Longdistance Dispersers

The precise year-to-year homing behaviour, shown by the species in Figure 17.3, is not universal in birds. It may be favoured only where habitats remain fairly stable from year to year, and where returning birds can expect to survive and reproduce. It would not be expected in species that depend on unpredictable habitats or food supplies, which are available in different areas in different years. This is the case, for example, in some tundra-nesting species affected by variable patterns of spring snow conditions (Tomkovitch & Soloviev 1994), in some waders and waterbirds affected by fluctuating water levels (Frith 1967, Johnson & Grier 1988, Nager et al. 1996, Robinson & Oring 1997), in desert species affected by irregular rainfall (Davies 1988, Zann & Runciman 1994, Dean 2004), in some boreal finches that exploit sporadic tree-seed crops (Svardson 1957, Newton 1972), or in some predatory birds that exploit locally abundant rodents (Newton 1979, 2002, Saurola 2002). The local population densities of such species often fluctuate greatly from year to year, in line with fluctuating habitat or food supplies (Chapters 16, 18 and 19). The speed with which local numbers increase in response to improving conditions has led to the view that such species are nomadic, with individuals concentrating in different areas in different years, wherever conditions are good at the time. For some such species, ring recoveries have now confirmed that some individuals do indeed breed at hundreds or thousands of kilometres from their natal sites and in widely separated areas in different years (Chapters 18 and 19).

Among various duck species nesting on the North American prairies, the extent and depth of wetlands varies greatly from year to year, according to previous rain and snowfall. Diving duck species, such as Redhead Aythya americana, Canvasback A. valisineria and Lesser Scaup A. affinis, that occupy the deepest and most stable wetlands, show the greatest degree of site-fidelity; while dabbling species, such as Northern Pintail Anas acuta and Blue-winged Teal A. discors, which use the shallowest and most ephemeral wetlands, tend to settle wherever conditions are suitable at the time (Chapter 19; Johnson & Grier 1988). The opportunist settling behaviour of dabbling ducks is reflected in their long natal and breeding dispersal distances and by their low return rates to particular areas, as measured by ringing (Table 17.5). In years of extreme drought on the prairies, many Northern Pintail and other ducks continue their spring migration northward and settle to breed on the tundra. This means that many Pintails that were raised on the prairies move much longer distances in drought years to breed further north, as shown by many ringing recoveries (see Figure 19.7; Smith 1970). Dabbling ducks, and to a lesser extent some species of freshwater diving ducks, contrast with some sea-ducks, which breed in more stable habitats and typically show much higher natal and breeding site-fidelity (Dow & Fredga 1983, Savard & Eadie 1989, Cooke et al. 2000).

Similarly, among shorebirds, species that nest in habitats that normally remain stable from year to year, such as Eurasian Oystercatcher Haematopus ostralegus, Dunlin Calidris alpina and Common Sandpiper Tringa hypoleucos, show strong phil-opatry and site-fidelity, while species that nest in patchy and ephemeral habitats, or are affected by variable patterns of snow-melt, such as Curlew Sandpiper Calidris ferruginea, Sanderling Calidris alba and Little Stint C. minuta, show much less natal philopatry and less adult site-fidelity (Evans & Pienkowski 1984, Tomkovich & Soloviev 1994). In such species, the numbers nesting in any one locality may fluctuate greatly from year to year, depending on how many birds move to or from

Table 17.5 Sex and age differences in the percentage return rates of waterfowl to particular study areas in successive breeding seasons

Natal dispersal Breeding dispersal

Table 17.5 Sex and age differences in the percentage return rates of waterfowl to particular study areas in successive breeding seasons

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