Intraspecific aggression is known to be an important behavior structuring bird communities, but interspecific aggression has been studied less frequently. Because of a high degree of similarity in foraging niches, I hypothesized that American redstarts (Setophaga ruticilla) and Adelaide's warblers (S. adelaidae) would interact aggressively during the non-breeding season in southwest Puerto Rico. I used a crossover experiment to determine whether these species were aggressive to heterospecifics, presenting decoys and vocalizations of these two species, along with a control, to individuals and observing their vocal and physical responses. However, the study used only a single playback and decoy for each species, limiting the generality of the conclusions. Both species responded aggressively to decoys of conspecifics and heterospecifics, while no individual responded to the control. Responses to conspecifics were stronger than responses to heterospecifics as hypothesized, although the differences were significant only in Adelaide's warblers. The interspecific aggression observed in this study, combined with previous studies showing a high degree of overlap in space use and foraging niches and probable food limitation, strongly suggests that these species are competing for food during the non-breeding season.