Changes in the numbers of migratory birds, either long-term or year-to-year, may be caused by changes in conditions in the breeding or wintering areas or both. The strongest driver of numerical change is provided in whichever area the effects of adverse factors on mean survival or fecundity are greatest. Examples are given of some species whose numbers have changed in association with conditions in breeding areas, and of others whose numbers have changed in association with conditions in wintering areas, either year to year or long term. In a few such species, the effects of potential limiting factors have been confirmed locally by experiment.

In some species, habitats occupied in wintering and migration areas, and their associated food supplies, can influence the body condition, migration dates and subsequent breeding success of individual migrants. Similarly, poor weather or stress during breeding can lower the body condition of breeders, and reduce their subsequent survival. In addition, the numbers of young produced in the breeding range could, through density-dependent processes, affect subsequent overall mortality in the non-breeding range. Events in breeding, migration and wintering areas are thus interlinked in their effects on bird numbers. Such effects are apparent at the level of the individual, and at the level of the population.

Some species breed on continents, but winter on islands or other small areas. The limited carrying capacity of such areas could in turn limit the sizes and distributions of breeding populations. In many bird species, probably through male dominance, the sexes occupy partly different winter ranges and winter habitats. These differences may contribute to the surplus of males apparent in many ducks, passerines and others in their breeding areas, with nesting populations limited by the numbers of females.

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Bewick's Swans Cygbus columbianus, which migrate from western Europe to northern Siberia and are greatly influenced by conditions at crucial staging sites
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