Lazy males and hardworking females? Sexual conflict over parental care in a brood parasite host and its consequences for chick growth

Due to the costs of parental care, a conflict of interests often arises between mates wherein each prefers the other to invest more. As with parents raising their own offspring, hosts of brood parasites also exhibit negotiations over investment, becoming particularly intensive when parasite demands are high. Lack of cooperation between the partners may eventually affect the condition and fledging success of the young. Here, we investigate the magnitude of sexual conflict over food provisioning in socially polygynous great reed warblers (Acrocephalus arundinaceus) rearing either a parasitic common cuckoo (Cuculus canorus) or their own nestlings and its consequences for chick growth. We found that, overall, males provided less food than females, and that polygynous males provided less food per nest than monogamous males. Moreover, polygynous males provisioning two simultaneous broods supplied their own offspring in relation to age and type (cuckoo/host) of the other brood. Females, unlike males, delivered food amount almost irrespective of social status. The difference in contribution between polygynous males and their mates was most pronounced in nests with a cuckoo. In any case, reduced paternal assistance had no significant effect on growth performance of nestlings. In cuckoos, however, this result may be biased as we could not consider a relatively high proportion of secondary cuckoos that died before their growth parameters could be ascertained. Although not detected in chick growth, host sexual conflict over food provisioning may impose a cost on cuckoos in terms of increased mortality in secondary nests.