Despite a long history of anatomical studies in birds, the genitalia of most avian species remain undescribed. Birds are the only vertebrate taxon with internal fertilization where an intromittent phallus has been lost in most species. Studying the anatomical transitions of the avian phallus in those species where it is still present, allows us to test evolutionary hypotheses of why the phallus was lost in the ancestor of modern birds. As part of an anatomical survey of the evolution of avian phallus morphology, we have examined some avian species whose genitalia have not been described. Previously, there were only two known events of phallus reduction in birds: one transition from intromittent to non-intromittent in the Galliformes, and a complete loss of phallic structures in the ancestor of Neoaves. Here we report three additional cases of phallus reduction in birds: a transition from intromittent to non-intromittent phallus in Tinamiformes (Crypturellus, Tinamidae), the presence of a non-intromittent phallus in Alectura (Megapodidae), and a complete loss of the phallus in Leipoa (Megapodidae). In addition, we report on the unique morphology of the Crypturellus non-intromittent phallus. These new records of phallus reduction highlight the dynamic nature of phallus evolution in birds. Our findings provide evidence against the hypothesis that the phallus in birds is maintained to insure paternity in taxa with exclusive male parental care, since both groups where we report phallus reduction provide predominately male-only care.

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