Nestling provisioning in polygynous great reed warblers (Acrocephalus arundinaceus): do males bring larger prey to compensate for fewer nest visits?

Most studies of social polygyny in birds have examined male provisioning on the basis of the number of feeding visits. This may be misleading if males compensate for infrequent visits by bringing larger prey at each visit. We investigated nestling provisioning in the socially polygynous great reed w arbler, Acrocephalus arundinaceus, in south Central Sweden in 1996-1997. We collected data on rare of feeding visits, prey size and the amount of biom ass delivered by males and females. Males had lower rates of feeding visits and provided smaller prey to nestlings in secondary than in monogamous and primary nests. Secondary females had higher rates of feeding visits and br ought larger prey than monogamous and primary females. These results confir m that secondary females face a potential cost of polygyny through a lower rate of male feeding, and that this cost was reinforced by the significantl y lower male provisioning rate (biomass h(-1)) at secondary nests. Secondar y females compensated for the lack of male assistance by increasing their r ate of feeding and bringing larger prey. As a result, offspring in nests of secondary females received as much food as did those in nests of primary f emales. Prey load size increased with the parent's proportion of feeding vi sits, suggesting that parents use different feeding strategies depending on their amount of responsibility for nestling provisioning. We suggest that parents which take the main responsibility for nestling feeding have to for age further away from the nest, and based on optimal-foraging theory, they should then on average bring larger prey to their nest.