The aggregation of nonbreeding insectivorous songbirds into multispecies feeding flocks during migration and on their wintering grounds is a well-known and important aspect of their ecology. The establishment of multispecies feeding flocks on the temperate breeding grounds of North American Neotropical migrants, however, remains poorly known or understood. To address this gap, we investigated the composition and timing of flocking behavior among several species occurring in the southern boreal mixed-wood forest of western Canada. Of 67 species observed in 216 flocks, the most abundant were Tennessee Warbler (Vermivora peregrina) and several resident species: Black-capped Chickadee (Poecile atricapillus), Red-breasted Nuthatch (Sitta canadensis), and Boreal Chickadee (Poecile hudsonica). Consistent with previous work on Eurasian boreal species, residents appeared to play a pivotal role in flock occurrence and cohesion. Flocking tended to begin in late June, and flock sizes increased throughout the summer. This suggests that unsuccessful breeders, early breeders, and early migrants are the first to join flocks, whereas later-nesting species may delay joining flocks until after their young fledge. We also investigated the propensity of several species to display flocking behavior in areas with and without a superabundant food source—the spruce budworm (Choristoneura fumiferana). These data provided some support for the hypothesis that flocking facilitates foraging, as species tended to flock in areas where food abundance was lower.