For everything there is a season, and a time for every purpose. (Ecclesiastes 3: 1-2.)
In our present context, migration cannot be considered in isolation from breeding and moult - the other major events in the annual calendar of birds. Comparing species, these three activities vary in their timing, in the sequence in which they occur, and in the extent to which they overlap with one another. These variations are in turn linked with features of the species themselves, and the circumstances in which they live. Most bird species have only one breeding period each year, during which they raise one or more broods, but some (mainly tropical) species have two separate breeding periods. Many species also have only one moulting period each year, but some have two or more; and most species migrate twice each year (to and from their breeding areas), while others move three or more times or only once (from one breeding area to another). This chapter is concerned with how the variations in annual cycles between bird species are regulated by both external and internal influences. In this context, migratory birds are of special interest because, unlike most other animals and plants, they are exposed within each calendar year to conditions in more than one part of the world.
In matters of timing, whether of migration, breeding or moult, it is helpful to distinguish between ultimate and proximate causal factors (Baker 1938, Lack 1954). The ultimate factors include those aspects of environment, such as seasonal fluctuations of food supply, that influence the timing of various events through their effects on the survival and reproductive success of individual birds. They thereby influence the optimal period for the bird to undertake particular activities and, through the action of natural selection, favour individuals that organise their annual cycles in the most effective manner. The proximate factors are those, such as daylength, that birds can use as reliable cues to begin their preparation for migration, breeding and moult at appropriate dates each year.
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