereas members of the same species that breed at lower latitudes may be entirely sedentary (i.e., non-migratory). 1 Further, even within a discrete population, there can be systematic differences in the distance, routes, endpoints, or seasonal timing of migrations among male versus female or younger versus older individuals. 2 Such variation in migratory behaviors can emerge rapidly over “evolutionary time scales” (e.g., thousands of years)—including over contemporary times (e.g., years or decades) in response to human activities such as habitat alteration and climate change. 3 Therefore, effective conservation agendas for animal migrations must consider the implications of both spatial and temporal variation in migratory behavior, even within a single “migratory species” or a single local population.

Our primary goal in this paper is to introduce the following three biological topics to nonspecialists, and to discuss their potential implications for legal, policy, management, and research agendas related to the conservation of migrations: 1) geographic variation in migratory behavior within-species (i.e., inter-population variation in migration), 2) variation in migration of different individuals within a single population (i.e., intra-population variation in migratory behavior among individuals), and 3) the sensitivity of migratory behavior to environmental change—with dramatic changes observed even over relatively short time scales.

To illustrate these topics, we use a common “backyard” North American songbird species, the Dark-eyed Junco ( Junco hyemalis ). 4 We chose the “sometimes migratory” junco, not because this species’s migration is of immediate conservation concern (it is not), 5 but because past scientific research has revealed the complexity of its migration, 6 allowing it to serve as a model to convey why intra-species variation in migration—and the sensitivity of migratory behaviors to environmental change—provide important challenges and opportunities for policy efforts to protect migrations. Principles derived from the junco almost certainly apply to many other migratory species, including those of immediate conservation concern, and we provide selected examples. 7 However, for most species, intra-specific variation in migration or the potential impacts of changing environments on migration have not been well characterized. Even for the junco, which has received much research attention from biologists studying migration, there remain many unanswered questions about migratory variation within and among junco subspecies and populations. These types of information gaps have the potential to confound or frustrate conservation initiatives and should be dealt with by future research efforts.

The rate at which migratory processes can be altered by environmental changes—habitat destruction or alteration, climate change, construction of barriers to migration, pollution, or anthropogenic food or water supplementation—underscores the need for immediate conservation action and the articulation of ongoing research agendas—both of which must be drafted to accommodate intra-species variation and rapidly changing biological systems. Although both intra-species variation in migratory biology and the sensitivity of migrations to environmental change challenge the desire to generalize in the context of conservation law, policy, management, and research, these topics must be considered if the most effective migration conservation strategies are to be developed.

In Part II, we introduce the Dark-eyed Junco, providing relevant background information for this species, which we subsequently use as an example to illustrate our key points throughout the following three Parts. In Part III, we explain the extent of geographic (inter-population) variation in migration as a general phenomenon, and we consider the implications of this type of variation for conservation agendas. Part IV introduces the topics of “differential migration” and “partial migration” (both types of intra-population variation in migratory behavior among individuals), and we discuss the implications of such intra-population variation for conservation. In Part V, we highlight two recent landmark studies which demonstrate how contemporary environmental changes have rapidly altered migratory biology in the junco, and we consider how these types of studies can inform approaches to conservation. In Part VI, we conclude by summarizing our key points, emphasizing that future research, along with improved communication and collaboration among scientists, policymakers, and managers, could proceed to more effectively consider intra-species variation and response to environmental change in the context of conservation of animal migrations.

II. “Sometimes Migratory Songbird”: The Dark-Eyed Junco

The Dark-eyed Junco ( Junco hyemalis ) is a medium-sized (18–22 gram average) sparrow that breeds primarily in montane or higher latitude coniferous and mixed forest habitat throughout the northern part of North America and south through the Western United States. 8 The winter range of the junco includes lower latitudes and lower altitudes throughout North America. 9 The breeding and wintering ranges of Dark-eyed Juncos are illustrated in Figures 2a through ​ through2d, 2d , and discussed in more detail below. Juncos forage and nest primarily on the ground, they form conspicuous wintering flocks of ten to fifteen individuals, and they flash their white outer tail feathers when they fly. 10

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The breeding, wintering, and year-round ranges of the Dark-eyed Junco species ( Junco hyemalis ) as a whole is shown. Throughout their range, Dark-eyed Juncos vary extensively in migratory behavior and plumage coloration, including long-distance, short-range, and altitudinal migrants, as well as sedentary populations (see Figures 1 , ​ ,2b, 2b , ​ ,2c, 2c , & 2d ).

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The Oregon Junco group of Dark-eyed Juncos includes long-distance, short-range, and altitudinal migrants throughout its range, and even a recently established sedentary population in San Diego County (see Part V.B.). The Red-backed group of Dark-eyed Juncos is mostly altitudinal in its migration.

Wintering flocks of juncos are common at birdfeeders and juncos actively feed during winter conditions, earning them the colloquial nickname “snowbirds.” 11 Where it occurs, the Dark-eyed Junco is typically one of the most common and abundant songbirds in both its breeding and wintering range, which visualized together, cover most of the United States and much of Canada ( Figure 2a ). 12 Thus, for millions of North Americans, the junco is arguably the easiest songbird to observe in their backyards and local habitats, including its seasonal arrival and departure on breeding and wintering grounds. The Dark-eyed Junco is technically considered a single species by current taxonomic criteria, 13 but as we discuss below, there are many subspecies and races that differ in feather plumage coloration, body size, life-history and social behavior, and importantly, in their migratory tendencies. 14

We refer to the Dark-eyed Junco as “sometimes migratory” because some junco populations are long-distance migrants while others are non-migratory (i.e., sedentary), and yet others are regional “short-range” or “altitudinal” migrants. 15 This geographic variation among junco groups is introduced at the end of this Part and expanded upon in Part III. Further, even within particular junco populations, some individuals (e.g., females) migrate farther than others (e.g., males), 16 and we discuss this intra-population variation below in Part IV. Recent studies, highlighted below in Part V, reveal how climate change and habitat alteration are associated with rapid changes in migratory behavior in some junco populations. As described above, we chose the junco to illustrate our main points regarding the ubiquity and relevance of intra-species variation and sensitivity of migrations to environmental change not because junco migrations are of immediate conservation concern, but because prior research in this species allows it to serve as a helpful model to convey the importance of understanding the variable and dynamic nature of migratory behaviors over space and time.

A. Diversity Within the Genus Junco

The genus Junco has historically been considered to include three species: Volcano Junco ( Junco vulcani ), found in Costa Rica; Yellow-eyed Junco ( Junco phaeonotus ), found in Mexico and Guatemala; and Dark-eyed Junco ( Junco hyemalis ), found in North America north of Mexico. 17 The Dark-eyed Junco, which is the primary focus of this Article, is divided into at least fifteen subspecies, 18 including a group of eight western subspecies collectively known as the “Oregon” Junco. 19 The range of the Oregon Junco complex extends along the west coast of North America from Alaska into Martir Mountains in Baja California. 20 Each junco subspecies has distinctive markings and can be distinguished from the others based on size and coloration. 21 Juncos are conspicuously patterned, and their plumage varies significantly with geography. 22 Some of this variation is illustrated in Figure 1 . For example, in the Oregon complex of the west, juncos have a dark hood, white breast, and rusty flanks. 23 In the East, the heads, backs, and flanks are more often gray, as found in the Slate-colored Junco and White-winged Junco groups of the Dark-eyed Junco species. Junco phaeonotus (the Yellow-eyed Junco) is also divided into at least three subspecies, 24 which means that, depending on how one counts, there are at least nineteen distinguishable groups of juncos. Because of the large geographic variation, as well as the species’ abundance and ease of study, the junco has been used as a classic model for speciation in progress. 25 Recent molecular evidence indicates that the diversity exhibited among the Dark-eyed Juncos has emerged very rapidly in evolutionary time, as birds re-colonized North America following glacial maxima in the last 10,000 to 100,000 years. 26

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Variation in the appearance and migratory behavior within junco species is shown, along with a reference to figures of the geographic range for each group. Juncos exhibit extensive variation in migratory behavior among groups, ranging from long-distance to sedentary, with geographic overlap in the wintering ranges. Such migratory variation exists within many species, but it is more easily observed in juncos due to the striking differences in feather plumage color of the various sub-species and races.

B. The Junco as a Model in Science

The junco was the first vertebrate animal in which photoperiodic time was measured. 27 William Rowan held juncos outdoors in Edmonton, Alberta during the winter and exposed them to artificial light at the end of the day. 28 Despite the cold and snow of Alberta, the light caused the birds to enter the “reproductive state,” while birds held under much milder conditions, but on shorter days, remained in the “winter state.” 29 The role of day length in regulating seasonal changes associated with both reproduction and migration has since been demonstrated in many species. 30 An area of intense research addresses the relative importance of day length as a cue that regulates events of the annual cycle, in concert with temperature and food as supplementary cues that contribute to the timing of migration and reproduction. 31 The junco has also been a model for scientists studying hormones and behavior, 32 neurobiology, 33 social dominance, 34 sexual selection, 35 trait evolution, 36 eco-immunology, 37 chemical ecology, 38 sex differences, 39 speciation, 40 and, of course, migration, as we will highlight in the following Parts. Therefore, although most junco populations are not facing extinction risks that would make them populations of conservation concern under traditional paradigms, 41 protecting the geographic ubiquity and diversity of juncos while they are still abundant and common is of fundamental importance to science.

C. Conservation Issues and Status

With respect to conservation, the Dark-eyed Junco as a whole is a species of least concern, 42 but at least one subspecies, the non-migratory Guadalupe Junco ( Junco hyemalis insularis ) was classified as critically endangered on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List as recently as 2008, when the list stopped including subspecies. 43 The question of subspecies is beyond the limits of consideration here, but the junco’s phylogeny is fluid, and the true status of the Guadalupe Junco remains to be determined, although recent molecular work indicates it should be listed as a separate species. 44 Although the Dark-eyed Junco is still abundant in most parts of its range, the species declined at a rate of almost two percent annually between 1980 and 2002 according to the National Breeding Bird Survey project. 45 Declines were particularly notable among breeding juncos in western Canada, where both Oregon and Slate-colored Junco subspecies migrate long distances to and from wintering grounds at more southerly latitudes. 46 Similarly, from 1959 to 1988, Dark-eyed Junco abundance declined nearly everywhere based on data from Christmas bird counts, and significant increases were observed only in Quebec and a few northern locations, perhaps indicating a shift in winter distribution in response to climatic warming. 47 Because juncos prefer areas of partial tree cover, including recent clear-cuts, logging is probably not a direct cause of junco population declines. 48 Hypothesized causes of junco declines include aerial application of insecticides and forest regeneration and succession on the breeding grounds as well as unknown factors during migration and winter. 49

D. Migration in the Junco

With respect to migration, the biological diversity represented in the junco species complex is striking, as described in Figure 1 and illustrated in Figures 2a through ​ through2e. 2e . As we will describe below in Part III, there is great variation among junco subspecies and populations in their migratory dispositions. Further, within discrete subspecies and local populations, juncos exhibit differences in migratory behavior among both age and sex cohorts. 50 Finally, two recent studies presented in Part V highlight how junco migrations are sensitive to environmental change, including in response to recent climatic shifts and urbanization. Despite decades of research on this particular species, some of it focused directly on migration, there is still much that remains to be learned, and greater knowledge gaps exist for most migratory species.

