Departure from breeding areas

In many species of birds, failed breeders that are freed from parental duties leave their nesting areas earlier than successful breeders, which can depart only when their young are grown. In consequence, in years of widespread breeding failure, as sometimes occur in arctic-nesting shorebirds, the bulk of the post-breeding migration occurs noticeably earlier than usual. It illustrates the dependence of post-breeding migration time on previous events in the annual cycle. More significantly, however, in some bird species, only one sex looks after the young, which frees the other sex to leave the breeding areas at earlier dates. This is evident, for example, in most duck species in which the males play no part in parental care, and leave their breeding places up to several weeks before the females and young (Cramp & Simmons 1977). Typically, the males assemble at special moulting sites, where they pass the flightless period (when they replace all their flight feathers), before moving on to the wintering areas (Chapter 16). Many females stay with their young in the breeding areas, where they moult their flight feathers, and only later in the year move directly to wintering areas, arriving later than the males. Other females move to the moulting sites, but later than the males.

Some shorebird species show a similar pattern to ducks in that the females remain with the young until they are full-grown, allowing the males to leave the breeding areas at earlier dates (as in Curlew Sandpiper Calidris ferruginea and Ruff Philomachus pugnax). In other shorebirds, however, the males raise the young to this stage, thus allowing the females to depart at earlier dates (as in Spotted Redshank Tringa erythropus, Wood Sandpiper Tringa glareola and Grey Plover Pluvialis squatar-ola). In yet other shorebirds, both partners help to the same stage with parental care, and the two sexes migrate at about the same time (as in Lapwing Vanellus vanellus and Black-tailed Godwit Limosa limosa). In all these shorebird species, the young tend to depart after the adults, requiring longer to prepare themselves. Hence, the general sequence of post-breeding migration in shorebirds begins with failed breeders, followed by the successful breeders of the non-parenting partner, then the parenting partner (or both partners), and finally the young. Departure dates in a population can thereby spread over several weeks, as can the subsequent arrival dates in moulting or wintering areas (Cramp & Simmons 1983).

Among many small raptor species, the adults migrate after completing moult. Females start moult around the time of egg-laying, while the males, who provide the food, delay the start of moult until around the time of hatch. This in turn enables the females to finish moulting and depart on migration earlier, leaving the remaining parental care to the male (Kjellen 1992). In the Honey Buzzard Pernis apivorus, in which the sexes share breeding duties equally, the two sexes depart at the same time. Other bird species which show uniparental care show marked sexual divergence in autumn migration dates. The fact that this pattern is repeated independently in different taxonomic groups further emphasises the link between migration and breeding system. Moreover, amongst species which show bi-paren-tal care, some show only minor sex differences in autumn departure dates, while others show no such differences (e.g. Murray 1966 for various passerines).

In addition to the sex difference in arrival and departure times, young adults nesting for the first time typically arrive on their spring breeding areas some days later than older and more experienced birds of the same sex (Figure 15.1). Such age-related differences have been noted in a wide range of species, from passerines to shorebirds and seabirds (see later). Two explanations have been offered, which are not mutually exclusive. The first is that, because young adults cannot compete effectively with older ones for nesting territories, they are better to arrive later in the season, when most old birds have already settled, thereby saving the energy that would otherwise be wasted on futile battles. On this basis, later arrival is under endogenous (genetic) control. Secondly, because of their inexperience and subordinate status with respect to older birds, young adults cannot feed as efficiently before departure from wintering areas, or at stopover sites en route, so are delayed on migration (Chapter 27). On this basis, later arrival is under external influence, and results partly from prevailing conditions. In single-brooded species, a late start to breeding would be expected to result in a late finish, and hence in a later departure from breeding areas by young adults compared with earlier nesting older ones. In multi-brooded species, young adults might attempt fewer broods and thus be ready to migrate from breeding areas no later than older ones.

Age differences in autumn migration dates have been recorded in a wide range of bird species, from passerines to raptors and shorebirds (Table 15.1, Figure 15.2). In some species of long-distance migrants, the difference in departure dates between age groups is substantial. Among passerines, the adult-juvenile difference seems to depend on whether a wing moult occurs in breeding areas before the start of migration, or whether migration occurs immediately after breeding, with moult either started and then suspended, or delayed altogether until after arrival in winter quarters (Chapter 11). Most species that moult completely before autumn migration are short-distance or partial migrants, while most of those that arrest or delay moult are long-distance migrants that winter in the tropics.1 In the former,

1 While some long-distance passerines moult none of their feathers before leaving their breeding areas in late summer, others moult some of their body feathers, or body feathers and tertials, while others also moult some of their flight feathers, before moult is arrested for migration (Jenni & Winkler 1994). Moult is then resumed either at a staging area part way through migration, or at the ultimate destination in winter quarters. There is considerable variation in these patterns within species, depending on breeding area and individual breeding dates (Chapter 11).

5 10 15 20 25 30 5 10 15 20 Day of departure

Figure 15.2 Migration departure schedules of (a) juvenile (N=241) and (b) adult (N=199) White-crowned Sparrows Zonotrichia leucophrys based on the pooled data for seven years from Toiga Meadow Pass, California. No sex difference in departure dates was detected, but juveniles left 3.2 days earlier, on average, than males. The range of departure dates was 45 days for juveniles, and 37 days for adults, with most of the variation traceable to inter-year variation in breeding dates. Departure was delayed about one day for every two days that nesting had been delayed by environmental conditions, such as persisting snow cover earlier in the summer. Over the seven-year period, mean departure date varied by about 14 days in juveniles and eight days in adults. From Morton (2002).

5 10 15 20 25 30 5 10 15 20 Day of departure

Figure 15.2 Migration departure schedules of (a) juvenile (N=241) and (b) adult (N=199) White-crowned Sparrows Zonotrichia leucophrys based on the pooled data for seven years from Toiga Meadow Pass, California. No sex difference in departure dates was detected, but juveniles left 3.2 days earlier, on average, than males. The range of departure dates was 45 days for juveniles, and 37 days for adults, with most of the variation traceable to inter-year variation in breeding dates. Departure was delayed about one day for every two days that nesting had been delayed by environmental conditions, such as persisting snow cover earlier in the summer. Over the seven-year period, mean departure date varied by about 14 days in juveniles and eight days in adults. From Morton (2002).

juveniles migrate first, presumably because they replace only their body feathers, a process which takes less time than the adults take to replace their entire plumage, including their flight feathers. But in species that suspend or postpone moult, the adults can leave their nesting areas soon after their young are independent, although the young themselves take another week or more before they are ready to undertake their first migration. This dichotomy holds in all the passerine species listed in Table 15.1, whether Eurasian or North American. In addition, of 18 species of passerines studied in Alaska, 11 moulted in their breeding areas, and the juveniles left before the adults. One species (Alder Flycatcher Empidonax alno-rum) postponed moult until later in the year, and the adults left around 13 days before the juveniles. In the remaining six species, which also moulted later in the year, no significant difference in departure dates occurred between adults and juveniles (Benson & Winker 2001). Likewise in Idaho, in nine passerine species that migrated immediately after breeding, adults left significantly earlier than

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