Bird song has been extensively studied in oscine passerines, but far less is known about song in hummingbirds, which are distantly related to passerines. Song is generally thought of as a male trait. Here we document song in 2 wild female Costa's Hummingbirds (Calypte costae) of 10 examined. Their vocalizations closely resembled male song and were produced, undirected, on territories just prior to the breeding season. Both birds were confirmed as female and not young males, which resemble females: one had an enlarged ovary, and the other laid eggs. Our results imply that at least some female hummingbirds may learn song as well as males and have similar song capacity as males. The function, if any, of female song was not immediately clear. Female song seemed rarer in this population than in males and thus easy to overlook. Presence of female song could also be an overlooked phenomenon in other hummingbird species.

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