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The Yellow-eyed Junco Species includes both altitudinal migrants and sedentary groups throughout its range, which extends from the highlands of Guatemala northwards into the mountains of Mexico and the southwestern US. The southernmost junco species, the Volcano Junco, is found in the highlands of western Panama and Costa Rica and is entirely sedentary.

III. Geographic and Subspecific Variation in Migration

“Migration is rarely a unitary phenomenon even in those species usually regarded as classic migrants.” 51

Distinguishing between migratory and non-migratory species might sound like an ordinary exercise for field biologists, but if one looks closely, many species elude such simple categorizations. In a few cases—such as the long-distance, 52 complete, 53 and obligate migrations 54 exhibited by Arctic Terns ( Sterna paradisaea )—all members of the species must migrate or die. 55 For the Arctic Terns, the limited geographic distributions of their specific breeding and wintering habitat, as well as the dramatic environmental fluctuations at the poles, ensure that staying behind or making only part of the journey is not an option. 56 Even among Arctic Terns, however, variation exists in the form of the migratory routes taken between the contiguous polar ranges—the species splits into tracks, traveling down different coasts of the Americas, Eurasia, and Africa. 57

A. Migratory Diversity Among Junco Groups

In many species, however, distances of migrations vary, as exemplified by the junco species complex, in which some subspecies and populations can be entirely sedentary, while others migrate long distances. This geographic variation is described in Figure 1 and illustrated in Figures 2a through ​ through2e. 2e . The Slate-colored group of Dark-eyed Juncos ( Junco hyemalis hyemalis ) (SCJU), which breeds throughout boreal forests ranging through Alaska, Canada, and New England, is generally considered a relatively long-distance migrant, with most SCJUs migrating hundreds of kilometers to spend the winter across the southern United States (see Figure 2b ). 58 In stark contrast, another Dark-eyed Junco, the Guadalupe Island Junco ( Junco hyemalis insularis ) (GUJU), is entirely sedentary, with its range restricted to “an island within an island”: small groves of cypress trees (<5 square kilometers) on a small island (<400 square kilometers) more than 300 kilometers off the coast of Baja California, Mexico (see Figure 2c ). 59

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The map shows stylized geographic ranges of the Slate-colored Junco group of the Dark-eyed Junco species. Slate-colored Juncos are either long-distance or short-range migrants (northern breeder) or altitudinal migrants (Appalachian Mountains).

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Breeding ranges of the Pink-sided Junco, White-winged Junco, and Grey-headed Junco groups of the Dark-eyed Junco species are shown, with arrows indicating the direction of long-distance or short-range migration to overlapping wintering ranges. Also shown is the year-round range of the sedentary Guadalupe Junco, also part of the Dark-eyed Junco species complex.

Between these extremes, natural populations of the junco exhibit a full range of variation in the distances and directions they migrate. Another distinct group, the White-winged Junco ( Junco hyemalis aikeni ) (WWJU), which breeds in a small region of the Black Hills, migrates regionally towards the Southwest, probably some few hundreds of kilometers (see Figure 2c ). 60 Both Pink-sided Juncos ( Junco hyemalis mearnsi ) (PSJU) and the Gray-headed Juncos ( Junco hyemalis caniceps ) (GHJU) that breed in the Rocky Mountains and the Southwest, respectively, migrate variable distances westward and southward towards coastal mountains or lower elevations ( Figure 2c ). 61 The Red-backed Juncos ( Junco hyemalis dorsalis ) (RBJU) as well as most Yellow-eyed Junco subspecies ( Junco phaeonotus palliatus ) (YEJU) seem to be primarily facultative altitudinal migrants, leaving their higher elevation breeding sites only during harsh winter conditions (see Figures 2d and ​ and2e, 2e , respectively). 62

Among the different varieties of Oregon Dark-eyed Junco ( Junco hyemalis ) (ORJU), different subspecies span the full range of migratory dispositions—from long-distance migrants that breed in the northern part of the breeding range (e.g., British Columbia) and winter in southern and coastal California, to altitudinal migrants that breed in southern California and migrate less than fifty kilometers during periods of harsh winter weather (see Figure 2d ). 63 As detailed below, another distinct population of Oregon Juncos has become entirely sedentary following its recent colonization of a novel urban and climatically mild breeding habitat in San Diego, California. 64 Similarly, within the Slate-colored Juncos, which are typically referred to as long-distance latitudinal migrants, there exists the Carolina subspecies ( Junco hyemalis carolinensis ), which breeds in the Appalachian Mountains and migrates only short distances attitudinally (see Figure 2b ). 65

B. The Generality of Intra-Species Variation in Migrations

Importantly, although the geographic diversity of migratory groups exhibited within the junco complex is extraordinary, it is apparently not unique. Within species variation in migration appears to be quite common. 66 However, in most species, extensive efforts have not been made to characterize this variation, or logistical challenges (e.g., tracking small birds) has made it very difficult. In Mr. Hugh Dingle’s discussion of population and species differences from his text Migration: the Biology of Life on the Move , he presents examples from a diverse array of taxa in which intra-specific and geographic variation have been observed when scientists have looked closely. 67 These include milkweed bugs, 68 grasshoppers, 69 Old World warblers of the genus Sylvia , 70 killer whales, 71 Chinook salmon, 72 Brown trout, 73 and American shad. 74

In the case of the junco, as opposed to other songbirds, investigators were able to observe and characterize this variation more readily because it is associated, at least roughly, with distinct differences in plumage coloration. 75 We are confident that far less would be known about the variable migratory propensities of junco subspecies with overlapping wintering ranges if they all looked the same through binoculars or in the hand. How could it be known which ones migrate, how far, and to where? Thus we suspect, and studies have shown for a few, 76 that there are many species that share similar biogeographic history and breadth of diversity among migratory subspecies and types, but without the same dramatic diversification in plumage, 77 their migratory diversity has been harder to detect.

Another axis of variation in the junco, wing shape and wing length, is associated with differences between migratory and sedentary groups, with migrants exhibiting larger wings better suited to long sustained flights. 78 This biogeographic variation in wing morphology has been documented in many other avian species, 79 but is far more subtle than plumage variation and can typically only be detected with a captured bird.

Other methods for establishing geographic variation have also presented difficulties in the past. The small body size of most songbirds has made large-scale radio-tracking impossible. 80 Banding individual birds of local populations over small geographic scales allows the tracking of altitudinal or shorter distance migrants, but banding programs are not effective over long distances. 81 There are simply too many birds and too few banders. For larger birds and other larger animals, radio- and satellite-telemetry have been available for some time, but in many cases their widespread implementation has been prohibited by high costs, and they are deployed on few individuals from a limited number of populations. 82 Only recently have technological advances in radio-, satellite-, and photosensitive-tracking devices and stable isotope analyses provided the tools to elucidate biogeographic diversity of migrations. 83 Thus, obtaining a detailed understanding of geographic variation among populations, races, or subspecies has been quite challenging for most animal groups, but we have reason to expect significant breakthroughs in the coming decades for many species, which should prove empowering for those committed to the successful conservation of migrations.

C. Geographic Variation in Migrations: Implications for Conservation

The existence of geographic and intra-specific variation in migrations described above—that is, the degree to which the distances or routes traveled by migrants vary among different subgroups (i.e., subspecies, races, and geographically-separated populations)—poses both challenges and opportunities for conservation of migrations that must be considered.

With respect to challenges, for example, the research efforts required to obtain knowledge of patterns of migratory connectivity 84 between breeding and wintering grounds and to characterize the migratory routes and stopover requirements for particular animal groups becomes greater, more expensive, and more logistically demanding when intra-species variation must be considered. However, once this knowledge is obtained, only certain subspecies or races may prove to exhibit migratory behavior or traverse imperiled landscapes on their journey. Here lies the opportunity: armed with the knowledge of which populations migrate, conservation specialists will be better able to design policies and management strategies that are targeted, smaller-scale, more efficient, and ideally more effective at protecting the most important migrations per se rather than the “migratory species” as a whole. If limited funds and resources are mandated to protect a particular migration, prioritization efforts would be aided by a clear understanding of which geographic variants exhibit the most ecologically or culturally valuable, or the most imperiled migrations.

Many conservation laws, policies, and management strategies include species-level mandates and goals, which has forced scientists and conservationists to merge theory and practice in confronting the challenges associated with defining “species” and species’ boundaries. 85 These scientific and policy debates are beyond the scope of this article, 86 but to generalize, some consensus has been reached recognizing subspecies and local populations as independent targets of conservation if they represent “evolutionarily significant units” that contain unique genetic or biological characteristics that distinguish them from other such groups. 87 However, defining criteria that effectively discriminate between groups on separate evolutionary trajectories is challenging and contentious, because in most species, the extent of phenotypic and genetic variation is not well-sampled, and consensus on quantitative genetic divergence thresholds is lacking. 88 Elucidating intra-specific variation in migratory disposition and geographic connectivity could provide highly relevant criteria for the distinct evolutionary history and evolutionary future of particular migratory (or sedentary) populations, 89 perhaps allowing for their inclusion in targeted species-level legal, policy, and management mandates such as those invoked by the Endangered Species Act. 90 For example, in his recent monograph, Kevin Winker argues that geographic variation in migratory behavior has been a key ecological driver of differentiation leading to speciation throughout the evolutionary history of avian lineages, suggesting that populations that differ in their migratory behavior are likely to be on divergent evolutionary trajectories. 91

IV. Intra-Population Variation: Partial and Differential Migration

Even within a local population or subspecies, migratory animals can vary in two important ways. “Partial migration” refers to those populations in which some individuals migrate and some remain sedentary. 92 “Differential migration” refers to those migrations where different groups of migrating individuals, such as age and sex cohorts, move varying distances. 93 These two categories of migratory variation are not mutually exclusive. 94 We bias our subsequent attention here to focus more on differential migration, though most of the implications for conservation are the same—both invoke the need to consider types of individuals within a population differently, with respect to both the biogeographic and demographic consequences that ensue for conservation policy and planning.

