Pygmy Nuthatches, White—breasted Nuthatches, and Mountain Chickadees travel in mixed flocks in winter in Colorado. The species exhibited different by overlapping foraging niches. Both intra— and interspecific aggression occurred, supporting the idea of niche overlap. The ratio of intra— to interspecific encounters was higher than expected if encounters were in proportion to flock composition, indicating that niche overlap was not complete. At artificial feeding stations, where found niches were identical for all species, the level of interspecific aggression initially was higher than in nature. However, the dominance of Pygmy Nuthatches at the feeders soon resulted in a "competitive exclusion" of the other birds and a lessening of interspecific contact.