Relationship between breeding moult and autumn migration

Many birds lay a repeat clutch if the first is lost, or raise more than one brood in a year. This means that adult pairs get increasingly out of synchrony with one another, and that some breed much later into the summer than others in the same area. In species that moult in their breeding areas, individuals that finish breeding late also moult and migrate later, but may begin moult earlier with respect to a nesting cycle, well before their young have fledged, and may also moult more rapidly than earlier birds (for Dunlin Calidris alpina see Johnson & Minton 1980, for White-crowned Sparrow Zonotrichia leucophrys see Morton & Morton 1990, Morton 2002). By these means, individuals breeding until late in the season reduce the delay in their migration. In some passerines, the delay in moult caused by late breeding is greater in females than in males, producing differences in departure dates between the sexes, including members of the same pair (Ginn & Melville 1983, for Cetti's Warbler Cettia cetti see Bibby & Thomas 1984, for Willow Warbler Phylloscopus trochilus see Norman 1990).

Similar things happen in the juveniles. In multi-brooded populations, young are produced over periods of several weeks. Compared to early-hatched young, later ones start to moult at a later date but at an earlier age, and may also replace their feathers more rapidly or less completely (for Chaffinch Fringilla coelebs see Dolnik & Gavrilov 1980; for various warblers see Berthold, 1988, Mukhin 2002, for White-crowned Sparrow Zonotrichia leucophrys see Morton 2002, for Stonechat Saxicola torquata see Helm 2003). Among Great Tits Parus major in northern Europe, first brood young start moulting in July, and second brood young in mid-August, the mean age at the start of moult decreasing over this period from 56 to 42 days, and the mean duration of moult from 67 to 59 days (Bojarinova et al. 1999).

Because of these differences, late-hatched young also migrate at an earlier age than early-hatched ones, with a mean difference of four weeks between their departure dates (Bojarinova et al. 2002). Within populations, a shortening of moult duration is achieved by shedding successive feathers at shorter intervals, so that more are growing at one time; with no obvious change in the growth rates of the feathers themselves (e.g. Newton 1967, Pienkowski et al. 1976).

In addition, while early young normally prepare for migration after completing moult, late birds may begin to fatten well before the end of moult (for Garden Warbler Sylvia borin and Blackcap S. atricapilla see Berthold 1975, for Reed Warbler Acrocephalus scirpaceus see Mukhin 2002). Among Mountain White-crowned Sparrows Zonotrichia leucophrys oriantha in California, autumn fattening usually took 8-9 days, as found by the repeated trapping and weighing of individuals (Morton 2002). In a few early individuals, moult had been finished for up to two weeks before fattening began but, in some late birds, fattening began up to two weeks before moult ended (Figure 12.3).

In various species, late individuals may also accumulate greater fat reserves before departure from breeding areas or stopover sites than early ones. Among Reed Warblers Acrocephalus scirpaceus migrating through southern France, mean stopover duration increased from 6.1 to 11.1 days, and rate of fat deposition from 0.29 to 0.40 g per day over the period late July to late October (Balança & Schaub 2005). The potential flight range of Reed Warblers migrating later in the season was therefore substantially longer than that of early migrants, enabling them to make longer flights and perhaps achieve a higher overall migration speed. Increased fattening rates or greater fat reserves in late season have been recorded in many other species, including Bluethroat Luscinia svecica (Ellegren 1991), Greater Whitethroat Sylvia communis and Lesser Whitethroat S. curruca (Ellegren & Fransson 1992), Blackcap Sylvia atricapilla (Izhaki & Maitav 1998a, 1998b), and Temminck's Stint Calidris temminckii (Hedenstrom 2004). Similarly, in several passerine species studied at Ottenby in Sweden, body mass and fat level were found to increase during the course of the several week autumn migration season (Âkesson et al. 1995, Fransson 1998, Danhardt & Lindstrom 2001). Evidence from ring recoveries revealed that birds which migrated late in the season also travelled more rapidly than earlier ones from the same population (for various species see Hildén & Saurola 1982, for Sylvia warblers see Fransson 1995). Speed of migration may therefore be another aspect of the annual cycle that responds to date, faster migration late in the season being achieved by faster and greater refuelling rates.

The time-saving resulting from quicker development of late young can be substantial. For example, in German Blackcaps, the earliest young leave the nest in late May and the latest in August. Their hatching dates are spread over about 72 days, but owing to accelerated development (especially of moult), the late young develop migratory activity only 18 days later than the early ones. Late young are ready to migrate in September, but without accelerated development they could not depart until mid-November, a dangerously late date (Berthold 1988). One consequence of the relationship between hatching date and development rate is that, in years of late breeding, an entire population can moult, on average, later and more rapidly than usual in preparation for migration (for Lapland Bunting Calcarius lapponica see Fox et al. 1987). Conversely, if birds fail in their breeding,

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