Forest fragmentation is held to have promoted increases in the numbers of predators and brood-parasitic Brown-headed Cowbirds Molothrus ater. In eastern North America, as in Europe, many small generalist predators, which eat eggs and chicks, reach much higher densities in suburban and farming areas than in more natural areas (Wilcove et al. 1986, Small & Hunter 1988, Hoover et al. 1995), benefiting from the additional food provided by human activities. Such predators include mammals, notably Grey Squirrels Sciurus carolinensis, Raccoons Procyon lotor and feral cats, and birds such as American Crows Corvus brachyrhynchos and Blue Jays Cyanocitta cristata. All these species tend to concentrate their hunting along woodland edges (Gates & Gysel 1978, Whitcomb et al. 1981, Paton 1994).
Several studies have shown that the nesting success of songbirds is higher in large forests than in small woods (Figure 25.1). Others have shown that in large
10 100 1000 Forest area (ha)
Figure 25.1 Proportion of Wood Thrush Catharus mustelinus nests that produced young in forest patches of different size, Pennsylvania (r2 = 0.86, P < 0.001). Nest success increased with size of forest fragments, mainly because of differential predation. Both avian and mammalian predators were more abundant in small fragments. From Hoover et al. (1995).
m en forests nest success increased from edge to interior. In total, overall predation rates decreased with increasing area of forest patch in eight out of eight studies, and decreased from centre to edge in 10 of 14 studies involving artificial nests, and in four of seven involving natural nests (Paton 1994). In each case, predators that entered the woods from nearby farmed or suburban areas were held responsible for most of the losses. In one study in southern Wisconsin, for example, the overall nest success of 13 species was only 18% within 100 m of forest edge (N = 96), compared with 58% at 100-200 m within forest (N = 98) and 70% at more than 200 m into forest (N = 82) (Temple & Cary 1988). The entire area of most small woods lay within the most vulnerable zone. It is not hard to imagine, therefore, that as forest becomes more fragmented, overall nest success could decline, eventually to the point where insufficient young were produced to offset the annual adult mortality, leading to regional population declines (Brawn & Robinson 1996). Other (but not all) studies using artificial or natural nests have given similar results elsewhere in North America (Figure 25.2; Gates & Gysel 1978, Wilcove 1985, Small & Hunter 1988, Yahner & Scott 1988, Donovan et al. 1995, Hoover et al. 1995, Robinson et al. 1995, Weinburg & Roth 1998, Manolis et al. 2002). The same is true for studies with artificial nests in Europe (Andren & Angelstam 1988, Sandstrom 1991, Andren 1992), although findings from artificial nests may not always be typical of those from natural nests.
If nest predation has increased with forest fragmentation, why should it have affected some Neotropical migrants more than other birds? Three reasons have
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