The migratory habit enables a species to enjoy the summers of northern latitudes while avoiding the severity of the winters. (Frederick C. Lincoln 1935.)
Migration is the most spectacular of bird movements. It can be defined as a large-scale return journey, which occurs each year between regular breeding and wintering (or non-breeding) areas. Involving seasonal shifts of millions of individuals, it produces a massive twice-yearly re-distribution of birds over the earth's surface. High-latitude regions receive birds mainly in the breeding season, while lower latitude regions support wintering birds from higher latitudes, as well as year-round residents. Migration thus increases the numbers of species that occur in particular regions, even though some are present for only part of the year.
Throughout the world, migration is most apparent wherever the contrast between summer and winter (or wet season and dry season) conditions is great. Migration thus allows individual birds to exploit different areas at different times of year, whether to benefit from seasonal flushes of food or to avoid seasonal shortages. In fact, some migratory birds occupy habitats over winter that they could not use for breeding, and then occupy breeding areas that would not support them in winter. This applies to all arctic nesting shorebirds which spend the winter on coastal mudflats where, due to tidal flooding, nesting would be impossible, and then migrate north to breed on the arctic tundra which is frozen and snow-covered for the rest of the year. Thus some bird species exist over much or all of their range only by exploiting widely separated habitats at different seasons.
Although most marked at high latitudes, migration also occurs in the tropics, especially in the savannahs and grasslands exposed to regular wet and dry seasons. In the northern tropics, for example, many species move south for the non-breeding season, some crossing the equator, while in the other half of the year many species of the southern hemisphere move north. In contrast, birds confined to lowland equatorial rainforest are probably the least migratory, especially the small insectivores of the understorey where conditions remain relatively stable and suitable year-round. This year-round consistency in the rainforest environment removes any advantage in moving, and many individuals may remain within the same few hectares throughout their adult lives. In the same forests, however, some nectar-eaters and fruit-eaters move within small latitudinal or altitudinal bands in response to flowering and fruiting patterns, while other birds from higher latitudes move in for their 'winter' (Levey & Stiles 1992).
Most migratory bird species thus form parts of different communities at different seasons, and may interact with different species in their breeding and non-breeding homes. Each migratory species must therefore be compatible with the various species it encounters through the year, as well as with different climatic and dietary regimes. Overall, more than half the world's 10 000 or so bird species are likely to perform migratory movements in at least part of their range, causing a general shift in the centre of gravity of populations towards the north for the boreal summer and towards the south for the austral summer.
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