 The first direct investigation of this possibility, while providing no support for what came to be called the "genetic adaptation hypothesis" (GAH), which explains the vocal dialects of the brown-headed cowbird ("Molothrus ater") well.showed that the spatial organisation of song variation was very closely associated with the distribution of distinct habitat types.Moreover, the structural characteristics of the dialect variable (trill interval) showed variation largely consistent with the interspecific acoustic patterns described by E.S.