been commonly suggested. First, with few exceptions, the Neotropical migrants that breed in the eastern USA construct open nests, either on the ground or in low bushes, in sites that are most vulnerable to predators (Wilcove & Whitcomb 1983). In contrast, none of the residents or short-distance migrants that breed commonly in suburban settings places its nest on the ground, and many use tree-cavities, which provide relatively safe sites. Second, long-distance migrants start breeding later in the year (mostly June) and usually raise only one brood per year, occasionally two (Greenberg 1980, Whitcomb et al. 1981). In contrast, many resident species and short-distance migrants start in April or May and routinely attempt second and even third broods. Third, many Neotropical migrants are small and unable to drive off nest predators in the way that some larger resident species can. For these various reasons, then, Neotropical migrants are supposedly disadvantaged with respect to nest predators, which could together cause a disproportionate reduction in their breeding rates, and contribute to the selective disappearance of tropical migrants from fragmented eastern woodlands. Through nest predation alone, Wood Thrushes Catharus mustelinus in Pennsylvania produced insufficient young to offset the usual annual mortality (Hoover et al. 1995), and through a combination of predation and cowbird parasitism the same was true for Wood Thrushes in Illinois (Robinson 1992). Yet the latter population did not decline, presumably because local numbers were maintained by immigration.

In an analysis of the population trends of 47 North American insectivorous passerine species in relation to various features of their biology, Bohning-Gaese et al. (1993) found that migratory status, nest location and nest-type were the best predictors of population trends. Only residents and short-distance migrants increased during the years concerned (1978-1987), whereas long-distance migratory species with low nest locations declined. Also, species with closed nests did better than those with open nests. These are both aspects of nesting biology that influence rates of predation and probably also rates of parasitism, as discussed below.

Was this article helpful?

0 0

Post a comment