Background  Extra-pair paternity (EPP) in birds provides benefits in terms of more offspring, and characteristics for maintenance of this behaviour have been the subject of investigation. Microorganisms are known to be transmitted during mating, especially when mating with multiple partners, and factors reducing this cost of multiple mating are expected. Further, plumage brightness and colour intensity have been shown to be important traits to benefits from multiple mating as predicted by sexual selection. The aim of this study was to investigate the relationship between the rate of extra-pair paternity and the relative size of the uropygial gland at the interspecific level, as the uropygial gland is an exocrine gland hypothesized to produce antiparasitic substances and further identified to affect plumage brightness. Because of the expected benefits of large uropygial gland in scenarios of sexual selection, we predicted a positive correlation with EPP.

Methods  We collected information from the literature of uropygial gland size and frequency of extra-pair paternity of 60 avian species of different families and explored the predicted positive correlation between them. We did so with means of comparative analyses that considered phylogenetic relationship as random factor and included body mass as covariate. We used Markov chain Monte Carlo generalized linear mixed models that were weighted by number of nests used to estimate extra-pair paternity.

Results  We detected a positive relationship between level of extra-pair paternity and uropygial gland size at an interspecific level. This finding is consistent with the prediction.

Conclusions  We discuss the importance of this result in scenarios of sexual selection and argue that the detected relationship may have arisen by utilizing antiparasitic secretions through secondary sexual characters indicating parasite resistance.