Helping in greyish baywing decreases nest predation but does not ameliorate the costs of brood parasitism by screaming cowbirds

Abstract

In cooperatively breeding species one or more adults help raise offspring not their own. In the last decade studies unveileda geographic correlation between avian cooperative breeding and obligate brood parasitism that suggests an evolutionarylink between these breeding systems. However, the fitness consequences of cooperative breeding for brood parasites andtheir hosts are still poorly known. Here we examined potential benefits of cooperative breeding to host and brood-parasiticnestlings in nests of the greyish baywing (Agelaioides badius), main host of the specialist screaming cowbird (Molothrusrufoaxillaris). We assessed whether the presence of helpers can provide short-term benefits to baywing and screamingcowbird nestlings through additional nest provisioning and antipredatory defence. Provisioning rates increased with hostbrood size but did not differ between nests with and without helpers. Consistent with this, the asymptotic mass and bodycondition of host and parasitic nestlings were unrelated to helper presence. Moreover, baywings showed reduced mass andcondition in mixed broods, suggesting that helping would not fully mitigate costs of parasitism. On the other hand, nestswith helpers were defended more intensively against a predator model and more likely to succeed than nests of unassistedpairs. Our results do not evidence nutritional benefits of helping neither for baywing nor parasitic nestlings, but suggestthat cooperative breeding can enhance short-term nestling survival through collective nest defence. Further research onindividuals’ contribution to offspring care and the ecological correlates of breeding success in this host-parasite systemwill increase our understanding of the interplay between cooperative breeding and brood parasitism.