Many temperate zone breeding birds spend their non-breeding period in the tropics where they defend individual territories. Unlike tropical birds that use song for breeding and non-breeding territorial defense, vocal defense differs strikingly between breeding and non-breeding territories in migrants. Song, restricted to males, is used during defense of breeding territories but callnotes are used to defend non-breeding territories. To explain why callnotes and not songs predominate in the non-breeding context, we present an empirical model based upon predictions from motivational/structural rules, ranging theory and latitudinal differences in extra-pair mating systems. Due to sex role divergence during breeding that favors singing in males, but not females, females may be unable to range male song. Ranging requires a signal to be in both the sender and receiver’s repertoire to allow the distance between them to be assessed (ranged). Non-breeding territories of migrants are defended by both males and females as exclusive individual (androgynous) territories. Ranging Theory predicts callnotes, being shared by both males and females can, in turn, be ranged by both so are effective in androgynous territoriality. Where songs are used for non-breeding territorial defense both sexes sing, supporting the evolutionary significance of shared vocalizations in androgynous territorial defense.
Morton, E. S., & Stutchbury, B. J. M. (2012). Vocal Communication in Androgynous Territorial Defense by Migratory Birds. ISRN Zoology, 2012, 1–9. https://doi.org/10.5402/2012/729307