Interference Competition from House Wrens as a Factor in the Decline of Bewick’s Wrens

Abstract

Widespread declines in breeding populations of migratory songbirds have been attributed to the destruction of breeding habitat, destruction of overwintering habitat, interspecific brood parasitism by cowbirds, and increased rates of predation on nests (Wilcove 1985; Terborgh 1989; Askins et al. 1990; papers in Hagan & Johnson 1992). Evidence for interspecific competition as a cause of decline is negligible (Terborgh 1989), although competition has been suspected as the cause of decline in Bewick's Wren (Thryomanes bewickii [Audubon]) (Wilcove 1990). Bewick's Wren, a songbird once found across much of the U.S., is on the Blue List (species of concern) of the National Audubon Society (Ehrlich et al. 1992), and Breeding Bird Surveys show declines in Bewick's Wrens from 1966 to 1991 in several parts of the country. Declines have been most notable in the migratory, eastemmost subspecies, the Appalachian Bewick's Wren (T. b. altus). The Appalachian Bewick's Wren once bred from eastern Kentucky, eastern Ohio, and Pennsylvania, south to Alabama and Georgia, and it was considered common or abundant in most of that region (Bent 1948). Over the past century the Appalachian Bewick's Wren has almost disappeared; during the 1980s only 20 pairs of Bewick's Wrens were found in Maryland, Virginia, and West Virginia combined (Maryland Natural Heritage Program 1989). In 1986 the Maryland Natural Heritage Program petitioned the U.S. Fish and Wildlife Service (UJSFWS) to list the Appalachian population of Bewick's Wren as endangered (Bartgis 1986). The USFWS accepted the petition (Maryland Natural Heritage Program 1989) but ranked the Appalachian population of Bewick's Wren