Innate development of acoustic signals for host parent–offspring recognition in the brood-parasitic Screaming Cowbird Molothrus rufoaxillaris

Abstract

Young birds communicate their need to parents through complex begging displays that include visual and acoustic cues. Nestlings of interspecific brood parasites must ‘tune’ into these communication channels to secure parental care from their hosts. Various studies show that parasitic nestlings can effectively manipulate host parental behaviour through their begging calls, but how these manipulative acoustic signals develop in growing parasites remains poorly understood. We investigated the influence of social experience on begging call development in a host-specialist brood parasite, the Screaming Cowbird Molothrus rufoaxillaris. Screaming Cowbird nestlings look and sound similar to those of the primary host, the Greyish Baywing Agelaioides badius. This resemblance is likely to be adaptive because Baywings discriminate against fledglings unlike their own and provision nests at higher rates in response to Baywing-like begging calls than to non-mimetic begging calls. By means of cross-fostering and playback experiments, we tested whether the acoustic cues that elicit recognition by Baywings develop innately in Screaming Cowbird nestlings or are acquired through social experience with host parents or nest mates. Our results suggest that begging call structure was partially modulated by experience because Baywing-reared Screaming Cowbird and host nestlings were acoustically more similar as age increased, whereas acoustic similarity between cross-fostered and Baywing-reared Screaming Cowbird nestlings decreased from 4–5 to 8–10 days of age. Cross-fostered Screaming Cowbirds developed begging calls of lower minimum frequency and broader bandwidth than those of Baywing-reared Screaming Cowbirds by the age of 8–10 days. Despite the observed differences in begging call structure, however, adult Baywings responded similarly to begging calls of 8- to 10-day-old cross-fostered and Baywing-reared Screaming Cowbirds, suggesting that these were functionally equivalent from the host's perspective. These findings support the idea that, although rearing environment can influence certain begging call parameters, the acoustic cues that serve for offspring recognition by Baywings develop in young Screaming Cowbirds independently of social experience.