Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

URSINO, C.A.; DE MÁRSICO, M.C.; SUED, M.; FARALL, A.; REBOREDA, J.C.

Cusco

Congreso; IX Congreso de Ornitología Neotropical; 2011

Obligate avian brood parasites like Cowbirds (Molothrus spp.) lay their eggs in nests of other species (hosts), which raise the parasitic young. Parasitic nestlings often exhibit more exaggerated begging displays than host young for a given hunger level. A few studies in biparental host species showed that such intense begging may influence host parental behavior. In contrast, little is known about the effect of brood parasitism on parental behaviors in hosts that breed cooperatively. In these species parents are assisted by one or more individuals (‘helpers’) that contribute to nestling feeding and nest defense. In this study we used a model selection approach to examine the influence of brood parasitism on nest provisioning levels in a cooperative host, the Baywing (Agelaioides badius), parasitized by the Screaming (M. rufoaxillaris) and Shiny Cowbird (M. bonariensis). We monitored nest contents and group size (parent plus helpers) at baywing nests at Buenos Aires Province, Argentina between 2006 and 2010. We video recorded feeding visits by cooperative groups at 23 nests when nestlings were 3-4, 6-7 and 9-10 days of age. The most parsimonious model indicated that feeding visits increased with nestling age, group size and number of Cowbird nestlings in the brood. In addition, nests artificially parasitized by Shiny and Screaming Cowbirds recruited more helpers than unmanipulated nests having only host or Screaming Cowbird young. Our results showed that brood parasitism induced changes in brood demand and Baywings responded to these changes by adjusting overall provisioning rates or by recruiting additional helpers. These findings are in agreement with previous results showing an effect of parasitic nestlings on host’s parental behavior and indicate that brood parasitism and cooperative breeding may interact in determining the levels of parental effort in cooperatively breeding hosts.