BREEDING BIRD DENSITY IN WOODLOTS: EFFECTS OF DEPTH AND BUILDINGS AT THE EDGES

Abstract

A model is presented that describes the breeding bird density of a species in a woodlot, given that this density may change from edge to interior. The primary variable is a measure of depth in the woodlot that depends on distances to four woodlot edges. The model applies to forest-interior species and to edge species, and it also accounts for different degrees of threat or attraction at different edges. During 1993-1996, we collected bird abundance data using point counts at 387 sample plots in 176 woodlots, including two big woods (>5000 ha), in eastern Pennsylvania, USA. Models were developed for 36 species by using nonlinear regression guided by tests of residuals against other variables measured at each sample plot. Densities of 20 of the 36 species were affected by the depth in the woodlot. The density of one forest-interior species, Ovenbird (Seiurus aurocapillus), de- creased farther away from the big woods, which can be explained qualitatively by dispersal dynamics. Densities of 21 of the 36 species were affected by buildings near the woodlots: 10 species increased, and 11 decreased. One preliminary conservation conclusion is that it is important to preserve big woods (>5000 ha) because at least one common forest species (Ovenbird) might disappear from nearby woodlots if the big woods were to become fragmented. A second conservation conclusion is that certain species can persist only in the absence of nearby buildings. Perhaps these species would persist near buildings if the negative biotic interactions associated with buildings were removed, but more information is needed before this possibility can be evaluated. Our model-building procedure and sta- tistical approach might be useful for modeling a number of variables concerning birds or other animals in various habitats where edge effects are important.