In long-distance migrants that raise only a single brood each year and then depart from their breeding areas, the timing of arrival in breeding areas can influence the timing of all subsequent events up to post-breeding departure. This is because the breeding cycle, consisting of egg-laying, incubation and chick growth, is of fairly consistent duration, both from bird to bird and from year to year. Thus, in springs when birds arrive early in their breeding areas, they can usually breed earlier and depart earlier in late summer, the whole sequence of events being affected by spring weather (Nisbet 1957, Sokolov & Payevsky 1998, Sokolov et al. 1999). Moreover, in years of widespread breeding failure in arctic-nesting shorebirds and others, the post-breeding migration occurs much earlier than usual, as mentioned above.
Consider some specific examples. The Common Swift Apus apus migrates soon after breeding, postponing moult for winter quarters. Each pair leaves Britain within a few days after raising its brood. In this species, the date of departure is clearly determined primarily by the completion of breeding, which is in turn influenced by the date in May when breeding starts (Lack 1956). Since the latter is influenced by temperature, the mean date of departure of Swifts in August depends on the weather in the preceding May. Another well-known example is provided by the Spotted Redshank Tringa erythropus, which migrates between tropical Africa and northern Europe (Hilden 1979).
More recently, year-to-year correlations between spring arrival or laying dates and autumn departure dates have also been noted in the Bluethroat Luscinia svecica,
Willow Warbler Phylloscopus trochilus, Arctic Warbler P. borealis, Pied Flycatcher Ficedula hypoleuca and Little Bunting Emberiza pusilla, among others (Ellegren 1990a, Sokolov et al. 1999). The implication is that, in such single-brooded obligate migrants, autumn departure dates depend not so much on environmental conditions at the time, but on the completion of previous events in the annual cycle, whether the end of breeding or moult in adults or the end of growth or moult in juveniles, as the case may be. In line with this, the timing of peak autumn migration in different years at Rybachi on the Baltic coast was related to the preceding April temperature in 15 species examined, emerging as statistically significant in five such species (Sokolov et al. 1999). In none of these species was the timing of autumn migration related to local temperature at the time. Such close coupling between the timings of spring and autumn migration would not be expected in birds that can raise more than one brood each year, nor in some facultative migrants in which individuals may linger in breeding areas well beyond the end of moult in autumns when the weather is warm or food is plentiful.
The relationship between arrival and departure dates is also apparent in different populations of the same single-brooded species in different breeding areas. For example, those Ospreys Pandion haliaetus nesting in Florida that are migratory leave their breeding areas earlier in autumn, and return earlier in spring than do Ospreys from more northern breeding areas, but both populations spend about the same number of days in winter quarters (Martell et al. 2004). With only one brood raised, the breeding cycle also takes the same time in both regions. The wintering areas of both populations overlap, but southern breeding birds undertake both outward and return journeys several weeks earlier than the northern ones, neither spending more time than necessary in breeding areas.
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