Nest environment modulates begging behavior of a generalist brood parasite

Abstract

In young birds, the intensity of the begging signal varies according to the chick's internal condition (i.e., hunger or nutritional state) but may be additionally modulated by the external conditions experienced in the nest (e.g., nestmate competition or feeding success). Flexibility of begging in response to nest environment should be especially beneficial for the chicks of generalist brood parasites, which encounter diverse brood conditions depending on the host nest in which they are reared. We investigated variability in the begging behavior of an extreme generalist brood parasite, the shiny cowbird (Molothrus bonariensis). We compared visual and vocal begging variables of cowbird young of 5 days of age reared in nests of 2 common hosts, one smaller (house wren, Troglodytes aedon) and one larger (chalk-browed mockingbird, Mimus saturninus) than the parasite. Cowbird chicks were standardized for short-term hunger prior to recording and were similar in mass/condition in each host, but nevertheless begged more intensely in mockingbird than house wren nests, as indicated by increased stretching of the neck, lower latency to beg, increased time spent begging per bout and greater call rate. These results are consistent with young cowbirds adjusting their begging intensity based on the intrabrood competition of a given host, independent of need, an adaptability that likely contributes to their success as parasites utilizing diverse host species.