Song Repertoire and Mate Choice in Birds

In many species of birds, individual males possess “repertoires” of multiple versions of the species song. Females of several of these species have been shown to respond preferentially in courtship to larger song repertoires. The female preference for large repertoires usually has little effect on female settlement, but is likely to affect mate choice in extra-pair copulations. A number of hypotheses have been proposed to explain the evolution of the female preference. Some of these posit a natural selective advantage for the preference, in securing for the female a better territory, better paternal care for the offspring, or a mate with good genes. Another hypothesis suggests the male trait and female preference have coevolved in a process of runaway sexual selection. Here I show that female common grackles (Quiscalus quiscula) court preferentially for repertoires of four song types compared to equal numbers of repetitions of single song types.The female preference exists in common grackles despite the fact that males in this species sing only one song type each. None of the usual hypotheses, based on natural or sexual selection, can explain the occurrence of the female preference in a species in which males lack the preferred trait. Instead, the female preference may be a simple consequence of two properties of most response systems: habituation and stimulus specificity. If so, female preferences for repertoires may in general pre-date the evolution of male song repertoires, which evolve to exploit the pre-existing female response bias.