Edge Effect on Breeding Birds along Power-line Corridors in East Tennessee

Abstract

Breeding bird territories were mapped for 2 years along 1.75 km of forest edges bounding two power-line corridors through forest. Mean bird density (averaged over corridors and years) at the was 153 pairs/40 ha for a defined width of 37 m and 266 pairs/40 ha for a defined width of 5 m. Bird density averaged 236 pairs/40 ha in the corridor interior and 149 pairs/40 ha in the forest interior. Corridor species were less dense at the forest than in the corridor interior (P 0.05). Numbers of corridor and forest species were lower at the than in the community interiors (P <0.05). No species appeared to be dependent on the presence of the forest edge. INTRODUCTION Edge effect is the tendency for the number of species and the number of organisms per unit area to be greater at the borders (edges) between different plant communities than in the interiors of the communities (Odum, 1971; Gates and Gysel, 1978). Although effect is an important concept in wildlife management and is often emphasized in wildlife management texts (Gates and Mosher, 1981; Hanley, 1983), relatively little empirical justification for effect is available. Most research has been on birds, which have been reported to illustrate effect at man-made edges between forests and fields (Lay, 1938; Johnston, 1947; McElveen, 1979; Strelke and Dickson, 1980) and in habitat mosaics having much (Beecher, 1942; Good and Dambach, 1943). However, Gates and Gysel (1978) concluded that habitat suitability at narrow manmade edges was lower than in the community interiors, because of low fledging success caused by greater predation and cowbird parasitism at the edges. At natural edges or ecotones in some areas of the mountainous West, bird species richness shows little evidence for effect and few species if any inhabit primarily edges (Balda, 1975). Recent studies of mule deer (Odocoileus hemionus) and elk (Cervus elaphus), both often believed to be edge animals, did not show increased density at edges (Hanley, 1983; Kirchhoff et al., 1983). The construction of high voltage power lines through forests in the United States has greatly increased the amount of narrow man-made edge. Previous studies concerning birds and forest along power-line corridors addressed fledging success in relation to spatial distribution of nests in corridor, and forest (Chasko and Gates, 1982), variation in the degree of effect shown by five corridor species (Kroodsma, 1984a) and the effect of power-line corridors and their edges on forest birds only (Kroodsma, 1984b). No study, however, has addressed the effect of forest-corridor on the entire bird community in terms of the traditional edge-effect concept of increased species number (richness) and population density. The objectives of the present paper are to: (1) describe the effect of forest-corridor on species number and population density of breeding birds, and (2) evaluate the benefits of forest-corridor to breeding birds. In addition, the effect of on individual bird species found in the corridors is analyzed to complement previous analyses for individual forest bird species (Kroodsma, 1984b). Forest species and corridor species were readily classified as such in my studies, with few exceptions, because bird species overlap and territory overlap between forest and corridor were minimal.