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Studies of song and its function in suboscine passerines are rare. We examined spatial and temporal variation in song structure in a wild population of Olive-sided Flycatchers (Contopus cooperi) and tested among hypotheses explaining this variation. Repeatable variation in song type was observed both within and among individuals. More than 10% of territorial males expressed atypical song types, i.e., permutations of sequential missing and repeated elements of the typical adult song. Atypical songs were predominantly expressed by unpaired males independent of habitat type. A small fraction of males sang atypical song through the middle of the breeding season, but all males sang only stereotypical adult song by the end of the season. These results suggest the expression of atypical songs reflect protracted vocal development rather than evolution of new song types, geographic variation in song structure, or an extensive song repertoire in Olive-sided Flycatchers.