Ecological Factors Associated with Degree of Edge Effect in Breeding Birds

Abstract

The 5 most abundant breeding birds [Dendroica discolor, Geothlypis trichos, Icteria virens, Passerina cyanea, Spizella pusilla] on power-line corridors through forests in eastern Tennessee were censused on 13 study plots. The degree of edge effect (ratio of bird density at the edge to that in the corridor interior) was compared with vegetation features within the corridors. Bird density in the corridor was associated primarily with the patchiness of blackberry (Rubus sp.) and saplings and with the density of blackberry (P < 0.01). The degree of edge effect was for most species independent (P > 0.05) of bird density and of the vegetation features that apparently control bird density in the corridors. Edge effect was negatively associated with the width of the corridors (P < 0.001) and with the frequency of vegetation > 2 m high (primarily saplings) (P < 0.01) within the corridors. These 2 associations were interpreted to be a function of the availability of singing posts for males (territory mapping was based primarily on singing males). Thus as corridor width increased, singing-post trees at the edges became less available to all males, and as the frequency of vegetation > 2 m in height in the corridor increased, males sang proportionately more often from the corridor interior. Some edge effects on bird density reported in the literature may depend more on singing-post preferences than on increased densities along edges or on the presence of highly disturbed habitats of poor quality in one of the communities forming the edge.