The Brown-headed Cowbird Molothrus ater lays its eggs in the nests of a wide range of passerine host species, thereby reducing their production of young (Figure 25.3, Mayfield 1977, Lorenzana & Sealy 1999). Originally confined to the grasslands of mid-continent, following deforestation the species gradually spread to occupy most of the continent up to the boreal forest (Mayfield 1977, Brittingham & Temple 1983). Within this newly occupied terrain, its numbers continued to rise at least to the 1960s, as it benefited from the increased food supplies provided by waste grain in cereal fields, cattle feedlots and garden feeders (Figure 25.4). Its overall numbers probably increased more than 10-fold during the twentieth century (Brittingham & Temple 1983), spreading into areas with new host species. Despite this overall national trend, its numbers seem to have declined in the eastern USA since around 1980 (BBS data).
Forest fragmentation increased the amount of edge along which cowbirds have access to the nests of forest species previously immune to attack. Several studies have shown greater parasitism of nests in small than large woods, and near the edges of woods than in the interior (Gates & Gysel 1978, Brittingham & Temple
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