Costa's Hummingbird (Calypte costae) is a North American species that breeds in desert scrub in the southwestern United States. Males sing a whistled song that is relatively simple compared to the complex learned song of the species' congener, Anna's Hummingbird (C. anna). Because hummingbirds are one of only three groups of song-learners separated by intervening taxa of non-song-learners, this family is an important group in the study of song evolution. We characterize the structure of Costa's song and identify song features that vary among males. More than 300 songs from 26 individual male Costa's Hummingbirds were recorded during the 2006 spring breeding season in Anza Borrego State Park. We found that the song, which is, on average, 2.5 s in length, has a stereotyped structure consisting of four elements in an invariant sequence. The song can occur singly or be repeated multiple times in a bout. All song features measured show significant variation among individuals; however, it is not known whether the variation found is sufficient to allow individual recognition. The ability to identify individuals vocally is a function of song that may differ between Costa's and Anna's hummingbirds. Further behavioral studies of the differences in structure, function, physiology, and development of song in these two species will contribute to our understanding of the evolution of avian song.