Resource Use by Sympatric Kingbirds

Resource use by Cassin's Kingbirds (Tyrannus vociferans) and Western Kingbirds (T. verticalis) was studied during the breeding season in southeastern Arizona. The former nested predominantly in riparian creek habitat, while the latter nested in more open desert habitat. However, both species nested together where creeks flowed through open habitat. Here, they used similar nest sites, foraged in similar ways, and fed nestlings similar diets (taxonomically and by size). The nesting habits of the two species in the same habitat were more alike than those of members of the same species nesting in different habitats. Breeding success of Cassin's Kingbirds in open riparian habitat was lower than in denser riparian habitat, owing to more predation. However, we found no evidence of smaller clutch size or slower nestling growth, or more starvation in this habitat despite the lack of ecological divergence. These data indicate that competition for food is not likely to be a significant factor in maintaining the habitat separation between these two species.