A. Partial Migration

Partial migration has been documented in a wide variety of taxa from birds to fish, and is likely to be much more widespread than has historically been appreciated. 95 Partial migration appears to have a genetic basis in some taxa (i.e., the propensity for individuals to be migratory or sedentary is heritable), 96 but in other systems migration is known to be a conditional Demographic models and field data from certain species suggest that residents should gain major fitness advantages (e.g., earlier onset of reproduction in spring and perhaps reduced mortality associated with migratory journeys), for which the benefits of migration outweigh the benefits of staying put, despite any costs of making the journey. 98 In other species, residents (i.e., non-migrants) may pay high mortality costs by overwintering in harsh climates, so similarly, the net expected benefits of remaining (i.e., not migrating) in terms of overall fitness must be even greater. 99 Dominance interactions have been implicated as a major driver of both partial migration and differential migrations; both of these types of migration can segregate non-breeding populations with respect to age cohorts or sex cohorts, as older animals and males versus females, are typically more dominant. 100

In the junco, several populations that are altitudinal migrants (e.g., Yellow-eyed Juncos, Carolina Juncos, Red-backed Juncos, and southern races of the Oregon Junco group) also appear to be partial migrants to a large degree. Some individuals, especially males, stay on or very close to the montane breeding grounds, even during winter storms, while females and younger individuals are more likely to depart. 101

B. Differential Migration

Among migratory members of a population, differential migration refers to variation in distance traveled, some long, some short; and it can give rise to habitat-related segregation of classes of individuals (e.g., by sex or age). 102 If, for example, males are more likely to survive in one habitat type and females in another, then evolution will favor divergence in habitat preferences and dispersal behavior, causing the sexes to settle in different locations. 103 Diverse ecological mechanisms have been proposed to underlie the evolution of differential migration among sex and age classes, including: differences in nutritive requirements (e.g., in Northern Elephant Seals, Mirounga angustirostris ); 104 variable abilities to cope with thermally exposed habitats (e.g., in the Great Bustard, Otis tarda ); 105 differential predation risk (e.g., in Western Sandpipers, Calidris mauri ); 106 or intrasexual competition for breeding resources that may lead one sex (usually males) to travel shorter distances than the other sex so as to remain closer to the breeding range in autumn and to arrive earlier in spring to breeding grounds (e.g., in the White-Throated Sparrow, Zonotrichia albicollis ), 107 a pattern that is probably common in many avian migrants. Additionally, other selective factors, such as climate and disease prevalence, may play a role in mediating differential migration. 108 It is also important to note that differential migration can lead to differences among age or sex or dominance cohorts not just in distances traveled, but also in the seasonal timing of migration—e.g., those that migrate farther from the breeding grounds may begin the return journey sooner. 109

C. The Junco, a Differential Migrant

In the winter, the range of the of the Slate-colored Junco ( Junco hyemalis hyemalis ) in eastern North America extends from the northern United States and the extreme southeast of Canada to the southern United States (as illustrated in Figure 2b ). 110 Settlement of the winter grounds occurs between mid-October and early December more or less simultaneously as the sex and age classes that migrate farther also migrate earlier. 111 Junco males are generally larger and dominate females, limiting their access to food resources. 112

Data collected between 1950 and 1976 on migration schedules and population structure for Junco hyemalis hyemalis overwintering near Bloomington, Indiana, as well as data obtained from museum collections and from populations sampled at other locations throughout the winter range, revealed the historic pattern of geographic variation in winter population structure and dynamics of the junco. 113 These data showed that females and older birds move farther south (i.e., farther from the breeding range) in winter than did males or younger birds. 114 They also showed that overwinter survival was greater at lower than at higher latitudes but that annual survival did not differ by latitude. 115

Assuming equilibrium population dynamics (i.e., a stable and equal population-wide sex annual rate of survival over time), 116 the implication of these findings is that juncos making longer migrations to winter in the south (primarily females), where winter survivorship is higher, must face higher risk of mortality during migration; while juncos making shorter migrations (primarily males), given their observed lower winter survivorship due to extreme and unpredictable northern climates, must face lower risk of mortality during migration. 117 Subsequent research ongoing to the present has examined both the proximate and ultimate factors underlying differential migration in the junco. It appears that the propensity for males and females to migrate different distances has at least a partial genetic basis, but it also depends on the environmental conditions (both the social environment and climatic environment) that they encounter along the way. 118 And although behavioral data on dominance interactions indicate that male-dominance at feeding sites drives females southward, 119 it is less clear why older birds migrate farther south. One idea is that because juveniles start migrating earlier in the autumn while adults finish their molt, they are able to establish themselves sooner as “owners” of wintering sites farther north. 120 These studies of differential migration in the junco were some of the first to carefully document the phenomenon and examine its underlying causes and consequences in an ecological and evolutionary framework.

D. Differential Migrations: Implications for Conservation

Regardless of the specific ecological mechanisms, the key implication of both differential and partial migrations for conservation is that males versus females or young versus older birds may be found in different geographic regions during the non-breeding season and thus along different points in their migratory routes at different times of the year, and if one area were to be rendered uninhabitable or impassible, the consequences for the species or the migration could be far reaching and more damaging than the area involved might predict.

Thus, the task of conserving migrations in species that exhibit differential migration is made more complex and challenging. In extreme cases of segregation of sub-populations (e.g., males and females) for example, it has been proposed that to ease the implementation of conservation policy, management practices should consider the classes as completely different species. 121 At the very least, management policies must include strategies that protect habitat at breeding and winter locations, migration corridors, and stop-over sites that may differ in usable food and shelter resources by sex or age. In most cases, migration corridors are unknown, particularly for neotropical migrants, and in many cases basic knowledge of migratory connectivity is nonexistent with respect to breeding and wintering grounds. 122 Moreover, when conserving these habitats, policies must also take into account class differences in predator-prey dynamics, potential competitive interactions between classes, and effects of disease communities. Current data, however, are very limited with respect to the ecological mechanisms underlying differential migration. 123 Furthermore, while differential migration has been documented in a number of birds, 124 it is only recently that differential migration has been studied and fully characterized in Old World warbler families, 125 raising questions about even relatively well-studied species in which this phenomenon may be overlooked.

It is important to note that the benefits of conserving and the consequences of losing differential migrations are not clear. For example, is differential migration a mechanism to reduce competition between sex and age classes, or does it place a heavier burden (e.g., migration distance traveled) on subordinate sex classes such that relaxation of segregation may be beneficial to the subordinate sex? In the junco, differential migration apparently results in sex differences in overwinter mortality versus that incurred during migration. 126 Recent changes in climate, however, are reducing the level of segregation between the sexes. 127 Relaxing segregation could reduce migrational mortality for females while increasing overwinter mortality due to extreme weather events, which could lead to skewed sex ratios and subsequent selective sweeps, fundamentally changing the genetic or phenotypic architecture of a population. It is worth noting again, however, that despite the depth of knowledge about the junco, it is extremely difficult to predict the consequences of any potential change in demography or distribution. This holds to an even greater degree for the vast majority of species whose migratory biology remains undocumented.

V. Migrations as Dynamic Phenomena: Responses to Environmental Change

In addition to the spatial (i.e., geographic) variation in migratory biology observed both among and within populations, migrations also vary within species across another important axis: time. Over both contemporary and historical time scales, the characteristics of migrations are constantly changing in response to shifting environmental conditions. As anthropogenic climate change and habitat alteration progress at alarming rates, this reality must be an especially important aspect of research, policy, and management agendas for migration—both with respect to researching and mitigating the detrimental effects of altered environments on migrations, but also insofar as the habitat ranges and phenologies of migratory animals represent “moving targets.”

Below, we highlight two examples from the Dark-eyed Junco, both of which illustrate how human activities may have led to dramatic shifts in migratory processes over time, even over just a few decades. While these examples are striking because we can time their occurrence, the diversity of migratory phenomena across closely related species and populations (which indicates repeated and relatively recent evolutionary changes), as well as additional contemporary examples that also demonstrate recent and rapid shifts in migration in response to changing climates 128 and urbanization, indicate that migrations can be quickly gained, lost, or altered as environments change.

A. Shifts in Junco Winter Distribution and Sex Ratio Associated with Climatic Warming

Over the past 100 years, the Earth’s mean temperature has increased at least 0.6 degrees Celsius. 129 This increase has resulted in long-term, large-scale alterations in phenology, distribution, and population dynamics of eighty-five percent of the animal and plant species that have been studied. 130 Warmer temperatures have also been associated with a ten percent reduction in winter snow cover, permitting increases in available winter seed biomass and the migration of several plant species into previously unavailable northern habitats. 131 For ground-feeding birds, these changes may have lessened the cost of overwintering at higher latitudes by reducing the number of extreme temperature drops that induce fasting events or by decreasing competition for winter resources, 132 thereby decreasing the relative benefits of prolonged migration farther south.

To date, the main focus of study with respect to distribution changes following climate warming has not included differential migrants. 133 As we mentioned above, for differential migrants, the consequences of changes in climate and distribution may be especially problematic to predict, as relaxation in segregation may impact one sex more than the other, resulting in significant changes in demography, population dynamics, and possibly changes in abundance. 134 Furthermore, climate change can have significant effects on wintering physiology impacting survival as well as reproductive success. In the junco, where the correlation between historical sex ratio during winter (the result of sex differences in distance migrated) and climate measures is high, 135 one would predict that with milder climates females may no longer migrate as far south as they did previously.

When recent demography of wintering populations across the junco’s winter range was compared to data collected thirty years ago to assess whether recent warming has led to detectable changes in the population structure of the Dark-eyed Junco ( Junco hyemalis hyemalis ), the comparison supports this prediction. 136 Recent data suggest significant changes in sex ratio across the junco’s winter range with relatively more females at more northern latitudes and relatively fewer males at southern latitudes. 137 Additionally, these changes in sex ratio appear to be highly correlated with the milder present-day winter climate. 138 In association with increases in the proportion of females making shorter migrations, there has also been a distributional shift. The number of juncos wintering further north has increased and the number in more southerly regions has declined. 139 Current data also suggest that in years in which climate is milder, females truncate their migratory journey and remain in larger numbers at northern and intermediate latitudes. 140 “When climate is more like historic conditions (i.e., more days with extreme minimum temperatures and snow fall), females make longer migrations and winter sex ratios match historic data.” 141 Such instability in sex ratio between years suggests plasticity in junco migratory behavior and a possible adaptation to changing climate.

