In obligate migration, all main aspects are viewed as under firm internal (genetic) control, mediated by daylength changes, giving a high degree of consistency in timing, directions and distances of movements from year to year. For the most part, each individual behaves in the same way year after year. In contrast, facultative migration is viewed as a direct response to prevailing autumn conditions, especially food supplies. Within a population, the proportions of individuals that leave the breeding range, the dates they leave and the distances they travel, can vary greatly from year to year, as can the rate of progress on migration, all depending on conditions at the time. The same individual may migrate in some years, but not in others.

In general, obligate migration occurs in populations whose food supplies in breeding areas are predictably absent in winter, whereas facultative migration occurs in populations whose food supplies in breeding areas vary greatly from one winter to another, according to weather or other variables. Obligate and facultative modes can be regarded as opposite ends of a continuum, with predominantly internal control at one end and predominantly external control at the other. In addition, many migrants may change from obligate to facultative mode during the course of their journeys.

An endogenous programme influences the time-course (and hence distance) of autumn migration among obligate migrants. Evidence for endogenous control has come from findings that: (a) the timing and duration of migratory restlessness in captive birds resembles the temporal pattern of migration in free-living birds; (b) populations which migrate over different distances show corresponding differences in the amount and duration of migratory activity in cages (and hybrids show intermediate patterns); and (c) experimental interruption of migration or migratory restlessness is not subsequently compensated for. These findings apply primarily to juveniles, and do not necessarily hold in experienced birds returning to a known area.

In many species, departure dates from breeding areas on autumn migration are spread over periods of up to several weeks, because they are influenced by variations in the dates that individuals finish breeding and (in some species) moulting. Adult passerines breeding until late in the season also start moulting later, but earlier with respect to the stage of a nesting cycle, and replace their feathers more rapidly than earlier birds, thus minimising the delay in autumn migration. Similarly, young produced late in the season moult at an earlier age, and more rapidly, than earlier hatched young, again reducing the delay in migration. They also begin fattening before the end of moult and may migrate more rapidly. As confirmed experimentally, this acceleration in development in late birds is triggered by the shortening daylengths of late summer and autumn.

Preparation for spring migration appears to be influenced primarily by increasing daylengths in association with an endogenous rhythm, the latter being particularly important in populations wintering in equatorial and opposite hemisphere regions. The spread in departure dates between individuals wintering in the same area may be attributable partly to variations in completion of previous events in the annual cycle, and in their feeding and fattening rates. Adverse weather can cause further delays.

In some single-brooded populations, the dates of arrival or egg-laying in spring influence the dates of autumn departure because, with a breeding cycle of roughly constant length, early arrival (or egg-laying) allows early departure. Post-breeding departure dates in such populations thus depend more on preceding spring weather than on prevailing weather.

In some bird species with deferred maturity, individuals remain in wintering areas and do not return to breeding areas until they are two or more years old; other individuals may return part way towards breeding areas, or may visit breeding areas only for a short time each year, leaving wintering areas later and returning earlier than breeding adults. Mechanisms that control the occurrence and timing of migration in such species during the early years of life await study.

Sex hormones may help to promote preparation for spring migration but delay autumn migration. They may also influence direction. Corticosterone is involved in migratory fattening and restlessness, and melatonin in nocturnal activity.

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Part Three

Large-Scale Movement Patterns

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Northern Lapwings Vanellus vanellus on migration

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