In some long-lived bird species, individual migrants leave their natal areas towards the end of their first summer, and do not return in the next spring, but only in a later one, when they are two or more years old (up to four years in Bristle-thighed Curlews Numenius tahitiensis, Marks & Redmond 1996). These young birds either remain in their 'wintering' areas year-round over one or more years, or they may return only part way towards the breeding areas, or they may visit the breeding areas for only a small part of the breeding season, migrating later in spring and earlier in autumn than older birds. They perform both migrations in less hurry than the breeding adults, and in more favourable conditions. Such patterns are shown by various raptors, seabirds, shorebirds and others in which individuals do not breed until they are several years old (Chapter 15).
Most of the first-year shorebirds that stay in 'wintering areas' show no sign of pre-migratory fat deposition or spring moult into breeding plumage, but remain light in weight and in well-worn winter plumage until the next 'post-breeding' moult in late summer into new winter plumage. In other individuals, 'pre-breeding' moult and fattening are much delayed, sometimes into July, too late for the birds to breed that year (McNeil et al. 1994). Lack of both weight gain and 'pre-breeding' moult was apparent among juvenile Curlew Sandpipers Calidris ferruginea in South Africa, among Turnstones Armaria interpres in Scotland, and among Western
Sandpipers Calidris mauri in Panama, while adults wintering in the same places began moult, accumulated body fat and left in spring in the usual manner (Elliott et al. 1976, Metcalfe & Furness 1984, O'Hara et al. 2002). Although the birds that stay year-round in 'wintering areas' do not always undergo the 'pre-breeding' moult into summer plumage, they undergo the late summer 'post-breeding' moult up to several weeks earlier than do adults returning from breeding areas. Turnstones Arenaria interpres over-summering in England moulted seven weeks earlier than adults returning from their arctic nesting grounds (Branson et al. 1979), and Western Sandpipers Calidris mauri over-summering in Panama moulted 3-4 weeks earlier than returning adults (O'Hara et al. 2002). They provide an example of birds moulting at a more favourable time of year when not constrained by breeding to a less favourable time later in the year.
The failure of immatures to migrate, or their tendency to return later and travel shorter distances than adults, might result from inability to obtain enough food and accumulate fat at the same rate as more experienced adults (Chapter 15).
Alternatively, it might result from an inherent, endogenous response that matures with age, leading birds of some species not to return to nesting areas in their earlier years or to visit nesting areas only to prospect for suitable sites but not to breed. Resolving these questions must await more research, but whatever the answer, endogenous factors are presumably involved in initiating moult and movements at appropriate dates, at least in birds that winter in equatori al or opposite-hemisphere regions. A proximate factor of possible importance is the reproductive state of the individual at the time of spring migration. This migration may be triggered only when the bird achieves reproductive capability, with high levels of gonadal hormones, at two or more years of age (similar to the attainment of hormone-dependent breeding plumage in some species). In first-year gulls and others, the testes develop only partially in the first year of life, and not until late in the breeding season (for review, see Lofts & Murton 1968). Moreover, in passerines, experimental castration has revealed the dependence of spring moult and migratory fattening on gonadal hormones, as explained in the next section. After their first nesting attempt, most individuals of species showing deferred maturity evidently return to breeding areas every year, as judged by the annual recurrence of marked individuals at their usual nest-sites (Chapter 17).
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