B. Rapid Loss of Junco Migration Following Establishment in a Novel Urban Environment

As discussed above with respect to geographic variation, a full range of migratory diversity is represented within the western Oregon Junco group, with northern breeding populations migrating thousands or hundreds of kilometers southwards to spend the winter, whereas southern groups are facultative altitudinal migrants, leaving their montane breeding grounds to winter on the coast. In San Diego County, California, the thurberi race of the Oregon Junco ( Junco hyemalis thurberi ) has historically been found breeding only in higher elevation (e.g., >1500 meters) forest habitats in the mountains seventy kilometers inland from the coast. 142 These juncos migrate variable distances to lower elevations and coastal areas during harsh winter weather. 143 In the early 1980s, however, a small isolated breeding population (approximately eighty breeding pairs), colonized an atypical and previously unoccupied habitat: the urban and coastal campus of the University of California-San Diego—presumably as the result of some wintering individuals failing to return to the breeding grounds, but instead remaining on the coast to breed. 144 Since then, a stable breeding population has persisted as an effective biogeographic island. Many biological changes have since been documented, 145 including that the colonist population is entirely sedentary, remaining on or near its breeding territories year round. 146

Preliminary data suggest that the differences in migratory disposition (i.e., the lack of migration) in the colonist population have a genetic basis, as evidenced by differences in migratory restlessness behaviors in captive birds from the colonist (sedentary) population and a population from the nearby ancestral-range (altitudinal migrants) when raised under identical environmental conditions. 147 Though colonist birds still exhibited seasonal migratory restlessness behaviors in the common captive environment, the intensity of these behaviors were greatly reduced in the captive birds originating from the colonist population when compared to birds originating from the montane ancestral-range. 148

Although the exact sequence of events leading to population establishment and the cessation of migration in this system is not entirely clear, one thing is certain: the Mediterranean climate and natural coastal sage scrub habitats of San Diego County do not include suitable junco habitat (which is typically seasonal and forested). However, the presence of thousands of landscaped eucalyptus trees as well as ornamental vegetation and abundant anthropogenic water and food resources, has created an “artificial habitat,” allowing juncos to establish this population in a climatically mild and urban environment. Increased sedentary behavior (i.e., loss of migration) following colonization of urban habitats has also been documented in European blackbirds, apparently over the course of just a few hundred years since these birds recolonized cities. 149

C. Migration as Dynamic in Response to Changing Environments: Implications for Conservation

The sensitivity of migrations to environmental change has several important implications for conservation agendas. As the above examples from the junco indicate, climate change and habitat alteration have the potential to lead to geographic range shifts, changes in intra- or inter-species competition, and even the cessation of migration altogether. Thus, it is easy to imagine scenarios in which a species could be “conserved” from a demographic standpoint, but the phenomenon of its migration could be lost or dramatically altered in response to changing environments.

Scientists generally consider such changes in the biology (e.g., physiology, behavior, appearance) of animals in response to environmental change to be the result of either genetic changes (evolution), developmental changes induced by the environment (phenotypic plasticity), or perhaps most typically, some combination of both. 150 This is true for migrations, as evidenced by prior research that indicates both a genetic and an environmental basis for the timing and distance of migrations. 151 This means that observed shifts in breeding or wintering ranges of migratory animals, or changes in the onset or duration of migration, could result from either 1) rapid genetic evolution of the population in response to new (natural) selective forces, or 2) from individual organismal responses to new environmental regimes.

In most cases, these alternatives can be difficult to distinguish, but they have important implications for conservation. In short, species that lack the genetic variance in migratory disposition (e.g., complete, obligate migrants) to adapt via natural selection may be more imperiled, as might species that exhibit strong but maladaptive developmental responses to environmental change (e.g., “mis-timing” of migration due to shifting temperature cues). Although migratory species that exhibit sufficient genetic variation or adaptive plastic responses to environmental change may be less at risk of extinction, adaptation to novel climatic regimes or altered habitats could include range shifts, or attenuation, or cessation of migratory behavior. In some cases, it is plausible that the effects of changing environments on migrations could be reversed by management efforts, and this would probably be more likely and occur more quickly in the case of plastic responses.

Future efforts to conserve migrations should incorporate the temporally dynamic nature of migrations in two possible ways. First, following from current and future science-based understandings of how changing climates and altered habitats influence migratory biology, policy mandates and management activities could be pursued that minimize or reverse these impacts. For example, food supplementation, predator or competitor removal or mitigation, or management of local vegetation could aid breeding animals who have “mis-timed” their migrations to no longer coincide with the emergence of local food supplies or who have shifted their ranges maladaptively in response to changing climates. Similarly, planning of suburban development could include restrictions and policies that prevent the availability of ad libitum anthropogenic food and water sources to migratory wildlife on wintering grounds that could induce them to become sedentary. Second, an awareness of how migrations might respond to changing environments can allow conservationists to use predictive tools to prioritize the best habitats or mitigation strategies based not on current environmental conditions and habitat ranges, but based on those conditions that may be predicted by climatic or land-use models.

VI. Conclusion

Characterizing and incorporating the spatially and temporally variable nature of animal migrations into conservation agendas is a formidable objective, and one that adds layers of complexity to an already complex set of legal, political, and management challenges. Even for a single migratory species, achieving the tasks of documenting geographic variation among populations, evaluating whether there is variation among sex or age cohorts within populations, and investigating how habitat alteration or climate change might likely impact migratory behaviors is daunting. Nevertheless, these pieces of information are essential considerations for the development of conservation strategies, and even limited knowledge—generated with the assistance of emerging technologies and collaborative approaches—can present opportunities to develop targeted, smaller-scale, more efficient, and ultimately more effective conservation laws, policies, and management plans for animal migrations.

Acknowledgments

The authors would like to thank the leaders, presenters, and participants in the Animal Migration Conservation New Knowledge Seminar Series and Workshop, spearheaded by Professor Robert Fischman and funded by the Institute for Advanced Studies at Indiana University, for inviting us to participate in the discussions and workshops that lead to this manuscript. We would also like to thank the students and staff of the Environmental Law editorial team for their guidance during manuscript preparation and revision, especially Mr. Samuel Terpstra for his contributions to our workshop and comments on the manuscript. We would also like to acknowledge Libby Swanger for her work compiling data and preparing the figures for the manuscript. Questions and comments related to the Article can be directed to Jonathan Atwell ( ude.anaidni@llewtawj ).

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Tens of millions of bison once rumbled across the Great Plains on a quest for grazing. By the late 1800s nearly all had been slaughtered. Today most of the half million remaining bison are in captivity, like these on the Triple U ranch in South Dakota.

Animal Migrations

What is it that makes animal migration such a magnificent spectacle for the eye and the mind? Is it the sheer abundance of wildlife in motion? Is it the steep odds to be overcome? Is it the amazing feats of precise navigation? The answer is all of the above. But there’s another reason why the long-distance journeys of wildebeests, sandhill cranes, monarch butterflies, sea turtles, and so many other species inspire our awe. One biologist has noted the “undistractibility” of migrating animals. A nonscientist, risking anthropomorphism, might say: Yes, they have a sense of larger purpose.

Animal migration is a phenomenon far grander and more patterned than animal movement. It represents collective travel with long-deferred rewards. It suggests premeditation and epic willfulness, codified as inherited instinct. A biologist named Hugh Dingle, striving to understand the essence, has identified five characteristics that apply, in varying degrees and combinations, to all migrations. They are prolonged movements that carry animals outside familiar habitats; they tend to be linear, not zigzaggy; they involve special behaviors of preparation (such as overfeeding) and arrival; they demand special allocations of energy. And one more: Migrating animals maintain a fervid attentiveness to the greater mission, which keeps them undistracted by temptations and undeterred by challenges that would turn other animals aside.

An arctic tern on its way from Tierra del Fuego to Alaska, for instance, will ignore a nice smelly herring offered from a bird-watcher's boat in Monterey Bay. Local gulls will dive voraciously for such handouts, while the tern flies on. Why? "Animal migrants do not respond to sensory inputs from resources that would readily elicit responses in other circumstances," is the dry, careful way Dingle describes it. In plainer words: These critters are hell-for-leather, flat-out just gonna get there. Another way, less scientific, would be to say that the arctic tern resists distraction because it is driven at that moment by an instinctive sense of something we humans find admirable: larger purpose.

The arctic tern senses that it can eat later. It can rest later. It can mate later. Right now its implacable focus is the journey; its undivided intent is arrival. Reaching some gravelly coastline in the Arctic, upon which other arctic terns have converged, will serve its larger purpose, as shaped by evolution: finding a place, a time, and a set of circumstances in which it can successfully hatch and rear offspring.

But the process is complex as well as various, and different biologists define it differently, depending in part on what sorts of animals they study. Joel Berger, of the Wildlife Conservation Society and the University of Montana, who works on the American pronghorn and other large terrestrial mammals, prefers what he calls a simple, practical definition suited to his beasts: "Movements from a seasonal home area away to another home area and back again." Generally the reason for such seasonal back-and-forth movement is to seek resources that aren't available within a single area year-round. But daily vertical movements by zooplankton in the ocean—upward by night to seek food, downward by day to escape predators—can also be considered migration. So can the movement of aphids when, having depleted the young leaves on one food plant, their offspring then fly onward to a different host plant, with no one aphid ever returning to where it started.

Dingle, an evolutionary biologist who studies insects, offers a more intricate definition than Berger's, citing those five features (persistence, linearity, undistractibility, special start-and-stop behaviors, stored energy) that distinguish migration from other forms of movement. For example, aphids will become sensitive to blue light (from the sky) when it's time for takeoff on their big journey and sensitive to yellow light (reflected from tender young leaves) when it's appropriate to land. Birds will fatten themselves with heavy feeding in advance of a long migrational flight. The value of his definition, Dingle argues, is that it focuses attention on what the phenomena of the wildebeests and the sandhill cranes share with the phenomenon of the aphids and therefore helps guide researchers toward understanding how evolution by natural selection has produced them all.

Rattlesnake migration on the Great Plains of western Canada is a peculiar but illuminating case. A young Canadian biologist named Dennis Jørgensen, now employed by the World Wildlife Fund, studied movements of the prairie rattlesnake (Crotalus viridis viridis) on the outskirts of Medicine Hat, Alberta, near the northern limit of its range, and found the snakes migrating ambitiously each spring and fall. The average round-trip by his animals was about 5 miles, though an earlier study detected Canadian rattlesnakes migrating as far as 33 miles. In Arizona, by contrast, rattlers don't travel nearly so far, because they don't have to. The driving logic of the Canadian migrations is related to cold winter temperatures (always difficult for reptiles) and the scarcity of really good den sites in which to survive hibernation.

