Nest predation on woodland songbirds: When is nest predation density dependent?

Nest predation, if density dependent, may structure avian communities by selecting for nest-site characteristics that dilute the predators' perception of nest density (e.g., by increasing the number and types of potential nest sites) thereby reducing losses to predators. We tested for density dependent nest predation at a site dominated by predation from raccoons, Procyon lotor, and blue jays, Cyanocitta cristata. We used cohorts of 20 artificial ground nests placed on experimental grids at four densities (1.7, 7, 65, and 167 nests/ha). Predation was density dependent for one of two temporal replicates. In a second experiment, we tested for density dependent predation on both ground and shrub nests using more realistic nest densities (3.5 and 8.9 nests/ha) and excluding avian and small mammal predators by placing nylon screening over the nests. Predation was density dependent on artificial ground nests and density independent on shrub nests. Finally, we analyzed predation on natural shrub nests separating out avian (i.e., jays) and large mammalian predation based on disturbance to the depredated nest. In each of three years there was no evidence for density dependent predation. An outbreak of periodic cicadas in the first experiment and the maturation of a large mulberry crop during the second experiment demonstrated the importance of the distribution and abundance of alternative foods on the response of predators toward heterogeneity in nest abundance.