NESTING ECOLOGY OF TOWNSENDS-WARBLERS IN RELATION TO HABITAT CHARACTERISTICS IN A MATURE BOREAL FOREST

We investigated the nesting ecology of Townsend's Warblers (Dendroica townsendi) from 1993-1995 in an unfragmented boreal forest along the l ower slopes of the Chugach Mountains in southcentral Alaska. We examin ed habitat characteristics of nest sites in relation to factors influe ncing reproductive success. Almost all territory-holding males (98%, n = 40) were successful in acquiring mates. Nest success was 54% (n = 2 4 nests), with nest survivorship greater during incubation (87%) than during the nestling period (62%), Most nesting failure (80%) was attri butable to predation, which occurred primarily during the nestling per iod. Fifty-five percent of nests containing nestlings were infested wi th the larvae of bird blow-flies (Protocalliphora braueri and P. spenc eri), obligatory blood-feeding parasites. The combined effects of Prot ocalliphora infestation and inclement weather apparently resulted in n estling mortality in 4 of the 24 nests. Nests that escaped predation w ere placed in white spruce with larger diameter than those lost to pre dation; nests that escaped blow-fly parasitism were located higher in nest trees and in areas with lower densities of woody shrubs than thos e that were infested. The availability of potential nest sites with th ese key features may be important in determining reproductive success in Townsend's Warblers.