ARE DECLINES IN NORTH-AMERICAN INSECTIVOROUS SONGBIRDS DUE TO CAUSES ON THE BREEDING RANGE

Previous studies bave reported a recent decline in breeding populations of migratory songbirds in eastern and central North America Several explanations bave been suggested: deforestation on the wintering grounds in the tropics and habitat loss, increased predation pressure, and increased cowbird parasitism on the breeding range We used these factors to assign 47 species of insectivorous passerines to groups with contrasting vulnerability, and then used the North American Breeding Bird Survey to analyze population trends in these groups on a large continental scale. Variables indexing susceptibility to predation on the breeding ground were most strongly correlated with population trends from 1968 to 1987 During the period from 1978 to 1987, migratory status was also significantly associated with population trends: long-distance migrants to the neotropics exhibited a small, nonsignificant decreasing trend, whereas residents and short-distance migrants increased strongly. During the same time period, the group of species with low nest location, open nests, and high cowbird parasitism declined significantly. Although it is difficult to separate the effects of multiple factors, our analyses suggest that predation on the breeding ground in North America bas played a larger role in the decline of migratory songbirds than deforestation on the wintering grounds in the tropics.