Influence of Food Abundance, Nest-Site Habitat, and Forest Fragmentation on Breeding Ovenbirds

Between 1994 and 1996, we determined the density and pairing success of territorial male Ovenbirds (Seiurus aurocapillus) in 31 forest fragments in southern Ontario. The density and pairing success of territorial males increased significantly with area of the woodlot core. We tested the hypotheses that area-related changes in food abundance and the availability of suitable nest sites are the causal mechanisms limiting female settlement in small forest fragments. Our results indicated that Ovenbirds chose territories with significantly higher prey biomass than occurred at randomly selected sites in the woodlot. Within Ovenbird territories, prey biomass was 10 to 36 times higher in large woodlots than in small woodlots. Invertebrate biomass at randomly located quadrats in large woodlots was more than twice that found at random sites within small woodlots, and was slightly greater than prey biomass within Ovenbird territories in small fragments. Leaf litter was deeper within Ovenbird territories in large woodlots than at random sites in both large and small forests, and within Ovenbird territories in small woodlots. Ovenbirds in large woodlots selected nest sites that were more than 250 m from the forest edge, distances that were not obtainable in small forest fragments. The lack of potential nest sites, combined with lower food abundance in small fragments, may explain why female Ovenbirds find small fragments unsuitable as breeding sites, and hence why so few males secure mates in small fragments. Understanding the cause of deterioration of breeding sites with decreasing woodlot area should underscore efforts to preserve larger tracts of forest to provide adequate nesting habitat for forest-interior migrants.