Is shared male assistance with antiparasitic nest defence costly in the polygynous great reed warbler?

Polygyny typically has negative fitness consequences for secondary females, but may equally impose costs on primary females or even on polygynous males. We investigated how polygynous and monogamous great reed warbler, Acrocephalus arundinaceus, males assist their mates with aggressive nest defence against the common cuckoo, Cuculus canorus, and whether the females adjust their nest defence intensity according to male investment in aggression. Additionally, we investigated whether host social mating status affects host vulnerability to parasitism. We presented taxidermic cuckoo mounts at nests of primary, secondary and monogamous females, and recorded aggressive responses of nest owners. We found that monogamous males defended their nests most aggressively while polygynous males allocated their nest protection effort unevenly between their two mates, responding more vigorously on the primary than secondary nests. In contrast, nest defence intensity of females did not differ with respect to their social status, indicating that females of polygynous males did not compensate for low levels of male aggression. Similarly, we found no differences in natural cuckoo parasitism rates between monogamous, primary and secondary nests. Our results thus suggest that while monogamous females receive more assistance with nest defence than females of polygynous males, this has no effect on the probability of parasitism.