Effects of Forest Fragmentation on Breeding Tanagers: A Continental Perspective

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 conti- nental 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 el- evation. 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 gradi- ents 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 drop- ping 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 com- mon 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 demo- graphic and landscape factors interact to create population sources and sinks.

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