The Effect of Aggregated Nesting on Red-Winged Blackbird Nest Success and Brood Parasitism by Brown-Headed Cowbirds

Abstract

We examined temporal and spatial nesting aggregations in a prairie-nesting population of Red-winged Blackbirds (Agelaius phoeniceus). In particular, we were interested in the effects of aggregated nesting on blackbird nest success and brood parasitism by Brown-headed Cowbirds (Molothrus ater). Most cowbird parasitism occurred early in the breeding season during the peak in Red-winged Blackbird nesting. As a result, more parasitized nests were active simultaneously with other nests than were unparasitized nests, even when we controlled statistically for the effects of nest initiation date. The probability of parasitism across the entire study site did not decrease on days when many nests were initiated, suggesting no "swamping" effect of aggregated nesting on cowbirds. However, individual parasitized nests were less synchronous with their nearest neighbors and farther from the nearest simultaneously active nest than were unparasitized nests. These data are consistent with the hypothesis that brood parasitism by Brown-headed Cowbirds may select for aggregated nesting in Red-winged Blackbirds as a result of greater parasitism pressure on isolated nests. Nests that succeeded in producing at least one fledgling were initiated significantly earlier in the breeding season than were nests that failed. Nest aggregation had no effect on nest success; successful nests were no different from failed nests in the number of simultaneously active nests, their distance to nearest simultaneously active nests, the laying interval with their nearest neighbors, and the distance to their nearest neighbors. These data suggest that nest predation (the primary cause of nest failure), in contrast to nest parasitism, does not necessarily select for aggregated nesting, or that predation and nest aggregation are related only at high nest-densities.