"There aren't many dens that can support survival over winter on this landscape," Jørgensen told me. An ideal den must be deep underground, where the earth is warm, but accessible from the surface via burrows or natural fissures. Such refuges are few and far between. "Because of that, what you get is very large aggregations of snakes at these communal dens." Picture a serpentine tangle of a thousand snakes, piled together cozily, calm and sleek in their subterranean nook, jointly awaiting the signals of spring. When surface temperatures rise to a comfortable threshold, they emerge. For a while they bask in the sunlight, crowded like bronzed tourists on the Costa del Sol. But the rattlesnakes are hungry. What's their next imperative? To get away from one another, find food, and mate. So they migrate radially—in all possible directions away from the den—like starburst embers from a Fourth of July rocket.

Jørgensen used small radio transmitters, surgically implanted, to chart this pattern, tracking the individual routes of 28 different rattlesnakes during 2004 and 2005. More recently, on a blazing summer day, he took me back to one of the den sites, in a slumping bank above the South Saskatchewan River. The slumping had opened deep underground cracks in which roughly 60 prairie rattlers had wintered. From the riverbank we turned toward the uplands and began retracing the migration route of one of his animals, an ambitious female he had labeled E.

Not far upslope were three rounded boulders, lichen-covered, with a hole beneath. Snake E had arrived here on May 8, Jørgensen said; she rested, basked, and took off again on May 27. She ascended this steep bench (we started climbing) amid the sage and crumbling gray mud, then slithered back down the slope (we plunged after her), crossed this dirt road, crossed this moist gulch full of goldenrod and skunkbrush (we thrashed through), and climbed again. Back atop the bench, we ducked between strands of barbed wire into the corner of a crop field irrigated by center pivot. The crop had been alfalfa when E came through; this year, potatoes. We politely circumvented the spud field and picked up her track on the far side, between several more center-pivot circles, blooming bright yellow and rank with canola. The midday air, hot and thick, smelled like baked fish from an oven.

Having sprinted across two pivot fields in a single day, brave lady, E had then picked up the security of a fence line, where the weeds were dense and the discs of a tiller, the blades of a swather, never touched. By late June she was making 200 yards daily, still along the fence line, amid a hospitable jumble of rocks, weeds, and rodent burrows. At that point Jørgensen and I paused in the shade of a cottonwood to rest. We had covered eight weeks of rattlesnake migration in four hours and were drenched with sweat.

Hereabouts was where E had spent most of her summer that year, mating at least once and fattening herself on rodents for the homeward migration, another winter in the den, and pregnancy. It was productive habitat but also risky, Jørgensen said, what with all the agricultural machinery that could dice a snake like zucchini, all the farm-road traffic that could flatten her like an alligator belt. The changes that had come to this landscape did not favor long-distance rattlesnake migration. At that moment, as though to embody those changes within one human memory, a man named Aldo Pederzolli pulled up on his four-wheeler.

Pederzolli was the farmer on whose land we stood and who had genially welcomed Jørgensen's study. He wore a black polo shirt, rubber boots, and a cap that read "Cee-Gee Earthmoving." He was a fit-looking man of 80, with squinty brown eyes, a high voice, a sun-ripened Canadian smile. Introduced to me, hearing the reason for my visit, he said, "Oh, I just love rattlers." This wasn't irony. Got enough good snakes, he added, and you don't need to worry about gophers. Back when he was young, Pederzolli recalled, he would see nice fat old rattlers, that big around, when he seeded a fallow field. Don't see such big ones anymore. There was a den near the river, he said wistfully, and they'd migrate six miles up to a nice patch of open prairie full of gophers. Not anymore.

Although it's only a hypothesis, Dennis Jørgensen suspects that natural selection—in this case, the death of the venturesome—may be turning his migrating rattlesnakes into a population of stay-at-homes.

Biological diversity entails more than a gross tally of species. Diversity of ecosystems, behaviors, and processes are important too, contributing richness and beauty, robustness and flexibility and interconnectedness to the living communities on Earth. To lose the long-distance migrations performed by some species would be a grievous diminution. Joel Berger has made this point in the journal Conservation Biology and elsewhere, with reference to migrating species around the world and one creature close to home: the pronghorn (Antilocapra americana), North America's only endemic species of ungulate.

Loose talk sometimes mistakes the pronghorn for an antelope, but in fact it belongs to a family all its own. Its extreme speed (fastest land mammal of the New World), more than necessary to evade any living North American predator, probably reflects adaptation for escaping the now extinct American cheetah of the Pleistocene. Besides traveling fast, though, the pronghorn also travels far. One population migrates hundreds of miles across the Great Plains from north-central Montana into southern Saskatchewan and Alberta. Another population follows a narrow, tenuous route from its summer range in Grand Teton National Park, across a divide at the headwaters of the Gros Ventre River, and down onto the plains south of Pinedale, Wyoming, in the Green River Basin. There the pronghorn mingle with thousands of others arrived from other parts of Wyoming, where they try to distance themselves from the natural gas wellheads and drilling teams, and wait out the frozen months, feeding mainly on sagebrush blown clear of snow.

The Grand Teton pronghorn are notable for the invariance of their migration path and the severity of its constriction at three critical spots, known as Trappers Point, the Red Hills, and the Funnel. Field research by Berger and colleagues has charted that path and illuminated its jeopardies. If the pronghorn can't pass through each of the three bottlenecks during their spring migration, they can't reach their bounty of summer grazing in Grand Teton National Park; if they can't pass through again in autumn, escaping south onto those windblown plains, they'll likely die trying to overwinter in the Jackson Hole area or get fatally stuck in the deep snows of the divide. On a bright day in November, in company with a biologist named Renee Seidler, I went for a look at the details of their dilemma.

Seidler, also employed by the Wildlife Conservation Society, works mostly on habitat issues in the booming gas fields between Pinedale and Rock Springs, an area that supports perhaps 20,000 pronghorn each winter. The northward migrants constitute just a fraction of that total but are special, she noted, because without them, one of America's great western parks, Grand Teton, would entirely lack one of America's great western species. On a knoll at Trappers Point, we read the historical marker about fur trappers and Nez Perce and Crow peoples gathering here to trade and gazed down at the modern manifestations of growth and commerce alongside Highway 191: a sprawling little community known as Cora Junction. There were about 50 houses, trailers, and other buildings, including a Jehovah's Witnesses meeting hall, all nested within a grid of streets and lanes, fenced yards, dogs, chickens, real estate signs, old tires, boats on trailers, a weathered trampoline, and a rusting green Chrysler from the 1940s. Right about here, Seidler said, pointing to a gap of sage between our knoll and the houses, is where most of the pronghorn seem to cross through.

We drove north on a county road about 20 miles, along willowy bottomlands of the upper Green River, tracing the migration route. Pronghorn, dependent on distance vision and speed to keep safe from predators, do not like willowy bottomlands, Seidler explained. They don't like dense forest either, so they traverse these high, open shoulders between the river and the woods, where they can see and run. Then we came to a place where forested hills rose on both sides of the river to form a soft V, leaving a corridor of open ground only about 150 yards wide. "That's the Funnel," Seidler said. It was private land, dissected by the driveways, the buck-and-rail fences, the arched gateways of people wealthy enough to have a second home, or a third, on the headwaters of the Green. On this day, in the off-season, there was no sign of anyone around.

One more yard fence, one more house, one or two large barking dogs, could make a bad difference. As at Trappers Point so here at the Funnel; incremental human activities are accumulating toward a crisis for Grand Teton's pronghorn—threatening to choke off their passageway.

Conservation scientists such as Berger, along with some biologists and land managers within the National Park Service and other agencies, are now working to preserve migrational behaviors, not just species and habitats. The Bridger-Teton National Forest has recognized the path of the Grand Teton pronghorn, much of which passes across national forest land, as the first federally protected migration corridor. But neither the Forest Service nor the Park Service can control what happens on private land at a bottleneck, nor on Bureau of Land Management parcels within the drilling fields south of Pinedale. And with certain other migrating species, the challenge is complicated further—by vastly greater distances traversed, more jurisdictions, more borders, more dangers along the way.

Imagine, for instance, that you're a lesser sandhill crane (Grus canadensis canadensis), setting off on your spring migration from southwestern Texas. You might have to fly across a corner of New Mexico and Oklahoma, then Kansas, Nebraska, South Dakota, North Dakota (most of which allow hunting of sandhills), then over the Canadian border into Saskatchewan, angling northwest across Alberta and British Columbia, across Yukon Territory, then the breadth of Alaska, and finally across the Bering Strait to your summer breeding grounds in northeastern Russia. This would be a trip of roughly 5,000 miles. Needing to pause somewhere and replenish yourself, you would probably stop on the Platte River in Nebraska, near the town of Kearney. If so, you’d have company. About 500,000 northbound sandhills make the same stopover every year.

There they linger for two or three weeks, maybe four. Some depart onward as others arrive, keeping the average crane count during March and April at around 300,000. By night they roost in the gently flowing shallows of the Platte, shin-deep in cool water, or else on sandbars, giving them warning against any predator that might come splashing out. Each morning they rise up in vast, graceful waves and fly to fields nearby, where they spend their days assiduously feeding on waste corn the harvesters missed and earthworms and other invertebrates. Such a stopover period is no exception to the undistractibility of migrating animals, as defined by Hugh Dingle; it's a part of the whole program, repeated by generations of cranes. During this stopover, a six-pound lesser sandhill adds about a pound and a half of fat to its weight. (The greater sandhill, another subspecies also present on the Platte, is larger.) The birds need that fat between Nebraska and Russia. Therefore, they need the stopover habitat—the shallows, the sandbars, the security, the corn and invertebrates—to complete their arduous yearly cycle.

I stood overlooking that habitat, on a morning in late March, and watched wave after wave of cranes rise from the river. Each group climbed clumsily off the water, gained elegance as their wings caught more air, turned in formation, and flew out to their daily feeding. Meanwhile, they called to one another in their distinctive, creaky trill. There were maybe 60,000 sandhills just within the sweep of my binoculars. It was a spectacle of extraordinary abundance, a reminder of what America looked like back when John James Audubon stared up at sky-clogging flocks of passenger pigeons, when George Catlin saw the thunderous migrations of bison.

I had watched fly-in of the cranes too, on an earlier evening, when they arrived back through the twilight and settled onto their shoals for the night. But I found fly-out more deeply affecting—because, I suppose, the birds at daybreak are headed off with a purpose, not just home to rest. They would fatten themselves for another long leg of their journey. Their travel would take them to safe and bountiful breeding grounds. Their prodigious efforts, their resistance to distraction, would yield new cohorts of sandhill cranes, extending and rejuvenating the species. I almost wrote "perpetuating the species," but no, we can't be sure of that. Nothing alive is perpetual.

It was the accrued wisdom and resoluteness of evolution that I was witnessing, airborne above the Platte. If we humans have accrued equal wisdom and can summon equal resoluteness, I thought, maybe we'll allow them to continue their journeying a while longer.

