Population Biology of Cavity Nesters in Northern Arizona: Do Nest Sites Limit Breeding Densities?

Abstract

Breeding densities of secondary (i.e., nonexcavating) cavity-nesting birds are often assumed to be limited by availability of nest sites. We investigated this assumption for species breeding in northern Arizona's ponderosa pine forests. In 1979, we installed nest boxes on three treatment plots that differed in habitat structure and monitored breeding densities of six species through the 1983 breeding season. The effect of nest boxes was evaluated by comparing breeding densities on three treatment plots from 1980 to 1983 with: (1) pretreatment densities (1973 to 1975, 1979), and (2) densities on control plots from 1980 to 1983. We observed variation in the importance of nest-site limitation among treatment plots and species. Overall breeding densities (all species combined) increased significantly on only two treatment plots. Individual species' responses were influenced by habitat structure, and breeding densities of only three species were apparently limited by nest sites before boxes were installed; Violet-green Swallows (Tachycineta thalassina), Pygmy Nuthatches (Sitta pygmaea), and Western Bluebirds (Sialia mexicana). A given species' breeding density in northern Arizona is nest-site limited if it is locally common and reliant on dead trees for nest sites. Availability of food or foraging substrate and territoriality may determine an upper limit to breeding densities if nest sites are in ample supply.