Migration is most pronounced in environments in which food supplies vary greatly through the year. It enables birds to exploit seasonal abundances and to avoid seasonal shortages. Broadly speaking, birds move so as to keep themselves in favourable habitat for as much of the year as possible, allowing for the fact that their requirements may differ between the breeding and non-breeding seasons. The proportions of breeding species that leave for the winter increase with latitude, as seasonal changes in food availability become more marked. At any particular latitude, migration is also related to diet; only species whose food remains available year-round remain in their breeding areas year-round.

Migrations cause huge seasonal latitudinal and, to some extent, longitudinal changes in the distributions of birds. From low to high latitudes, a progressively greater proportion of breeding species leaves for the winter, and few species remain in winter north of the tree line. In mountain regions, many bird species migrate from high to lower ground for the winter, and back again in spring. Most latitudinal migrants occupy larger geographical ranges in summer than in winter when populations become more concentrated, but across species, the sizes of breeding and wintering ranges are correlated. The main exceptions include many shorebirds which breed over wide continental areas, switching in the non-breeding season to a linear distribution along coastlines.

Among landbirds, migration is much more marked in the northern than in the southern hemisphere. This is associated mainly with the far greater land areas in the northern hemisphere, especially between latitudes 30 and 80°N, where migration is most developed. Many birds from the northern continents migrate into the southern continents for the non-breeding season, but none from the southern continents migrate into the northern continents. In the southern continents, most migration is short-distance and partial, altitudinal or nomadic. Among pelagic birds, in contrast, migration is more marked in southern hemisphere species. This is associated with the greater sea areas, and more abundant nesting islands in the southern hemisphere, which support much greater numbers there. Some northern hemisphere seabirds migrate beyond the southern tropics, but more southern ones migrate beyond the northern tropics.

Appendix 13.1 Relationships between species numbers (or % migrants) and various environmental variables in western Europe

Latitude Land area Elevation range

Mean Mean Mean Seasonal Annual annual January July temperature precipitation temperature temperature temperature range range

Number of breeding species Number of wintering species % breeding species that leave for winter % wintering species that leave for summer

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