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  • Published: 06 August 2015

Climate change impacts on animal migration

  • Frank Seebacher 1 &
  • Eric Post 2  

Climate Change Responses volume  2 , Article number:  5 ( 2015 ) Cite this article

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This is the first in a regular series of mini-review which highlight outstanding recently published papers that shed new light on biological responses to climate change. We chose migration as the topic for the first mini review because its global geographical scale makes migrating individuals particularly vulnerable to climate change, and at the same time, the process of migration has fundamental impacts on ecological processes and biodiversity.

Movement is an integral part of the ecology of many animals, and it can affect individual fitness and population persistence by enabling foraging and predation, behavioural interactions, and migration [ 1 ]. Migration, in particular, affects biodiversity at regional and global scales, and migratory animals affect ecosystem processes. Animals use predictable environmental cues for the timing and navigation of migration. A change in these cues will affect the phenology and extent of migration. Arrival date and hatching date are phenological markers in migrating birds, for example, that can be strongly affected by global warming. These dynamics have been incorporated into a mathematical model recently [ 2 ]. Higher temperatures cause earlier appearance of the insect prey of hatchling birds, which exerts pressure on birds to breed earlier so that hatchling development coincides with peak prey abundance. Advanced breeding is dependent on the arrival time of the adults at the breeding site, as well as the delay between arrival and the start of breeding. These traits can change synchronously or asynchronously, and a mismatch between prey abundance and hatching can cause population declines. The mathematical model [ 2 ] explores the dynamics of these interactions and their evolutionary trajectories, and it can explain patterns observed in European flycatchers. Conversely, departure from their non-breeding grounds in Africa also appears to occur earlier for at least some Palearctic migratory birds [ 3 ]. Hence, there is a global shift in departure and arrival times that affects migratory bird movement and local abundance as a result of climate warming.

Phenological shifts in migration of endothermic birds are linked to the abundance of their ectothermic prey. Although endotherms are also directly affected by changes in temperature, which affects their metabolic demands for thermoregulation, these direct effects are more pronounced in ectotherms. The body temperature and hence physiology of ectotherms such as invertebrates and fish is closely tied to environmental temperatures. Climate warming will therefore influence metabolism and other physiological processes directly in ectotherms, and this can have pronounced effects on movement and migration.

The proximate mechanisms that enable movement are the physiological functions that provide energy to the muscles and the muscles themselves that transform chemical energy (ATP) to work. All physiological processes are influenced by temperature, to varying degrees and usually optimal physiological rates are achieved within a relatively narrow temperature range. At extreme low or high temperatures, cessation of physiological functions leads to mortality [ 4 ]. But even at more benign temperatures that nonetheless diverge from the optimal range, decreases in physiological performance increase ecological failure by impairing movement [ 5 ]. Hence, changing environmental temperatures, including changes resulting from anthropogenic global warming, impact migration and other ecological processes via the thermal sensitivity of physiological processes.

A new theoretical model now suggests that in Chinook salmon, metabolic constraints exacerbate the effect of temperature on the metabolic costs of migration [ 6 ]. Salmon often migrate hundreds of kilometres from their natal riverine areas to the ocean and return to their natal areas for spawning. Salmon prefer relatively cool water, and increasing water temperatures compromise their cardiovascular and metabolic physiology [ 5 ]. If fish did not have to replenish metabolic substrates (in particular glycogen) during their migration, the effects of warm water could be mitigated by swimming through warm sections of river rapidly. However, the need to replenish substrates by resting in tranquil warm water increases migration times and therefore exposure to warm waters [ 6 ]. Replenishment of substrates and increased exposure to warm water reduce the metabolic scope, that is the energy available for migration, because metabolic maintenance costs increase in warmer water. These effects are amplified because cardiovascular efficiency is compromised as temperatures increase beyond optimal ranges, thereby further reducing metabolic scope. These physiological dynamics mean that climate warming along the North American west coast has already affected salmon migration [ 5 , 6 ]. The examples from salmon emphasise that predictions of future effects of climate change require detailed physiological studies.

Similar to birds, a recent study has shown that the phenology of migration of aphids in the UK has changed as a result of climate change [ 7 ]. Seven hundred seventy trap-years of data collected over the past 50 years showed that over 55 species of aphids started flying progressively earlier in the year, and most species showed increasing duration of their flying season. The severity of the previous winter was the best predictor for the onset of the subsequent flying season, and the number of days above 16 °C predicted flying behaviour later in the year [ 7 ]. Correlations of phenological changes with climate change are essential starting points that need to be followed up with experimental approaches to determine the cause-and-effect relationships between climate change and animal responses [ 8 ]. The strength of the salmon studies [ 5 , 6 ] is that at least some of the mechanistic bases for the climate-dependent change in migration pattern have been identified. Hence, it is possible to determine the thermal sensitivity of, for example, cardiovascular function and metabolism experimentally, and use these data to predict the effects of future or regional climate change.

In addition to shifts in phenology of migrating animals, some species have reduced their migratory behaviour or even formed sedentary populations as a result of anthropogenic changes to the environment [ 9 , 10 ]. A new study now shows that changes in migratory behaviour also alter the incidence of infectious disease and its transmission [ 11 ]. Migration can reduce the incidence of disease because individuals leave contaminated habitats periodically, individuals are more separated from each other during migration, and infected individuals are likely to succumb to demanding long-distance movement. Monarch butterflies in the US have drastically changed their migratory behaviour in recent years as a result of habitat alterations, and the incidence of sedentary, non-migratory populations is increasing. Non-migratory populations have a significantly greater rate of infection by the protozoan Ophryocystis elektroscirrha compared to migratory populations. Infected butterflies have significantly reduced lifespan [ 11 ]. Changes in animal movement and migration as a result of habitat modification and climate change may therefore alter lifetime fitness of individuals in addition to biodiversity and ecosystem processes at regional and global scales.

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Seebacher, F., Post, E. Climate change impacts on animal migration. Clim Chang Responses 2 , 5 (2015). https://doi.org/10.1186/s40665-015-0013-9

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animal migration essay

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  • Why Do Animals Migrate?

A large flock of migrating birds.

Animal migration refers to the movement of animals over a long distance, usually in line with changes in the seasons. This movement exists in all the main animal groups, which includes birds, fish, insects, amphibians, crustaceans, mammals , and reptiles. A simple movement of animals over a substantial distance cannot be considered a migration. A migration is animal movement due to reasons like changes in the season such as when birds in the Northern Hemisphere escape to the south during winter. A migration can also occur if there is a major change in the habitat of an animal such as when a young one leaves the habitat of birth and moves to adult habitats.

The definition of the phrase “animal migration” is more of a guideline than a definitive statement as migration can occur in a number of ways depending on the species. Four proposed concepts are usually used as the general guidelines for looking for signs of migration. These signs are: movement in a relatively straight line, relocation of a species on a massive scale, and a movement that redistributes members of a species in a population. The fourth sign is the aforementioned seasonal movement.

Animals that Migrate

About 18% of the 10,000 species of birds in the world migrate due to changes in the weather seasons. Most of these birds make a north to south journey. The summer in the north is usually a season for the birds to feed and breed while the winter sees them move south to warmer places. Other species make an annual migration from the north and south hemispheres. For example, the Arctic tern makes the migration from the north to the south every year, which I a distance of about 12,000 miles.

Unlike birds, fish do not always migrate over long distances since they may end up in the same location. For example, if fish inhabit a huge lake and end up switching habitats, then the migration is rather short because they are still in the same water body. However, there are fish species that go for longer distances of up to hundreds of miles. In total, at least 120 fish species such as salmons move between freshwater and saltwater habitats. Other fish species include forage species such as capelin and herring that migrate within the North Atlantic Ocean, sardines in South Africa, and many more.

Migration of insects usually happens among winged insects such as dragonflies, butterflies, and locusts. A species of the dragonfly known as the wandering glider or the globe skimmer (Pantala flavescens) makes the trip from Africa to India across the ocean. The glider’s migration is the longest crossing of its kind among insects. Other insects that migrate include the painted lady and the monarch butterflies. However, for these two butterflies, the group that begins the migration is not the same one that completes the journey. The reason for this is that the butterflies mate and reproduce along the way so the newer generations are the ones that complete the migration.

This group exhibits the largest terrestrial migration of mammals. A good example of this migration is the famous wildebeest migration in Africa’s Serengeti National park. Aside from the wildebeest, other species that migrate include zebras and gazelles. Interestingly, these groups can alter their direction depending on the environmental conditions such that they move towards the rain.

Other Groups

Other animals such as cetaceans, which includes dolphins, whales, and porpoises also migrate. Others include some species of bats (such as the Mexican free-tailed bat) and some reptile and amphibian species. Crustaceans that migrate include the stunning Christmas Island red crab, which migrates in the millions every year.

Reasons for Animal Migration 

Reproduction.

One of the most common reasons for migration is for animals to find suitable breeding grounds for reproduction. An example of such an animal is the Atlantic salmon, which begins life in a river and then moves to the ocean upon reaching maturity. However, it still heads back to the river when ready to reproduce and the cycle is repeated. Crustaceans such as many species of crabs live in the deep seas but come to shallow waters for breeding before going back to deeper waters. Amphibians such as frogs and toads alternate between ponds for breeding and larger lakes for living.

Hibernation and Escaping Harsh Weather

Hibernation is crucial to the survival of some animals. A good example of such an animal is the little brown bat. During the summer, these creatures live in trees while they migrate to caves for hibernation in the winter.

Most of the bird species that migrate do so because of changes in the seasons. The aforementioned Arctic tern is a perfect example of such a bird. Due to its migration, the bird gets to experience two summers in a year instead of one.

Look for Food

Another common reason is a decrease in food levels. A perfect example is the wildebeest migration in the Serengeti. During seasons when food is scarce on one side, the animals begin moving to greener pastures in other places. Along the way, the direction may alter depending on where it is raining, which is where food will be in abundance. By doing this, they make sure that they give time for the land they left behind to recover so that it will be able to provide food when the herds return. Food is linked to reproduction since most animal species will migrate to breed in places where there is enough food for the survival of their offspring.

How Do Animals Know Which Direction to Migrate In? 

Experts in the field are still not sure how animals know the right directions although a few theories have been suggested. One such theory states that animals use landmarks to tell the direction. Examples of such landmarks include rivers and lakes. Other scientists have suggested that the sun and the stars are used in determining the direction. The acute sense of smell of some animals may also be how the animals determine the proper course while others use the magnetic field of the earth.

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Some species are migrating straight out of existence

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As humans continue to destroy the habitats of migratory animals, their numbers continue to decrease. Many of these species play important ecological roles, and certain ecosystems will be altered dramatically as a result. In order to protect migratory species, nations will have to come together and take conservation action — after all, animals don't adhere to any political boundaries. 

