Migrants normally leave their breeding areas when conditions deteriorate, but before their continued survival there would become precarious. In association with the earlier onset of winter at high latitudes, many species withdraw from high-latitude parts of their breeding range first, and from lower latitude parts later. The assumption is that, in order to prepare for autumn migration, populations have evolved responses to different daylength regimes, appropriate to the latitude at which they breed. From any one area, departure dates also differ widely between species, depending largely on their type of food and when it becomes scarce (Chapter 14). As elsewhere in this book, I use the term autumn migration for the post-breeding return to wintering areas, even though in some species this migration occurs in the latter half of summer.
In obligate migrants, in which all individuals leave the breeding range each autumn, the dates of migration are in general fairly consistent from year to year. This is apparent not only in the dates that birds leave their breeding areas, but also in the dates they pass particular places on their migration routes and arrive in wintering areas. For example, among raptors migrating through Israel, the timing and duration of passage varied greatly between species, but within species the autumn passage dates were remarkably similar between years (as were spring dates). Over nine years, the confidence intervals of the mean autumn dates ranged between 1.5 and 3.4 days (versus 2.1-5.5 days for spring dates), depending on species (Leshem & Yom-Tov 1996a).
In many species of obligate migrants (in contrast to facultative migrants), adults leave the breeding areas before juveniles. This is especially marked in species which leave immediately after breeding, postponing their moult until later in the year, including many long-distance passerines, shorebirds and others (Chapter 11). Sex differences in timing are most evident in the adults of those species in which males and females have different roles in parental care, and one sex is free to leave before the other (Chapter 15). No such sex difference is apparent in the juveniles of these species. However, the net result of such age and sex differences in autumn migration timing is that they add to the spread in migration dates within populations.
In some species, marked individuals showed great year-to-year consistency in their arrival dates in staging or wintering areas (for Bewick's Swans Cygnus columbianus see Rees 1989, for Snow Geese Chen caerulescens see Maisonneuve & Bedard 1992). Individuals which arrived early with respect to other individuals in one year also arrived early with respect to other individuals the next year. Rees (1989) attributed this individual consistency to variation in their response thresholds to daylength, but other explanations were possible, including variation in their abilities to feed and fatten for the journey. In addition, individual swans that arrived early in autumn also departed late in spring. In summary then, obligate migrants show relative consistency in autumn migration dates from year to year, both as populations and in some species also as individuals, and some show age-related and sex-related differences in mean migration dates.
In facultative migrants the situation differs. Such populations generally moult after breeding and, in contrast to obligate migrants, individuals do not necessarily depart immediately after finishing moult. If conditions are favourable, they may linger longer in the breeding or stopover areas, leaving only when food supplies dwindle or are shut off by snow and ice. Mean autumn migration dates may therefore vary greatly from year to year, depending on local food supplies, as may rates of travel. This situation is exemplified by most short-distance migrants, especially irruptive seed-eaters (Chapter 18), and by waterfowl and others affected by frost (e.g. Hario et al. 1993). For example, the peak date for passage of Siskins Carduelis spinus through Falsterbo Bird Observatory in south Sweden during 1949-1988 varied from 15 August (in 1988) to 17 November (in 1958), the last date the station was manned that year (Roos 1991). At another site, Siskins passed in largest numbers, and at the earliest dates, in years when birch seeds (the main autumn food) were scarce (Svardson 1957; Chapter 18). Likewise, the date on which the last Whooper Swans Cygnus cygnus left Lake Chuna on the Kola Peninsula each year during 1931-1999 varied between about 20 September and 9 November, depending on when the lake froze over (Gilyazov & Sparks 2002). These observations illustrate the point that some migrants depart only when deteriorating conditions encourage them to leave. Together with variable conditions on migration routes, this facultative response gives wide variation in the dates that birds arrive in their various wintering areas, the more distant of which may be reached only in occasional years. Where an age difference is apparent, juveniles of facultative migrants generally leave before adults (Chapter 15).
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