Offspring sex ratio in birds is adjusted according to the relative fitness payoff of producing sons and daughters, which is known to depend on parental quality. Therefore, spatial patterns in offspring sex ratio should be consistent with the distribution of pair quality within avian colonies. In many colonial birds, central parts of colonies provide greater safety against predators and thus are occupied by high-quality pairs, who relegate conspecifics of poorer quality to the peripheral zones of colonies. For this reason, we expected that offspring sex ratio was likely to follow similar central–periphery gradients. This hypothesis was tested in a colony of tree-nesting Great Cormorants (Phalacorcorax carbo sinensis) in central Poland, where 204 nestlings from 53 broods were molecularly sexed. We found a clear central–periphery pattern in offspring sex ratio within the studied colony. There was a considerable bias toward male offspring in the broods of centrally nesting pairs, while edge pairs invested more in female progeny. We also found that broods with a higher proportion of male offspring were associated with higher nesting densities. A nonrandom distribution of offspring sex ratio within the studied colony of Great Cormorants indicates that special care should be taken by researchers to randomize sampling of broods when studying sex allocation in colonial species.
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Vol. 131 • No. 3