Geographic variation in the body mass and acoustic parameters of territorial ‘yodels’ recorded from male Common Loons (Gavia immer) were assessed for individuals breeding on territories across the eastern United States. Multivariate analyses incorporating male body mass, body size, the acoustic parameters of yodels, and geographic latitude and longitude indicated that males inhabiting lakes in northwestern regions were smaller and produced higher-frequency yodels. These relationships strengthen previous observations of clinal geographic variation in loon body size and vocal behavior across North America, but also support the hypothesis that the dominant frequencies of yodels are in part influenced by male body size. Therefore, the frequencies loons use for long-distance communication are apparently influenced, at least in part, by those selective forces responsible for shaping optimal body size.