2 Bird migration

The remarkable migratory behaviours of some bird species provide excellent experimental systems for studying the underlying processes, and the results have a much wider application to the study of migration than just bird behaviour. Studies on genetics, the influence of magnetic fields and physiological processes behind homeostasis illustrate general principles, but as birds have attracted so much scientific attention, they provide some of the best examples of migration studies.

Although migration is widespread amongst bird species, not all species are migratory and in some species, not all populations of the same species migrate and sometimes not all members of a population migrate. For example, in many regions, including the British Isles, some populations of birds are residents all year round and do not migrate. Other populations exhibit partial migration, where some individuals of the population in the same breeding areas remain all year round while others migrate to overwinter elsewhere..

Migration occurs to some degree in most bird species that live in seasonal environments. It is prevalent because in such environments (e.g. the Arctic tundra) food supplies vary markedly throughout the year, changing from abundance during the summer to scarcity during the winter (Figure 6).

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Figure 6 (a) Summer in the Arctic. Snow geese (Chen caerulescens) nest in tundra pools in the summer and migrate south to Mexico in the winter. (b) Winter in the Arctic.

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Part (a) of the figure shows the brown landscape of the Arctic tundra flanked by ocean. In part (b) the Arctic land is covered by snow and ice.

Figure 6 (a) Summer in the Arctic. Snow geese (Chen caerulescens) nest in tundra pools in the summer and migrate south to Mexico in the ...

Generally speaking, birds time their migrations so that they are present in breeding areas during periods of food abundance and are absent during periods of scarcity.

In lowland tropical forests, where food supplies remain stable throughout the year, many birds that breed there remain all year. However, these forests also receive a seasonal influx of other bird species from higher latitudes which migrate from colder regions for the winter.

This means that worldwide, up to half of the 10 000 or so species of birds, totalling about five billion individuals, are thought to migrate every year on return journeys between breeding and non-breeding areas.

Because migration is between warm and cool habitats, birds move mainly on a north-south axis. Some, however, have an easterly or westerly component to their movements, especially those that breed in the central areas of the northern landmasses and move to the warmer edges for winter. Figure 7 shows some examples of different migration routes used by birds.

Figure 7 Examples of different migration routes.

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This figure is a simplified map on which the migration routes of four birds are shown as arrowed lines of different colours. A green line shows the route taken by the brent goose, between northeast Greenland and sites in Southern Ireland and England. The route of the red knot is the long red line extending from beyond the northeast coast of Russia overland and in a southwesterly direction into northern France and then south along the west coast of France, Spain and Africa down to and around the Horn, terminating southwest of Madagascar. The migration route of the dunlin (yellow line) runs from beyond the northeast coast of Russia southwards as far as the southeastern corner of the Arabian Peninsula. White lines show the two migration routes for the white stork, both of which span parts of Europe and Africa. One route extends from northeast France down through eastern Spain to terminate in three short branches all within northwest Africa. The other, longer, route runs from two regions within central Europe, southwestwards through Greece and Turkey, then directly south into east Africa. The route then splits, with one branch terminating in central Africa and the other continuing down towards the south where it splits once more, the two forks terminating in Botswana and South Africa, respectively.

Figure 7 Examples of different migration routes.

The patterns of bird migration are very diverse. Passerines are perching birds (Figure 8). They make up almost half of all bird species and some of the small passerines travel at night.

The mass migration of large storks is a spectacular sight during daylight (Figure 9).

Figure 9 (a) Migrating white storks (Ciconia ciconia). (b) Migratory paths (lines with arrows at each end) of white storks that breed in Europe and migrate to the South, principally Africa, for the winter.

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Part (a) is a photo showing a flock of white storks taking off from the water. These are large birds with plumage that is mainly white on the body, but mostly black on the wings. They have long red legs and long pointed red beaks. Part (b) of this figure is a simplified map showing a number of migration routes of white storks, with the breeding ranges and wintering grounds shown in green and orange respectively. There is a large breeding range extending over eastern and southern Europe from which there is mostly migration to areas in eastern and southern Africa. Migrations to wintering grounds in central and western Africa are mainly from breeding ranges in Spain and the far northwestern strip of Africa. A further migration route runs from a breeding range in Central Asia to wintering grounds in northwest India.

Figure 9 (a) Migrating white storks (Ciconia ciconia). (b) Migratory paths (lines with arrows at each end) of white storks that breed in ...

In contrast, individuals of some species travel alone.

When the routes taken by migrating birds are plotted on a map, many long-distance migration routes are found to be clustered together, as the next section describes.