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