Divergent patterns

Another indication of the importance of winter conditions in influencing population changes comes when closely related species that breed in the same area show different trends according to where they winter. Of the various waterfowl species that breed in Siberia, those that migrate to western Europe have all increased in numbers in recent decades, following their greater protection from winter hunting. Examples include the western population of the Greater White-fronted Goose Anser albifrons, and the Dark-bellied Brent Goose Branta b. bernicla. By contrast,

Year Year

Figure 26.5 Reduced production of young by Greylag Geese Anser anser and Barnacle Geese Branta leucopsis, as their numbers have grown. The populations concerned breed in Iceland (Greylag) and Svalbard (Barnacle) and winter in Britain, where counts were made. The graphs show five-year moving mean values. Adapted and updated from Owen et al. (1986).

Figure 26.6 Proportion of first-year birds in wintering populations of various bird species that breed on the Taimyr peninsula of Siberia, shown in relation to densities of lemmings on the breeding grounds in different years. High production of young in Sanderlings Calidris alba, Curlew Sandpipers C. ferruginea and Brent Geese Branta bernicla occurred about every three years when lemming numbers were also high, providing alternative food for predators. From Summers & Underhill (1987).

Figure 26.6 Proportion of first-year birds in wintering populations of various bird species that breed on the Taimyr peninsula of Siberia, shown in relation to densities of lemmings on the breeding grounds in different years. High production of young in Sanderlings Calidris alba, Curlew Sandpipers C. ferruginea and Brent Geese Branta bernicla occurred about every three years when lemming numbers were also high, providing alternative food for predators. From Summers & Underhill (1987).

all those populations that winter in Southeast Asia have continued to decline, in association with rising persecution in that region (Syroechkovski & Rogacheva 1994). Examples include the eastern subspecies of the Bean Goose Anser fabalis serrirostris and A. f. middendorffii, and the Baikal Teal Anas formosa. These various waterfowl species share similar nesting habitats, which are largely undisturbed by people, and their divergent population trends are most plausibly attributed to conditions on their different wintering areas (Syroechkovski & Rogacheva 1994).

Another divergent pattern, evident in temperate regions after a hard winter, is that resident species are often found to have declined greatly, whereas summer visitors from tropical wintering areas have not (Dobinson & Richards 1964, Graber & Graber 1979, Cawthorne & Marchant 1980, Holmes & Sherry 2001). This difference provides another indication of the importance of winter conditions, at least for the resident species (Newton 1998b). Some summer visitors may in fact benefit from the scarcity of resident competitors. The larger than usual numbers of European Pied Flycatchers Ficedula hypoleuca that bred in Britain in 1917 and 1947 were attributed to the greater availability of nest cavities in those years, associated with the scarcity of resident tits caused by preceding hard winters (Elkins 1988). This early observation also suggested that flycatcher numbers were limited in some areas by shortages of nest-sites, an inference supported by subsequent experiments involving either the removal of competing tit species (Gustafsson 1988), or the provision of additional nest boxes (e.g. von Haartman 1971, Currie & Bamford 1982, Newton 1998b). Many other species of cavity-nesting migrants (excluded from Table 26.1) also increased in breeding density and distribution following the provision of artificial nest-sites, as did various hirundines and swifts that use buildings (Erskine 1979, Newton 1994, 1998b, Evans et al. 2003b). The implication is that nest-site availabilities (and hence a feature of the breeding area) are important in limiting the overall population levels of such species.

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