Nonbreeding Dispersal

In migrating to wintering areas, young birds differ from adults in that they have no prior experience of where they are heading. The young of some birds, such as swans, geese and cranes, accompany their parents on their first autumn migration, so in this way the young could learn the locations of specific wintering sites. This behaviour could help to maintain the integrity of particular breeding populations, most of which also keep to distinct wintering areas, separated from those of other populations of their species (Chapter 23). In most bird species, however, the young do not accompany their parents, and may migrate earlier or later in the season than adults. Such young therefore migrate to wintering areas unknown to them, and often unaided by experienced birds (Chapter 9). The same is not true of breeding sites, because in most species both young and adults are returning to localities they already know.

In some bird species, once individuals have wintered in an area, they return from year to year with the same consistency as they return to their breeding localities. Most detailed studies of winter site-fidelity, like breeding site-fidelity, have been conducted by individual researchers working in the same small areas year after year. So again, while such records are useful in confirming site-fidelity, most give little idea of the proportion of surviving birds that settle elsewhere. However, in some species, the proportion of individuals that returned to the same area in successive winters was close to the expected annual (or over-summer) survival, implying that almost all surviving individuals were site-faithful. Thus, annual return rates to particular wintering areas of 37-52% have been recorded from various small songbirds, and of 77-95% for various longer-lived shorebirds (Table 17.6, Figure 17.3). More generally, return to the same specific wintering localities has been recorded by casual records for a wide range of passerines, raptors, gulls, waders, waterfowl and others, including many intercontinental migrants (Schwartz 1964, Moreau 1972, Holmes & Sherry 1972, Pearson 1972, Ely et al. 1977, Raveling 1979, Finlayson 1980, Cuadrado 1992, Rappole 1995, Rimmer & Darmstadt 1996, Sauvage et al. 1998, Salewski et al. 2000).

Many species in winter base themselves on communal roosts, from which they range out into surrounding areas to feed. For example, Starlings Sturnus vulgaris and Barn Swallows Hirundo rustica can range more than 50 km from their winter roosts, so records of individuals several tens of kilometres apart in different winters may still represent 'site-fidelity' in a way that breeding season records at such distances would not. A survey of Barn Swallows wintering in South Africa showed that nearly all returned to the same place, or to within 100 km of it, from year to year (Oatley 2000). Many shorebirds are faithful to their roost sites in different years, and the median distance moved by ringed Dunlin Calidris alpina trapped in different winters (14 km) was well within their daily travel range in large estuaries (Rehfisch et al. 1996). Hence, the existence of communal winter roosts serving extensive feeding areas gives more flexibility to the notion of site-fidelity than do nesting territories in summer.

Site-fidelity in winter seems to show the same pattern as in the breeding season, with ring recoveries in subsequent winters being centred on the first-recorded site, and declining sharply with increasing distance. Such dartboard patterns have been documented in many species,8 but again the distance scale varied greatly between

8Examples include the Common Starling Sturnus vulgaris (Spaans 1977), European Greenfinch Carduelis chloris (Boddy & Sellers 1983), Bullfinch Pyrrhula pyrrhula (I. Newton, in Wernham et al. 2002), Harlequin Duck Histrionicus histrionicus (Iverson et al. 2003), Ruddy Turnstone Arenaria interpres (Clapham 1979), Eurasian Curlew Numenius arquata (Bainbridge & Minton 1978), Sanderling Calidris alba (Myers et al. 1988), Grey Plover Pluvialis squatarola, Dunlin Calidris alpina and Common Redshank Tringa totanus (Rehfisch et al. 1996, Burton 2000), among others.

Table 17.6 Annual return rates of migrant birds to specific study areas in successive winters (all except Blue Tits Parus caeruleus refer to migrants)

Species

Location (Size of study area)

Unsexed

Source

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