Variations within populations

In species in which only one sex looks after the young, or in which one sex gives up before the other, the two sexes also moult and migrate at different times. Such differences are especially marked in ducks and in some species of shorebirds (Chapter 15). In some shorebird populations, moreover, same-sex individuals from the same breeding population vary greatly in the relative amounts of moult undertaken in breeding, stopover and wintering areas. Some individuals suspend wing moult, while others go straight through. In some Dunlin Calidris alpina populations, some birds break migration for several weeks, when they moult rapidly, growing several feathers simultaneously, whereas others migrate by brief stopovers and shorter flights, moulting simultaneously but growing fewer feathers at once (Johnson & Minton 1980, Holmgren et al. 1993). Most Redshanks Tringa totanus that migrate along the coasts of western Europe and West Africa in autumn by relatively short flights seem not to arrest moult while on migration (Pienkowski et al. 1976). In some bird species, therefore, breaks in moult may be facultative, and dependent on individual circumstances.

An important feature of moult is that it can be arrested in a way that a successful breeding attempt cannot. In terms of control, it is as though the demands of breeding (and to some extent migration) can override those of moult, and slow or stop it when necessary (Wingfield & Farner 1979, Hahn et al. 1992). In some desert species that have flexible breeding seasons (see later), immediately following unexpected rainfall, breeding starts and moult stops at whatever stage it has reached (Payne 1972).

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