they usually start moulting or migrating earlier than usual and, in poor breeding years, many individuals can begin their post-breeding migrations weeks ahead of normal.
In experimental conditions, various passerine migrants started post-juvenile moult and autumn migration activity at a younger age if held under short photoperiods (Berthold et al. 1970, 1988, Gwinner 1972, Kroodsma & Pickert 1980, Jenni & Winkler 1994, Berthold 1996, Noskov et al. 1999, Coppack et al. 2001, Gwinner & Helm 2003). They also moulted and fattened more rapidly, or showed more overlap between moulting and fattening (for Bobolink Dolichonyx oryzivorus see Gifford & Odum 1965, for White-crowned Sparrow Zonotrichia leucophrys see Moore et al. 1982, for Bluethroat Luscinia svecica see Lindström et al. 1994, for Lesser Whitethroat Sylvia curruca see Hall & Fransson 2001). These findings indicated that it was the shortening days of autumn which accelerated the development of late-hatched young in the wild. In all these species, the ending of moult probably does not itself promote the start of fattening because the two events do not invariably coincide, and as fattening rates increase with reducing daylength (= advancing date), this can be regarded as another time-saving adaptation in late birds. Such a response to declining daylengths is, of course, not possible in birds breeding at such high latitudes that they are still subject to 24-hour days at the time they leave.
There are costs to more rapid development. Feathers grown during a period of rapid moult are sometimes of poorer quality, which presumably reduces a bird's chance of survival (Dawson et al. 2000). In general, under food restrictions, birds seem to reduce the quality of feathers produced rather than their individual growth rates (Payne 1972). Hence, the greater acceleration and overlap of different processes in late birds probably involves a trade-off. It saves time but only at the costs of additional daily energy need and reduced feather quality. Early birds can enjoy greater separation between the different processes, whereas in late birds time-saving becomes paramount. The greater fat levels accumulated by late migrants may also make them more vulnerable to predation.
While studied mainly in passerines, links between breeding, moult and autumn migration timing are also apparent in other species. For example, among arctic-nesting geese and swans, sub-adults and failed breeders moult and migrate earlier than breeders with young (for Canada Geese Branta canadensis see MacInnes 1966, for Snow Geese Chen caerulescens see Maisonneuve & Bedard 1992, for Tundra Swans Cygnus columbianus see Rees 1980, for Whooper Swans Cygnus cygnus see Black & Rees 1984); and in years of widespread breeding failure, the mean migration date of the whole population may be earlier than usual by a week or more (Maisoneuve & Bedard 1992), as also noted in shorebirds (see above) and ducks (Blomqvest et al. 2002).
In conclusion, response to daylength not only influences the timing of autumn migration; in late-breeding adults, and in juveniles hatched late in the season, it can also speed up the moult, and increase the overlap between breeding and moult (adults only), or between moult and fat deposition, and increase the rate and extent of fat deposition, thereby reducing the delay in migration caused by late breeding. The acceleration of development by short daylengths, known as the calendar effect (Berthold 1993), is of functional significance in causing late birds to moult and leave breeding areas before conditions deteriorate (Berthold et al. 1970, Berthold 1988). In species which depart immediately after breeding, findings with respect to moult do not hold, because moult is postponed until after arrival in a staging area or winter quarters.
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