Brown-Headed Cowbird Nestling Vocalizations and Risk of Nest Predation

Models concerning the evolution of avian begging behavior predict that nestlings of brood parasites should beg more loudly or frequently than nonparasitic nestlings and that the exaggeration of begging calls in general may be limited by the risk of nest predation. This study is the first to test experimentally for a link between brood parasitism, nestling vocalizations, and the risk of nest predation. Begging calls at Indigo Bunting (Passerina cyanea) nests parasitized by Brown-headed Cowbirds (Molothrus ater) were louder and more frequent than those at nonparasitized nests. Predation rates at Indigo Bunting nests were significantly higher at parasitized nests than at nonparasitized nests during the incubation period and over the entire nesting cycle, and there was a trend for such a difference during the nestling period as well. I performed an artificial nest experiment to test for an effect of nestling vocalizations on the risk of nest predation. Nests broadcasting cowbird begging calls (300 calls/h at 80 dB) experienced the highest predation rates, followed by nests broadcasting bunting begging calls (60 calls/h at 74 dB), followed by silent nests. The overall difference among predation rates for the three treatments was significant, but follow-up pairwise comparisons detected a difference in predation rates only between nests broadcasting cowbird begging calls and silent nests. Thus, nestling vocalizations may be partly responsible for the observed difference in predation risk during the nestling period at parasitized and nonparasitized natural nests, but other factors such as nest site or parental behavior are likely to influence the risk of both parasitism and nest predation.