Begging Behavior and Host Mimicry in Nestling Brood Parasites: Screaming Cowbirds (Molothrus rufoaxillaris) and Shiny Cowbirds (Molothrus bonariensis), Brood Parasites of Grayish Baywings (Agelaioides Badius)

Abstract

Avian brood parasites beg more than host chicks, possibly to compensate for a deficit in provisioning if host parents feed unfamiliar chicks less than conspecific chicks, or as a result of decreased selection against selfishness due to their lack of relation to the hosts. We studied the effects of host mimicry on parasitic begging behavior in Grayish Baywing (Agelaioides badius) nests, which experience parasitism by the specialist Screaming Cowbird (Molothrus rufoaxillaris) as well as the generalist Shiny Cowbird (Molothrus bonariensis). Screaming Cowbird chicks have evolved to mimic Baywings, exhibiting extremely similar appearances and begging calls to their hosts; this may help them overcome the barrier of kin selection in the host adults’ food provisioning decisions. Shiny Cowbird chicks do not resemble Baywing chicks. We analyzed video footage from nine Baywing nests, with chick groups consisting of all Baywing chicks, Baywing chicks with one Screaming Cowbird, and host chicks with one Shiny Cowbird, and recorded begging frequency and intensity. We hypothesized that Screaming Cowbirds, the specialist host mimics, would still beg more frequently and intensely than host chicks, but they would show reduced begging compared to the generalist, host-dissimilar Shiny Cowbirds. Consistent with this hypothesis, Shiny Cowbirds begged more frequently and intensely than Screaming Cowbirds or Baywings. Screaming Cowbirds begged more intensely, but not more frequently, than Baywings. These results suggest the existence of a negative relationship between host mimicry and begging intensity, supporting the hypothesis that parasitic begging is typically exaggerated due to reduced host sensitivity to unfamiliar visual and auditory begging cues.