Depending largely on the environments in which they live, bird populations show great variation in the timing and sequence of the major events in their annual cycles. In most species, for nutritional and other reasons, migration, breeding and moult occur at different times of year, but in the same sequence every year. In some species, wing moult overlaps extensively with breeding but, in general, neither process overlaps extensively with migration, although exceptions occur. In many migratory species, the autumn migration and moult are split into two or more stages, each occurring in a different area.

The main ultimate factor governing the annual cycles of birds is assumed to be the seasonality of the environment, and its effect on food supplies. The main proximate factor is apparently an endogenous rhythm within the bird, which is entrained by seasonal changes in daylength. This self-sustaining rhythm is revealed in captive birds kept for years on constant photoperiods. Such birds may depart from the usual annual periodicity, but typically show repeated periods of gonad activity, moult or migratory restlessness and fattening, depending on the photo-period on which they are kept.

In general, endogenous cycles continue for longer under constant conditions in tropical species and in long-distance migrants than in temperate zone residents or short-distance migrants. Species that breed and winter in opposite hemispheres, and experience long days in both summer and 'winter', apparently rely on this internal rhythm to suppress reproduction in winter quarters, and those that winter there or in equatorial latitudes (where daylengths are constant year-round) apparently rely on the internal rhythm to prevent homeward migration until an appropriate date in spring.

At particular latitudes (away from the equator), daylength alters in a consistent manner from year to year, and therefore gives a reliable indication of date. Evidence that birds respond to daylengths (through adjustment of the internal rhythm) comes from several findings. First, gonad growth, moult or migratory activity can be advanced or delayed by experimental exposure to longer or shorter photoperiods. Second, birds can be made to undertake more than one 'annual' cycle in a calendar year by alternating exposure to long and short photoperiods.

And third, birds adjust their annual cycles when transported to the opposite hemisphere, becoming about six months out of phase with conspecifics in the original home area (an observation that can be duplicated by experiment). Birds also respond to secondary stimuli, such as temperature, food supply or social conditions, to fine-tune the timing of various events to conditions at the time, and can delay or delete particular events, as the need arises.

Different species have different inherent cycles, and respond differently to the same daylength regime, so that their annual cycles are adjusted to their particular needs. Captive-bred hybrids behave intermediately, implying genetic control of the endogenous response mechanism. Similarly within species, birds breeding or wintering at different latitudes respond to the local daylength regime, in such a way as to reach breeding, moulting or migratory condition at times appropriate to the latitudes concerned.

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White-crowned Sparrow Zonotrichia leucophrys, commonly used in experimental work on migration

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