Interactions between brood parasites and hosts can lead to coevolutionary arms races, whereby hosts evolve defences such as ejection of alien eggs and nest defence, and parasites respond with mimetic eggs and shorter laying times. However, little is known about interactions between brood parasites and cooperatively breeding hosts. The presence of several caregivers may either benefit the brood parasite by increasing reproductive success or be a disadvantage, if helpers improve nest defence. We addressed these hypotheses in cooperative carrion crows, Corvus corone corone, and their brood parasite, the great spotted cuckoo, Clamator glandarius, whose chicks are raised together. Parasitism rate by great spotted cuckoos was inversely correlated with crow group size and unassisted pairs were most likely be parasitized, in spite of being relatively rare. Moreover, in the nests of unassisted pairs, cuckoo chicks enjoyed a greater age advantage with respect to crow chicks compared to nests of larger groups, suggesting that parasitic females could not optimize laying date in nests with helpers. Although multiple caregivers increased cuckoo chick survival, this merely compensated for the smaller age advantage that parasitic chicks enjoyed in nests with helpers compared to those of unassisted pairs. Our results indicate that the presence of helpers reduces the cuckoo's laying performance, providing the first indication that cooperative breeding may protect against brood parasitism. This might occur in crows because the presence of helpers allowed incubating females to increase the time spent at the nest, reducing the parasite's opportunities to lay eggs.