Nest Predation and Its Relationship to Habitat and Nest Density in Dickcissels

Analysis of the histories of over 500 Dickcissel (Spiza americana) nests found in eastern Kansas showed that those in old-field habitats suffered more predation than those in prairies. Predation rates on the prairie, were not correlated with the weeks of the nesting season, but those in old-fields varied significantly with time. Although both predation rates and nest densities increased concurrently to a peak during the middle of the nesting season in old-fields, an analysis of the relationship between nest densities per week and both daily predation rates and the percent of nests lost to predators each week indicated that predation was not density-dependent. Predation rates are higher in old-fields than in prairies, not because of greater nest densities, but perhaps because predators are more abundant in old-fields. Snakes are the most probable nest predator, and their method of hunting, by chance encounters rather than by directed search, is suited to the absence of a density-dependent effect of predation on Dickcissel nests.