Dropping numbers

Many animals must travel great distances across land, air and sea to meet their ecological needs. This phenomenon is called migration, defined as the "seasonal movement of animals from one habitat to another in search of food, better conditions, or reproductive needs," said National Geographic . Approximately 44% of the world's migratory species are declining in population, a new United Nations report said, and of 1,189 monitored species, over one in five are being threatened with extinction. 

Migratory species "face enormous challenges and threats along the way, as well [as] at their destinations where they breed or feed," the report said. "When species cross national borders, their survival depends on the efforts of all countries in which they are found."

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The threats they face

Migrating species are being threatened by habitat loss, illegal hunting and fishing, pollution, and climate change. "Migration is essential for some species. If you cut the migration, you're going to kill the species," Stuart Pimm, an ecologist at Duke University, said to The Associated Press . The report found that overexploitation was the biggest threat to migratory species, which includes "intentional removal, such as through hunting and fishing, as well as the incidental capture of non-target species." For marine animals, "bycatch of non-target species in fisheries is a leading cause of mortality."

Climate change has also played a big role in the decline of migratory species. Changing weather patterns and warming temperatures are causing some species to migrate further or earlier than usual, which may necessitate travel at unsafe times or into unsafe regions. "These animals are, first and foremost, part of the ecosystems where they're found," Amy Fraenkel, executive secretary of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), said to CNN . "And we have a lot of evidence showing that if you remove these species, if they decline, it will have impacts on the ecosystems where they're found, and not in a positive way."

Taking global action

"One country alone cannot save any of these species," Susan Lieberman, vice president of international policy at the Wildlife Conservation Society, said to The Associated Press . To put it another way: Animals are non-partisan. "Migratory species have a special role in nature, as they don't recognize political boundaries," Anurag Agrawal, professor of environmental studies at Cornell University, said to CNN . This makes it all the more imperative that nations share the conservation burden. 

Per the report , experts recommend that nations make commitments to "restore and establish well-connected networks of protected areas and other effective area-based conservation measures," make attempts to "halt human-induced extinctions and to ensure that any taking of wild species is sustainable, safe and legal," and promise to "address climate change and pollution." Ultimately, it is clear that ensuring the survival of migratory species is a collective responsibility. 

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 Devika Rao has worked as a staff writer at The Week since 2022, covering science, the environment, climate and business. She previously worked as a policy associate for a nonprofit organization advocating for environmental action from a business perspective.  

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1 in 5 migratory species are at risk of extinction, says a new UN report

Migratory birds.

From wild salmon to wildebeest, new research finds many migratory species could soon become extinct. Image:  Unsplash/Barth Bailey

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animal migration essay

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Stay up to date:, nature and biodiversity.

  • A UN report highlights the existential threats faced by the world’s migratory species.
  • Almost half of migratory creatures are in decline and 20% could become extinct.
  • The World Economic Forum’s Global Risks Report 2024 cites biodiversity and ecosystem collapse as a major threat.

Whether it’s a murmuration of starlings, herds of wildebeest crossing the plains of Africa, or shoals of salmon leaping up a cascading river, the mass migrations of animals, birds and fish are some of the most spectacular events in the natural world. But human activity and other pressures are pushing many of these creatures towards oblivion.

The State of the World's Migratory Species repor t, a first-of-its-kind assessment by the Convention on the Conservation of Migratory Species (CMS) – an environmental treaty of the United Nations – paints a stark picture. One in five migratory species listed by the CMS is at risk of extinction and almost half (44%) are decreasing in numbers.

Have you read?

8 endangered species that are being reintroduced around the world, here's how extreme weather is affecting animal migration, 6 charts that show the state of biodiversity and nature loss - and how we can go 'nature positive'.

State of the world's migratory species.

The biggest concern highlighted in the report is the threat to migratory fish, such as wild salmon. The report finds that 97% of migratory fish species are on the verge of extinction .

The CMS research reflects concerns outlined in the World Economic Forum’s Global Risks Report 2024 . In its longer-term forecast, the loss of biodiversity and ecosystem collapse is ranked as the third biggest threat the world will face a decade from now.

In the last 100 years, more than 90 percent of crop varieties have disappeared from farmers’ fields, and all of the world’s 17 main fishing grounds are now being fished at or above their sustainable limits.

These trends have reduced diversity in our diets, which is directly linked to diseases or health risk factors, such as diabetes, obesity and malnutrition. One initiative which is bringing a renewed focus on biological diversity is the Tropical Forest Alliance .

This global public-private partnership is working on removing deforestation from four global commodity supply chains – palm oil, beef, soy, and pulp and paper.

The Alliance includes businesses, governments, civil society, indigenous people and communities, and international organizations.

Enquire to become a member or partner of the Forum and help stop deforestation linked to supply chains.

The main threats to migratory creatures

The report cites a wide range of reasons for the growing threat to migratory species. The primary risks are overexploitation and habitat loss due to human activities.

Overexploitation includes the unsustainable and illegal taking of migratory species, such as hunting and fishing, and the incidental capture of non-target species - or bycatch - in fisheries.

The report finds 277 species are endangered by excessive hunting and collecting, and a further 217 are adversely impacted by the overfishing and harvesting of aquatic resources.

Overview of threats to CMS-listed species

The loss, degradation and fragmentation of ecosystems stems from agricultural expansion, urban development and infrastructure projects, which disrupt the natural habitats and migration paths essential for the survival of migratory species, the report explains.

Amy Fraenkel, head of the CMS secretariat, says the conservation of wildlife habitats is critical to the survival of migratory species. “They regularly travel, sometimes thousands of miles, to reach these places. They face enormous challenges and threats along the way as well at their destinations where they breed or feed.”

Climate change also exacerbates the danger to wildlife by altering the timing of migrations, causing heat stress, and driving more frequent and severe weather-related events like droughts and forest fires. It acts as an "amplifier" of existing threats, including pollution and invasive species, further endangering migratory species.

Migratory cranes.

Impact on ecosystems

Migratory species play essential roles in maintaining the world's ecosystems. The report emphasizes their contribution to pollination, seed dispersal, nutrient cycling, and the regulation of ecosystems through predation and grazing. Their decline not only threatens biodiversity but also jeopardizes natural processes that are vital to human agriculture and other economic activity.

The business world has a vital role to play. “Recent data suggests that approximately 55 trillion US dollars, or half of global GDP, is moderately or heavily dependent upon nature," says Jack Hurd, from the World Economic Forum's Centre for Nature and Climate.

He explains: "What that means for companies is they need to start taking steps to assess their impacts and dependencies on nature, to develop longer-term operational plans that seek to mitigate those impacts, and to start pricing impacts and dependencies on nature into their opportunities and into their risks.

"This will change their business models, and it will change the information that they're reporting as part of their environmental obligations.”

Protecting migratory species

Despite the bleak outlook, the UN report finds population and species-wide recoveries are possible with strong, coordinated action at all levels.

One example highlighted explains how bird populations in Cyprus have recovered as a result of local action to outlaw illegal netting. While, in Kazakhstan, the Saiga antelope, once on the brink of extinction, is making a comeback due to integrated conservation and restoration efforts.

The report's findings add up to an urgent call for measures to protect migratory species and their habitats. By addressing the root causes of their decline it is still possible to ensure the survival and recovery of migratory species that play a vital role in both animal and human ecosystems.

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Animal Movement Across Scales

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2 Patterns of animal migration

  • Published: August 2014
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This chapter discusses the variety of migratory movements that can occur, and how they are thought to have evolved. Migratory behaviours vary across taxa, in spatial and temporal scale in addition to the mode of locomotion, yet it is clear that migration is a commonly evolved response to predictable changes to the environment. Migratory variation has been reported within species, and even within populations, a phenomenon known as partial migration. Partial migration is ultimately a form of differential migration, in which individuals differ in migratory traits such as timing, destination, and propensity, and the various hypotheses to explain these patterns are discussed. Migration occurs across a wide array of temporal scales, and can even be a transgenerational process. Spatial patterns of migration, as well as diadromous migrations across the freshwater–saline interface, latitudinal trends, and altitudinal migrations, are delved into. Finally, how global change has influenced migratory patterns is examined.

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  • Animal Migration

ffImage

What is Migration?

Migration is a type of behaviour where animals travel from their native habitat to another location in search of food, better living conditions, and sometimes for mating purposes. Migration is different from emigration in the fact that emigration involves the permanent movement of animals into a different location from their native habitat and settling there, whereas migration is seasonal movement and involves a journey back home at the end of the migratory period. Species like amphibians, crustaceans, reptiles, Pisces, and mammals migrate to faraway places. They usually migrate in flocks and often travel through sea, air, and land to reach their desired destination.

Now that you know what is migration, let's know its reasons, types and also the cause of occurrence.

(Image will be uploaded soon)

One of the main reasons for animal migration is food. During the winter season, when the temperature is well below the freezing point, food becomes a scarce resource vital for survival. Animals living in these regions migrate to warmer places where food is abundant and more easily available.

Another major reason for animal migration is climate. Animals who are not naturally equipped to survive harsh winters migrate to warmer climates to survive. The Monarch butterfly cannot survive in freezing temperatures. Thus they migrate in large numbers from Canada, all the way to Mexico, and flock together for warmth. At the e nd of the winter season, they return to their habitat, laying eggs on milkweed plants along the way.

The final vital reason for animal migration is for reproductive purposes. Animals might migrate to find a suitable mate, give birth to young ones, or raise them. Salmon, a riverine fish, migrate to the oceans to feed and grow. After spending about seven years in the ocean, they return to the rivers to mate and reproduce.

Animal migration is probably one of the most fascinating and studied animal behaviours. When animals migrate from their native lands in search of food and safety, they must travel an enormous distance to reach their destination, which is a test of their endurance and fitness.

Types of Migration

Animals may be classified into two groups depending on their reasons for migration.

There may also be another classification of the type of migration depending on animal behaviour.

Complete Migration: Every individual of the migratory species migrates to a different terrain every year. Example- Arctic Tern

Partial Migration: Some individuals of the migratory species migrate, while others stay behind. Example- American Robin

Differential Migration: Species have different patterns of migration based on age or gender. Example- Male American Kestrels migrate a shorter distance than female American Kestrels.

Interruptive Migration: These species either do not migrate at all or migrate all at once when food runs out in their habitat. Example- Blue Jays

Concept of Migration in Animals

Since it is dangerous and energy-exhaustive for an animal to travel a very long distance, scientists have concluded that animals only migrate when the benefits of migration outweigh the dangers it might face en route( predators, lack of food on the way, etc.) to its destination.

