Blackburnian warblers are solitary during winter and highly territorial on their breeding grounds and do not mix with other passerine species outside of the migratory period. However, during migration, they often join local mixed foraging flocks of species such as chickadees, kinglets and nuthatches. Similarly, in the tropics they were found to be fairly social while engaging in migration but solitary from other passerines while wintering.Bent, A. C. 1953. Life histories of North American wood warblers. U.S. Natl. Mus. Bull. 203. These birds are basically insectivorous, but will include berries in their diets in wintertime. They usually forage by searching for insects or spiders in treetops. Their breeding season diet is dominated by the larvae of Lepidoptera, i.e. moths and butterflies.Morse, D. H. 1976. Variables determining the density and territory site of breeding spruce-woods warblers. Ecology 57:290-301. They may help control the spruce budworm (often considered a harmful pest) when breakouts occur, at the local if not at epidemic level.Crawford, H. S., R. W. Titterington, and D. T. Jennings. 1983. Bird predation and spruce budworm populations. J. For. 81:433-435. In one study from Ontario, 98% of the diet was made of insects, the remaining 2% being spiders.Kendeigh, S. C. 1947. Bird population studies in the coniferous forest biome during a spruce budworm outbreak. Dept. Lands Forests, Ontario, Canada, Biol. Bull. 1:1-100. Among the migratory Setophaga warblers, it is considered one of the specialist at foraging in the micro-habitat of the tree's top canopy.Morse, D. H. 1968. A quantitative study of foraging of male and female spruce woods warblers. Ecology 49:779-784.
