Autumn dates

Changes in autumn migration dates over recent decades have been generally less, and more variable, than changes in spring dates (Gatter, 2000, Bairlein & Winkel 2001, Fiedler 2001, Sparks & Mason 2001, Jenni & Keri 2003, Lehikoinen et al. 2004, Sokolov 2006), apart from a study in southern Canada in which changes in autumn dates were more marked and more frequent (Mills 2005). Two patterns have emerged, involving either earlier or later departure over the years. In some single-brooded populations, earlier arrival is followed by earlier breeding and moult, so that birds are ready to depart earlier. In such populations, the timing of successive events through the summer, from arrival, egg-laying, hatching, fledging, moult and autumn migration, are correlated with spring temperatures, and show little or no relationship with the prevailing autumn temperature (Chapter 12). An earlier spring arrival pulls the whole cycle forward to give an earlier autumn departure (Ellegren 1990b, Sokolov et al. 1998, Sokolov 2000a, 2001, Bojarinova et al. 2002). At Rybachi on the southern Baltic coast, warming in the 1960s and 1980s led to significantly earlier mean dates in spring passage, breeding and autumn passage. Conversely, colder springs during the 1970s caused a shift towards later spring passage, breeding and autumn passage (Sokolov et al. 1999). These trends occurred in both short-distance and long-distance migrants. Most of the migrants through Rybachy came from breeding areas that gave time for only one brood. Similar relationships were found for single-brooded long-distance migrants passing through the Swiss Alps in autumn (Jenni & Keri 2003). The long-distance migrants may have benefited in autumn from an earlier crossing of the Sahara before its seasonal dry period. In contrast, shorter-distance migrants passing over the Alps and wintering north of the Sahara mostly showed a later autumn passage. These are chiefly passerine species that can raise more than one brood per year, so could better take advantage of a longer season by remaining longer in their breeding areas. Further south and west in Europe, where individuals can make up to two or three breeding attempts in the same season, departure dates of passerines have tended to get later as local temperatures have risen (Bairlein & Winkel 2001, Sparks & Mason 2001), but it is not known whether this has been associated with a lengthening of the breeding season.

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