Some migratory animals examples are:

Humpback whales migrate in the summers and travel towards the polar ice to feed on krills and small fishes, while in winter, they travel back to warmer waters. 

Christmas Island red crabs live in the forest lifelong but migrate to the oceans to reproduce. 

Longfin eels are oceanic animals. However, their young migrate to freshwater and live their entire lives there. Towards the end of their lives, they migrate back to the ocean to reproduce. This migration happens over many years. 

The Canada goose migrates 1000 miles north in search of food in winter while travelling as far as Texas in the summers to breed.

Concept of Migration Birds

Birds native to the Northern Hemisphere tend to move northwards during spring to feed on the multiplying insect population, growing plants, and better nesting locations. With winter, the availability of food drops, and the birds again move southwards. Although climate may be a major motivating factor for migration, many birds can withstand harsh climates as long as there is an adequate food supply.

Some migratory animal examples are:

The Siberian crane, a critically endangered migratory species native to the Arctic tundra of eastern and western Russia, migrates to China, Iran, and India during the winter season.

The Ruby-throated Hummingbird migrates to the North from Central America during the breeding season. They travel for over 900 miles, of which they fly nonstop over the Gulf Of Mexico for 500 miles.

The Bar-tailed Godwit is one of the most common migratory birds name. It is native to Alaska and migrates down south to Australia and New Zealand. They cross the Pacific Ocean in a nonstop flight for over nine days.

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FAQs on Animal Migration

1. Why do birds migrate? What is the advantage of migration?

Migration often helps animals escape extreme weather conditions, have access to food, and breed in safety. Resources vary geographically depending on the season. Migration allows animals to take advantage of this varying of resources and use them to their own benefit. The amount and duration of sunlight a region receives vary seasonally. This affects the amount of food available in a particular place at a particular time. During the winter months of October to March, the Arctic regions have a sub-zero temperature which makes the availability of food extremely scarce. Animal habitats are destroyed due to excessive snoring. The Southern Hemisphere is experiencing summer during this time and is much warmer, with an abundance of food and availability of shelter. This is why many arctic birds and animals migrate to the Southern Hemisphere during these months to escape the harsh climate and to find food.

2. What is the disadvantage of migration?

With global warming and changing climates, the cost of migration is at an all-time high. Migrating animals and birds might face many kinds of danger while travelling from their native habitats. While travelling long distances, animals require a place to rest and recover. With the loss of habitat due to human exploitation and interference, they might not find a safe place to recover on their long commute. Migrating animals might also face predators on the way, as well as human hunters on their migration route. Due to rising levels of pollution, they might encounter polluted water and food, which might endanger their lives.

IMAGES

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  5. ANIMAL MIGRATIONS MIGRATION a Migration Is a Long Journey Made by a

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VIDEO

  1. Animal migration research at MSU

COMMENTS

  1. Landmark report details how human activities can disrupt animal ...

    The loss of habitat and overexploitation from hunting and fishing are two of the most pressing threats, according to the report. Human activities and structures can obstruct migration routes ...

  2. Animal Migration

    Some animals, such as the Galápagos tortoise (Geochelone nigra), live their entire lives in one place. Others, such as the monarch butterfly (Sanaus plexippus) and Arctic tern (Sterna paradisaea), migrate. Animals have adapted to migrate based on seasonal or geographic variations. Humans have added barriers to this process by building roads across major migration routes or eliminating or ...

  3. Migration

    Migration is a pattern of behavior in which animals travel from one habitat to another in search of food, better conditions, or reproductive needs. There are two important factors that make migration different from other types of animal movement: First, migration happens seasonally, and second, . migration involves a return journey.. This makes it different from emigration, when animals travel ...

  4. Humans are threatening the world's migratory species : NPR

    A major new report by the United Nations finds that humans are not only making those journeys more difficult, but have put many migratory species in a perilous state. Nearly half of the world's ...

  5. What are migratory species and why are they threatened?

    Aquatic animals are also endangered by pollution and fishing, while land animals face physical barriers such as roads and fences. Some wildebeest migration routes have completely collapsed due to ...

  6. Explaining and predicting animal migration under global change

    1 INTRODUCTION. Animal migration is a global phenomenon. Every year, billions of animals - from butterflies and bats to wildebeest and whales - travel long distances in the pursuit of resources in different habitats around the world (Bauer & Hoye, 2014; Dingle, 2014; Wilcove & Wikelski, 2008; Figure 1).Populations of many migratory species are, however, declining more rapidly than their ...

  7. The influence of social cues on timing of animal migrations

    First, social cues influence the timing of migration in a wide variety of taxa (Figs. 3 and 4) and across temporal scales (Fig. 4 ). Second, a number of different social cue types, ranging from ...

  8. Essay Going, Going, Gone: Is Animal Migration Disappearing?

    Essay A nimal migration surely ranks as one of nature's most visible and widespread phenomena. Every minute of every day, somewhere, some place, animals are on the move. The migrants span the animal kingdom, from whales and warblers to dragonflies and salamanders. But is migration an endangered phenomenon? Around the

  9. Animal Migration

    Up to 8,500 km each way. Humpback whale ( Megaptera novaeangliae) Longest insect migration. Up to 4,750 km in the autumn. Monarch butterfly ( Danaus plexippus) Longest recorded round-trip. 80,000 ...

  10. Animal Migration

    Many animals journey great distances, or migrate, as part of their life. They do this for various reasons, including to mate and find food. Use this idea set to learn about several migratory species, including monarch butterflies and the challenges faced by these insects. Additionally, introduce students to pronghorn migration through the ...

  11. Animal migration

    Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The cause of migration may be local climate, local availability ...

  12. Migrating species crucial to planet under threat, says UN

    By mapping migration corridors, it is hoped that animals can be protected from human activities. One of the biggest threats facing sharks and rays is being incidentally fished as bycatch.

  13. Animal Migration As a Moving Target for Conservation: Intra-species

    Partial migration has been documented in a wide variety of taxa from birds to fish, and is likely to be much more widespread than has historically been appreciated. 95 Partial migration appears to have a genetic basis in some taxa (i.e., the propensity for individuals to be migratory or sedentary is heritable), 96 but in other systems migration ...

  14. Animal Migrations

    Animal migration is a phenomenon far grander and more patterned than animal movement. It represents collective travel with long-deferred rewards. It suggests premeditation and epic willfulness ...

  15. Migratory Animals Couple Biodiversity and Ecosystem Functioning ...

    Our Review demonstrates that the highly predictable, seasonally pulsed nature of animal migration, together with the spatial scales at which it operates and the immense number of individuals involved, not only set migration apart from other types of movement, but render it a uniquely potent, yet underappreciated, dimension of biodiversity that is intimately embedded within resident communities.

  16. Introduction

    This introductory chapter provides an overview of the four main themes covered in the book. These are: the evolution of migration, the physical and energetic constraints that shape migratory movement, the issues of spatial and temporal scale, and migration in a broader context. Keywords: migration, migratory movements, physical constraints ...

  17. Climate change impacts on animal migration

    Migration, in particular, affects biodiversity at regional and global scales, and migratory animals affect ecosystem processes. Animals use predictable environmental cues for the timing and navigation of migration. A change in these cues will affect the phenology and extent of migration. Arrival date and hatching date are phenological markers ...

  18. How and Why Animals Migrate

    Some animals travel relatively short distances to find food or more favorable living or breeding conditions.Most animals that migrate do so to find food or more livable conditions. Some animals migrate to breed. The Atlantic Salmon begins its life in a river and migrates downstream to the ocean. After several years, it heads back upstream to ...

  19. Why Do Animals Migrate?

    A simple movement of animals over a substantial distance cannot be considered a migration. A migration is animal movement due to reasons like changes in the season such as when birds in the Northern Hemisphere escape to the south during winter. A migration can also occur if there is a major change in the habitat of an animal such as when a ...

  20. Animal migration: Impacts of extreme weather, climate change

    Climate change and extreme weather events are impacting animal migration patterns, with over half of all species on the move, researchers say. Many are heading north and to higher ground, but some are at risk because of the slower speeds at which they migrate. Biodiversity loss and ecosystem collapse is one of the most significant threats ...

  21. Migrating animals are at risk of extinction

    Migrating species are being threatened by habitat loss, illegal hunting and fishing, pollution, and climate change. "Migration is essential for some species. If you cut the migration, you're going ...

  22. Animal Migration: A Synthesis

    This book takes a comparative, integrated view of migration, linking evolution with ecology and management, theory with empirical research, and embracing all the major migratory taxa (including humans). The scope extends beyond the target organism to consider the ecosystem-level dynamics of migration. Rather than simply reviewing the field of ...

  23. Almost half of migratory animals are in decline

    A UN report highlights the existential threats faced by the world's migratory species. Almost half of migratory creatures are in decline and 20% could become extinct. The World Economic Forum's Global Risks Report 2024 cites biodiversity and ecosystem collapse as a major threat. Whether it's a murmuration of starlings, herds of wildebeest ...

  24. Patterns of animal migration

    Animal migration is a fabulously rich and varied tapestry of behaviour, encompassing a diverse range of movements. Migration often evokes images of the spectacular seasonal journeys of birds from their European breeding grounds to the food-rich African plains and forests; or the leaping of Pacific salmon as they fight the river currents to return to their natal streams to reproduce and then ...

  25. Animal Migration

    Facultative Migration. Animals who fall under this type of migration must migrate for survival. This type of migration is consistent year after year and often follows the same route and timing. Obligate migration is longer. Animals who fall under this type of migration have the choice to either migrate or remain in their native habitats.

  26. The effects of light pollution on migratory animal behavior

    Local scale effects. At the local scale, the negative effects of light pollution on migration have been most prominently measured by bird collisions with windows, communication towers, offshore vessels, and countless other built structures around the globe through data collected by collision monitoring, surveys, and citizen science programs [9, 17, 45., 46., 47.

  27. Landmark UN report: The world's migratory species of animals are in

    Samarkand, 12 February 2024 - The first-ever State of the World's Migratory Species report was launched today by the Convention on the Conservation of Migratory Species of Wild Animals (CMS), a UN biodiversity treaty, at the opening of a major UN wildlife conservation conference (CMS COP14). The landmark report reveals: While some migratory species listed under CMS are improving, nearly ...

  28. Migratory species are in a shocking state of decline, landmark UN ...

    They include species from all sorts of animal groups — whales, sharks, elephants, wild cats, raptors, birds and insects, among others. Some 44% of those species listed are undergoing population ...

  29. Animal Migration Essay

    Animal Migration Essay. Migration is the movements or travels made by animals to locations outside of their natural environment for an extended period of time (Dingle 2007). Typically when migrating, animals are searching for a sustainable location with many of the resources needed available. Some animals migrate back and forth between the same ...