Animal Migration

I

Introduction

Animal Migration, seasonal or periodic movement of animals in response to changes in climate or food availability, or to ensure reproduction. Migration most commonly involves movement from one area to another and then back again. This round-trip, or return migration, may be of a seasonal nature, as in the spring and autumn migrations of many birds. Or it may require a lifetime to complete, as in various species of Pacific salmon that are born in freshwater streams, travel to ocean waters, and then return to the stream where they were born to breed before dying.

Migration occurs in a wide range of animals, from microorganisms in freshwater lakes, which shift seasonally from deep to shallow water as a result of temperature changes; to whales, which move in autumn from subpolar to subtropical seas to have their young and then return in late spring to the colder, food-rich waters. Humans migrate as well: The Kung bushmen of the Kalahari Desert, for example, follow migrating game animals that they rely upon for food. They also leave drought-ridden areas to find other water sources.

In addition to round-trip migration, some migrations are nomadic in nature. Nomadic migrations involve irregular movement patterns that are dependent on temporary local conditions. For example, many of the large grazing animals that live in herds on the plains of eastern Africa move in response to varying local conditions of food and climate. In these migrations, the animals follow no regular route and do not return to any one place. Another type of migration—removal migration, or one-way migration to new sites—is exhibited by migratory locusts of Africa and Asia. These locusts are well known for their enormous mass movements when their populations peak and food becomes scarce. They move to new areas, almost blackening the sky as they pass overhead. Rarely do they return to their place of origin.

Irruption is a specific migratory cycle occurring in extreme climates. The best-known example of irruption is seen in lemmings of the arctic tundra. These small animals reach a peak in their population every three or four years, at which point they migrate overland in great numbers. Most of them die in the process; only a few survive to start the migratory cycle again. Another migration pattern is remigration, in which the round trip is divided between generations: the first generation of animals migrates to an area and reproduces, and the return trip is made by their offspring.

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II

Seeking Food and Water

Migration based on the availability of food is often dictated by seasonal climate change. When winter suddenly halts the supply of insects, for example, birds that eat insects must head for warmer climates where food is still bountiful. Similarly, as the cold settles in, small rodents and birds that are prey for predatory birds become scarce. This scarcity prompts the North American red-tailed hawk, for instance, to fly to Mexico or the Gulf Coast to find a more abundant food source. In winter, animals that depend on fish or aquatic plants from waters in the north find their feeding grounds sealed by ice. These conditions force the animals to travel south in order to survive.

Plant-eating mammals, such as buffalo and antelope, typically graze in herds, which can quickly deplete the grass in an area. In the summer, cropped grass regrows quickly. While waiting for this regrowth, grazing animals may wander a short distance to find new grass, circling back to the original area when grass is abundant again. But in winter, grass does not regrow, which forces these herds to travel longer distances to find fresh food supplies. When spring brings new growth, the herds move back to the areas where they found food the previous season.

When climatic changes cause drought to occur, water holes draw predators and prey alike, making these areas both overcrowded and dangerous. In Africa, wildebeests, zebra, and other prey species therefore migrate to areas where water is more plentiful. Although these animals burn valuable calories to make the journey, the tradeoff is worthwhile because they reduce their dependence on risky watering places.

III

Reproductive Migration

Another reason animals migrate is to bear their young in places relatively safe from predators and rich in resources. Although reproduction may be the primary factor in these cases, the other elements of food availability and seasonal climate change are often involved as well. Some right whales, for example, leave their Antarctic feeding grounds where their primary food resource of krill (tiny shrimplike crustaceans) is plentiful. They travel to the relatively barren shores of Patagonia, where they bear their young. Although krill are in Antarctic waters year-round, ice covers the surface of the ocean in winter, making it impossible for whales to surface for air.

The green turtle is another reproductive migrant. When the time for laying eggs draws near, female green turtles swim from their feeding grounds off the coast of Brazil. They swim to tiny Ascension Island over 2,000 km (1,242 mi) away, which they may not have visited since they were hatched. After their long swim, they haul themselves onto the sandy beaches, scrape out shallow nests, and deposit their eggs. Once this is done, they swim back to Brazil.

Freshwater eels spend most of their lives in the rivers of North America and Great Britain. However, for reproductive purposes, they trace ancient migratory patterns, swimming from each side of the Atlantic to the weedy Sargasso Sea between Bermuda and Puerto Rico. After breeding in the Sargasso, the eels return to the continental rivers. The young eels, or elvers, take a year or two to reach American shores, and they are often three years old before they reach European rivers. This movement between saltwater and freshwater involves a special migratory adaptation—a physiological shift in kidney function. Without this adaptation, eels could not make this dramatic change in environment without bodily harm.

IV

Prerequisites for Migration

In order to migrate successfully, animals must be capable of sustained movement for long periods of time. This movement requires considerable energy output, and different species of animals have evolved a variety of mechanisms to ensure sufficient body fuel for the trip. How an animal migrates is significant in determining how much energy is needed as well as how much must be stored in the body. Flying, for example, is more physically intensive than walking or swimming, so migratory birds must build up large energy stores before they set off. Just before spring and fall migrations, certain birds increase their body fat—up to nearly 40 percent of body weight in some songbirds. Before migrating, the 10-cm- (4-in-) long ruby-throated hummingbird gains about 2 g (0.07 oz) of fat. This extra fat provides sufficient energy for this tiny migrant to fly 800 km (500 mi) from North America, across the Gulf of Mexico, to its winter home in Mexico. Some birds supplement this stored energy source with food along the way. Others make long, nonstop flights—the golden plover may travel 3200 km (2000 mi) over water without landing.

In contrast, land mammals are able to graze on plants along the way as they travel, so they have no need for sizeable fat stores. In fact, it is important that these animals travel light so that they can remain agile to escape predators. Land mammals, such as African wildebeest, may walk more than 1600 km (1000 mi) when migrating.

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