Effects of edge type and patch shape on avian communities in a mixed conifer-hardwood forest

Forest bird communities were sampled along line transects in northwestern Wisconsin during June of 1985 through 1992 to determine whether edge type and patch shape affect avian abundance. Landscape structural characteristics quantified along these transects included: (1) edges that defined the habitat patches they separated, (2) fractals to quantify patch shapes, and (3) areal extent of different patches. Three multiple-regression models were constructed for 10 bird species and the mean number of individuals and species using the following sets of independent variables: (1) edge variables and fractals, (2) area variables, and (3) the first six components from a principal components analysis based on all independent variables. Multiple-regression analysis indicated that edge variables and fractal dimension explained the most variation in abundance for Black-capped Chickadees (Parus atricapuillus). Red-breasted Nuthatches (Sitta canadensis), Hermit Thrushes (Catharus guttatus), and American Robins (Turdus migratorius). In contrast, area variables explained the most variation in abundance for Red-eyed Vireos (Vireo olivaceus), Chestnut-sided Warblers (Dendroica pensylvanica), and Ovenbirds (Seiurus aurocapillus). Abundances of Yellow-bellied Flycatchers (Empidonax flaviventris), Common Yellowthroats (Geothlypis trichas), and White-throated Sparrows (Zonotrichia albicollis) were equally correlated with both edge and area variables. Results of our study show that, for selected species, forest management strategies must be developed that consider not only stand characteristics, but also the edges created between these stands.