BREEDING-BEHAVIOR AND REPRODUCTIVE SUCCESS OF CERULEAN WARBLERS IN SOUTHEASTERN ONTARIO

Little is known about the breeding biology of the Cerulean Warbler (De ndroica cerulea), a species declining throughout much of its range. Ho wever, life history information can provide important insight into the vulnerability of a rare species to habitat disturbance. We studied th e breeding behavior of Cerulean Warblers at three different sites in s outheastern Ontario through the breeding season, from early May to lat e July 1995, and at one of the above sites in 1994. Twenty-seven nests , including three renests, were located within 27 territories. Average territory size was 1.04 ha (N = 18). Although both males and females displayed at potential nest sites, building was performed only by fema les and took five to six days. Nest trees were predominantly sugar map le (Acer saccharum) or oak (Ouercus spp.), with an average height of 1 7.7 m and an average diameter at breast height (DBH) of 40.2 cm. Avera ge nest height was 11.8 m. There were no incidents of brood parasitism by Brown-headed Cowbirds (Molothrus ater) and nest loss due to predat ion was low (14%). Incubation, performed only by females, lasted 11 to 12 days. Clutch size ranged from two to five eggs, with a modal clutc h size of 5 (N = 6). During the 10 to 11 day nestling period, both mal es and females fed the chicks equally. Reproductive success in this po pulation was high in both years. Twenty of the 27 pairs successfully f ledged young, with a mean of 3.2 fledglings per successful nest. Recor d increases, and the high reproductive success of Cerulean Warblers in the Frontenac Axis area of the Canadian Shield, may be the result of reforestation of agricultural lands abandoned at the turn of the centu ry.