We consider a number of hypotheses explaining the social use of song repertoires by territory-holding of male American warblers (Parulinae). Males vary by species in having from one to several songs that are used in station singing. In the latter case the songs are clustered into subsets or modes. The songs (= modes) vary in use by season and territorial position. Much of the organization appears to reflect a duality of function relative to mate attraction on the one hand and to interaction between males on the other. We focus especially on two hypotheses of male interaction that appear to make opposite and testable predictions, these being whether the males use the songs to honestly advertise their claims on the territory, or whether they are assessing their relative resource-holding potentials.