I Adult Juvenile
Figure 15.3 Numbers of Eurasian Sparrowhawks Accipiter nisus and Eurasian Hobbies Falco subbuteo seen migrating through Falsterbo, Sweden, at different dates in autumn. In the Sparrowhawk, which is a partial migrant, juveniles moved before adults and females before males, on average. Similar patterns were shown by other partial migrants, including Northern Goshawk Accipiter gentilis and Red Kite Milvus milvus. In the Eurasian Hobby, which is a complete migrant, adults moved before juveniles, on average, but sexes could not be distinguished. Similar patterns were shown by other complete migrants, including European Honey Buzzard Pernis apivorus, Montagu's Harrier Circus pygargus and Osprey Pandion haliaetus. From Kjellen (1992).
juveniles, while in 27 species that moulted before migrating, juveniles left before adults (except for the Pine Siskin Carduelis pinus, Carlisle et al. 2005).
Similar patterns occur among European passerines migrating to Africa. The Common Cuckoo Cuculus canorus is an extreme example, for the adults leave before their last young, reared by other species, have even left the nest, giving a difference in mean departure dates between the two age groups of about one month (Wyllie 1981). Both age classes moult in winter quarters.
Similar departure patterns are seen in raptors (Table 15.1, Figure 15.3). In short-distance and partial migrants that finish moult before migrating, such as the Eurasian Sparrowhawk Accipiter nisus, juveniles leave the breeding areas earlier than adults. The juveniles do not moult at all in their first autumn of life, but retain their juvenile plumage (acquired in the nest) for another year. But in most longdistance migrants that suspend moult and migrate immediately after breeding, such as the Osprey Pandion haliaetus, adults tend to leave before juveniles, departing as soon as the young are independent but still gaining the experience necessary to undertake migration. In the Black Kite Milvus migrans, the age difference is substantial, with adults leaving 3-4 weeks before juveniles (Schifferli 1967).
After nesting in the arctic, adult shorebirds usually leave the breeding areas well before the juveniles, and moult at a migratory staging site or in winter quarters. This temporal difference can be increased by the juveniles' slower rate of progress, resulting from longer and more frequent stopovers, use of poorer staging sites and less direct migration routes (Saurola 1981, Evans & Davidson 1990, Baccetti et al. 1999). In some populations in which the adults stop and moult at a staging site, the two age groups arrive in wintering areas at about the same date, as observed in Dunlin Calidris alpina in southern Europe (Baccetti et al. 1999). Hence, in all these species from different taxonomic groups, whether adults or juveniles depart first on autumn migration is linked to whether or not they migrate immediately after breeding, and where and when the moult occurs. Species whose summer food supply collapses soon after breeding have to leave before moulting, whereas those whose food supply lasts long enough beyond breeding can moult before migrating. The pressures are somewhat different on juveniles and adults because, while the juveniles have a shorter body moult (or in some species no moult), the adults have a longer complete moult, including flight feathers.
Although in geese, swans and cranes the young of the year migrate with their parents, the older immatures tend to migrate earlier than families in autumn (as do failed breeders) and later in spring (for Canada Goose Branta canadensis see Maclnnes 1966, for Snow Goose Chen caerulescens see Maissoneuvre & Bedard 1992, for Tundra Swan Cygnus columbianus see Black & Rees 1989).
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