| Female birds have been increasingly found to sing, and those who sing to attract mates appear to do so along with their male counterparts, not as the solo singers. Female-driven systems may exist, but the cryptic nature of many species hinders a better understanding of their mating systems. The coo is a mate attraction signal by yellow-billed cuckoos (Coccyzus americanus) that was previously assumed to be given by unmated males, though there is evidence that females also coo. We witnessed cooing in a color-banded population of yellow-billed cuckoos that we DNA-sexed from blood samples. Of 105 males and 80 females, 25 were confirmed cooing, all of them female. Despite the small sample size, our results are compelling that female and not male yellow-billed cuckoos coo, revealing a sex-reversed mating system in which unmated females sing to find available males. This result also complements our discovery of high rates of conspecific nest parasitism among yellow-billed cuckoos in which the nesting male is also the father of the parasitic young. Female cuckoos apparently not only coo to attract unmated males to nest with, but also to locate, mate with, and lay eggs in the nests of already-nesting males. |