RELATIONSHIPS AMONG NORTH AMERICAN SONGBIRD TRENDS, HABITAT FRAGMENTATION, AND LANDSCAPE OCCUPANCY^*

Abstract

Fragmentation of breeding habitat has been hypothesized as a cause of population declines in forest-nesting migratory birds. Negative correlations between the degree of fragmentation and bird density or fecundity at local or regional scales support the fragmentation hypothesis. Yet, in spite of reduced fecundity and densities in fragmented systems, many forest-nesting passerine species have increased in numbers over time. We hypothesized that range-wide population change in species for which habitat fragmentation negatively affects reproductive success should depend on the proportion of the population that actually occupies fragmented landscapes. We predicted that fragmentation-sensitive species (e.g., species that occur in reduced densities in fragmented landscapes) should increase globally in numbers at a greater rate than species that readily occupy fragmented landscapes, because fragmentation-sensitive distributions place a large proportion of the global population in contiguous landscapes that are superior for breeding. We used Breeding Bird Survey (BBS) data and associated landscape metrics to test this prediction for 10 species of forest-nesting passerines in the United States that experience reproductive dysfunction associated with habitat fragmentation: Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo olivaceus), Wood Thrush (Hylocichla mustelina), Northern Cardinal (Cardinalis cardinalis), Worm-eating Warbler (Helmitheros vermivorus), Kentucky Warbler (Oporornis formosus), Indigo Bunting (Passerina cyanea), Scarlet Tanager (Piranga olivacea), Acadian Flycatcher (Empidonax virescens), and Hooded Warbler (Wilsonia citrina). Our approach was to: (1) quantify landscape features associated with BBS routes across the eastern United States, (2) classify landscapes around BBS routes as “fragmented” or “contiguous,” (3) estimate the proportion of detected individuals that occurred in fragmented landscapes on a species-by-species basis, and then (4) associate 10-yr trends for each species with the proportion of breeding individuals occupying fragmented landscapes. Regression analysis indicated a significant, negative relationship between the proportion of the breeding population occupying fragmented landscapes and the population trend from 1970 to 1980. Although this result links habitat fragmentation to population change and provides support for the fragmentation hypothesis, other factors (e.g., land use change, weather, varying life history traits, varying winter survivorship, habitat amount thresholds) could generate similar results. More work is needed to partition the relative influence of these factors on regional bird population dynamics if conservationists are to understand more clearly the effects of fragmentation on the distribution and abundance of species across their ranges.

Keywords

Affiliated Institutions

• Rocky Mountain Research Station US
• State University of New York US
• York University US
• SUNY College of Environmental Science and Forestry US

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