Sex-specific defence behaviour against brood parasitism in a host with female-only incubation.

Nest protection against intruders is an indispensable component of avian parental care. In species with biparental care, both mates should evolve nest defence behaviour to increase their reproductive success. In most host-parasite systems, host females are predicted to have more important roles in nest defence against brood parasites, because they typically are primarily responsible for clutch incubation. Male antiparasitic behaviour, on the other hand, is often underestimated or even not considered at all. Here we investigated sex-specific roles in four aspects of great reed warbler (Acrocephalus arundinaceus) nest defence against a brood parasite-the cuckoo (Cuculus canorus), namely (1) mobbing, (2) nest attendance/guarding, (3) nest checking and (4) egg ejection. Using dummy experiments, simulating brood parasitism and by video-monitoring of host nests we found that males took the key roles in cuckoo mobbing and nest guarding, while females were responsible for nest checking and egg ejection behaviours. Such partitioning of parental roles may provide a comprehensive clutch protection against brood parasitism.