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The discovery that socially monogamous birds copulate outside the pair-bond compels us to view mating and parental care relationships separately. The operation of sexual selection via extra-pair copulation suggests that the mating behaviors of monogamous species are more similar to those of promiscuous species than previously thought. Males in many promiscuous species aggregate in display arenas called leks, which form in response to females seeking copulation. This chapter reviews and synthesizes evidence that the mating systems of monogamous birds that pursue extra-pair copulations resemble those of promiscuous species. Razorbills (Alca torda) demonstrate that males in a monogamous species form leks for extra-pair copulation. Results from studies of Purple Martins (Progne subis) indicate that lek mechanisms may contribute to colony formation. These studies, combined with other evidence …