Habitat Fragmentation and Pairing Success in the Ovenbird (Seiurus aurocapillus)

ABSTRACr.-In 1990 and 1991, we determined the proportion of Ovenbird (Seiurus aurocapillus) territorial males that were paired in a 25-ha plot in an extensive forest (>350 kM2) in Quebec, and compared it to pairing success in populations breeding in nearby forest fragments (4.5-53.0 ha) in Quebec and Ontario. We tested the hypotheses that pairing success would increase with population density, and decrease with the degree of fragmentation of the habitat. Pairing success was higher in the extensive forest in both years, although the difference was only marginally significant (P 0.25) in 1991 (76.5 vs. 58.3%). The density of territorial males was twice as high in the extensive forest as in the forest fragments. When including data from similar studies conducted in New Jersey and Missouri, we found that pairing success increased and gradually levelled off with the density of territorial males. Although we lacked a common parameter for measuring habitat fragmentation across all three studies, pairing success appeared to decrease with the isolation of forest fragments from surrounding woodland, while it remained stable around 80% in extensive forests of all three studies. The effect of habitat fragmentation on pairing success was particularly severe in the Missouri study area, at the periphery of Ovenbird's breeding range. A general linear model showed that woodland configurationthe location of a study site in an extensive or fragmented forest-was the best predictor of pairing success, and that geographical location also had a significant effect. We conclude that habitat fragmentation reduces pairing success by altering dispersal dynamics or habitat selection by females. Received 6 April 1992, accepted 17 November 1992. THE OVENBIRD (Seiurus aurocapillus) has been