In this descriptive study we performed an acoustic comparison of mimetic and non-mimetic song in a highly conspicuous mimic, the Northern Mockingbird Mimus polyglottos. Using automated acoustic software we measured 13 acoustic features from the mimetic and non-mimetic song of 13, free-living, male mockingbirds recorded during the breeding season. We found that when mockingbirds mimic they extend the maximum frequency of their song by over 600 Hz, which increases the frequency bandwidth by 42%. These differences might reflect female preference for the acoustic features present in mimetic song, or they might represent an artefact of which particular acoustic models are imitated by mockingbirds when developing a mimetic repertoire.