To address the question of the role played by acid rain in population declines of eastern North American songbird species, we combine data from several sources. We use a multiple logistic regression model to test for adverse effects of acid rain on the Wood Thrush, while controlling for regional abundance, landscape-level habitat fragmentation, elevation, soil pH, and vegetation. We show a strong, highly significant, negative effect of acid rain on the predicted probability of breeding by this species, and interactions with elevation, low pH soils, and habitat fragmentation that worsen these negative effects. Our results suggest an important role for acid rain in recent declines of some birds breeding in the eastern United States, particularly in high elevation zones with low pH soils, and show the need to consider other large-scale influences, in addition to habitat fragmentation, when addressing bird population declines.

We studied the effects of habitat fragmentation on four species of North American tanagers (  Piranga spp.) at 1107 study sites in 1995. Volunteer researchers used the standardized protocol of Project Tanager to select sites, census for breeding tanagers, and measure habitat and landscape features on a continental scale. In a principal components analysis, we used measures of forest patch size, distance to nearest other forest patches (isolation), percent forest cover and length of linear edge in a 1000‐ha landscape, and elevation. Our results showed that three independent habitat gradients affected tanager occurence: (1) overall amount of fragmentation; (2) forest configuration (the amount of edge in a landscape); and (3) elevation. Multiple logistic regression indicated significant variation among the tanager species in how habitat gradients affected tanager occurrence, presence of parasitic Brown‐headed Cowbirds (  Molothrus ater), and various nest predators. Scarlet, Western, and Summer Tanagers (  P. olivacea, P. ludoviciana, and P. rubra, respectively) responded similarly to the fragmentation gradient, with the probability of finding breeding tanagers dropping below 0.50 in highly fragmented sites. Response to the other habitat gradients was more variable, and effects on cowbirds and predators were usually opposite those of breeding tanagers (i.e., they were more common at highly fragmented sites). Response to habitat gradients also varied significantly among four regions within the range of Scarlet Tanagers. The strongest response to fragmentation, by breeding tanagers as well as by cowbirds and nest predators, was in the largely deforested Midwest and Atlantic Coast regions. In the heavily forested Northeast region, fragmentation had a significant but less severe effect on breeding tanagers. Effects were not significant in the Northern Forest region, where tanager abundance was low. Results of our continent‐wide study suggest that (1) forest fragmentation can be described similarly across most regions of North America; (2) three widespread tanager species exhibit clear and similar negative responses to habitat fragmentation; (3) local sensitivity to fragmentation varies geographically and may be lower in regions with greater overall forest cover; and (4) results from single‐species or local studies cannot be extrapolated to other species or regions. Conservation strategies to reverse the declines of forest bird populations will require knowledge of habitat requirements across the entire ranges of widespread species, as well as how demographic and landscape factors interact to create population sources and sinks.

Summary 1.Populations of formerly continuously distributed species subdivided by habitat fragmentation may show distributions in space and time that are consistent with predictions of metapopulation theory. Local extinctions and recolonizations should result in the most fragmented sites being infrequently occupied and the least fragmented sites being continuously occupied by sensitive species. The probability of extinction is predicted to be negatively correlated with patch size and the amount of habitat in the landscape. Conversely, recolonization is predicted to be negatively correlated with the isolation of the patch, and positively correlated with the amount of habitat in the landscape. 2. Data from a 3-year study of the effect of fragmentation were used to test whether these predictions from metapopulation theory apply to populations of the long-distance migrant forest bird Piranga olivacea (Scarlet Tanager) in fragmented North American landscapes. 3. Principal components analysis was used to derive a composite measure of fragmentation. This measure was used in a logistic regression as a predictor of the number of years that territorial males would occupy a site, given that it was occupied at least once. More fragmented sites were more likely to be occupied only once; the least fragmented sites were more likely to be occupied in all three years. Data on fragmentation were necessary, but not sufficient, to predict site reoccupation, and were poor predictors at medium levels of fragmentation. 4. The univariate measures of fragmentation (patch size and isolation, proportion of forest, and forest/non-forest edge), were also used in logistic regressions to predict the separate probabilities of local extinction or recolonization. Local extinctions were negatively correlated with patch size and amount of forest in the landscape, as predicted. Recolonizations were negatively correlated with isolation of the patch as predicted, and surprisingly, also with the amount of edge in the landscape. This suggests that stochasticity may drive extinctions, but that habitat selection may play an important role in recolonization. 5. Demographic data are usually required to establish the suitability of habitat to support persistent populations, but multiple-year distributional data can provide information on habitat quality far above that obtained from single-year studies.

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