Regional variation in the onset of winter conditions can again be tracked by following particular isotherms as they move from higher to lower latitudes, bringing colder conditions southwards across the northern continents (Figure 14.11). In autumn, the 10°C isotherm takes nearly three months to spread southward through Europe. Although not studied in detail, the autumn withdrawal of migratory birds from the northern continents occurs, as expected, from the top down, as northern populations generally leave before southern ones (for exceptions see below).
As mentioned earlier, species tend to leave particular localities in autumn in reverse order to that in which they arrived in spring: that is, the latest to arrive are first to leave (Figure 14.2). This sequence is again broadly linked with the decline in their respective food supplies, associated with falling temperatures. The earliest species to leave particular localities also show less year-to-year variation in their departure dates than do the last to leave (Gilyazov & Sparks 2002). Insufficient data are available to plot the withdrawal from much of a continent of any species, and see how closely the process keeps step with moving isotherms. But in particular species the whole process can again take several weeks, or up to three months in species such as the Barn Swallow Hirundo rustica which breed over a wide span of latitude. Low-latitude populations may still be on their breeding areas when individuals from higher latitude breeding areas pass through again, en route to winter quarters (examples include Yellow Wagtail Motacilla flava and others in Europe and Wilson's Warbler Wilsonia pusilla and others in North America; Curry-Lindahl 1963, Kelly 2006).
There are exceptions to this general trend. They include some single-brooded species, which spend about the same amount of time on their nesting areas at all breeding latitudes, but can both arrive and depart earlier from southern than from more northern parts of the breeding range. For example, migratory Ospreys Pandion haliaetus nesting in Florida arrive in breeding areas about one month earlier than those in New York and New Jersey, and also depart for their wintering areas about one month earlier (Martell et al. 2001, 2004). It is as though the annual cycle of high-latitude breeders is shifted later relative to lower latitude breeders. Other North American species in which southern populations migrate earlier than northern ones in autumn include Orange-crowned Warbler Vermivora celata and Common Yellowthroat Geothlypis trichas (Kelly 2006).
Early departure from breeding areas also occurs in some species that breed in drought-stricken parts of western North America (Rohwer & Manning 1990, Butler et al. 2002), while their conspecifics from further north leave later. The same holds in arid parts of southeast Europe, as exemplified by the Eurasian Reed Warbler Acrocephalus scirpaceus (Akriotis 1998). Trans-Saharan migrants generally benefit from early departure because they can then reach the Sahel zone at
its greenest, and while feeding conditions are optimal. By the time most migrants arrive in the Sahel, from September on, the region has already begun to dry.
Patterns of autumn departure seem to differ between obligate (complete) migrants, in which all individuals leave every year, and facultative (partial) migrants in which the proportions of individuals that leave, and their dates of departure, vary with local food supplies at the time (Chapter 12). In obligate (often long-distance) migrants, individuals seem to leave their breeding areas as soon as they can after breeding or after breeding and moulting, as the case may be (Lack 1960b, Preston 1966). In many such species, departure occurs before the collapse of local food supplies. If birds do not have time to raise another brood, they probably benefit by leaving as soon as possible. Unlike the situation in spring when food is scarce but increasing, in late summer food is plentiful but often declining, and weather is still fine. If the birds have finished breeding, and therefore have no reason to wait longer, they have every advantage in migrating while conditions are still good, and before food supplies on staging areas have been depleted. They might also establish themselves on wintering areas at an early date, perhaps benefiting in competition for the best habitat. This holds especially in species which have territories in winter quarters and is probably the main selective pressure for rapid departure and progress on the autumn journey. Hence, while spring arrival usually coincides fairly precisely with the reappearance of new food supplies, autumn departure of obligate migrants may precede the autumn collapse of food supplies by up to several weeks (Schneider & Harrington 1981).
Facultative migrants show less year-to-year consistency in autumn departure dates, and many individuals remain on breeding areas as long as food lasts, leaving up to several weeks later in some years than in others, and long after the end of moult (Chapter 12). This holds for seed-eating finches, which, in years of good tree-seed crops, stay all winter or leave much later than in years of poor crops (Chapter 18). They can stay after snowfall, providing that seeds remain on the trees, whereas other finch species, which pick seeds from low herbaceous plants or the ground, must leave by the first heavy snowfall. Likewise, many northern waterbirds leave as their wetland habitats begin to freeze over, which occurs much earlier in some years than in others. Moreover, because shallow waters freeze before deep ones, the shallow-water dabbling ducks usually migrate before deep-water diving ducks. In these facultative migrants, local food supplies seem to have a major influence on the rate of southward migration, with birds lingering en route while food lasts, and travelling markedly less far in some years than in others.
In species that compete for territories or feeding areas in winter, studies have shown an advantage of arriving early relative to other individuals. For example, wintering American Kestrels Falco sparverius arriving in Florida from the north occupied habitats in decreasing order of their quality in terms of food supply. Early-arriving birds (mostly females and juveniles) acquired most of the good places, while later arrivals took the less good places (Smallwood 1988). Hence, as in breeding areas, the best places were occupied first, and by the earliest birds.
It is not only territorial species that benefit from an earlier arrival in wintering areas. On an estuary in southern England, Oystercatchers Haematopus ostralegus accumulated first on the two most preferred mussel beds where feeding rates were highest (Goss-Custard et al. 1982, 1984). But as more birds arrived and interactions increased, birds progressively occupied the less favoured mussel beds. Later arriving adults displaced immatures already present on the favoured beds. Moreover, as mussel stocks declined on the favoured beds, more birds left, but in reverse order of their dominance rank, to feed on less preferred beds or in other habitats. In this species, therefore, the effects of arrival date were modified by dominance relations among age groups, but within age groups the benefits of early arrival relative to other individuals were still apparent. Further evidence of first arrivals taking the best habitats, and thereby having greater chance of surviving the non-breeding season, is available for Ruddy Turnstone Arenaria interpres (Whitfield, in Wernham et al. 2002).
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