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 severalof these species have been shown to respond preferentially incourtship to larger song repertoires. The female preferencefor 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 evolutionof the female preference. Some of these posit a natural selectiveadvantage for the preference, in securing for the female a betterterritory, better paternal care for the offspring, or a matewith good genes. Another hypothesis suggests the male traitand female preference have coevolved in a process of runawaysexual selection. Here I show that female common grackles (Quiscalus quiscula)court preferentially for repertoires of four song types comparedto equal numbers of repetitions of single song types.The femalepreference exists in common grackles despite the fact that malesin this species sing only one song type each. None of the usualhypotheses, based on natural or sexual selection, can explainthe occurrence of the female preference in a species in whichmales lack the preferred trait. Instead, the female preferencemay be a simple consequence of two properties of most responsesystems: habituation and stimulus specificity. If so, femalepreferences for repertoires may in general pre-date the evolutionof male song repertoires, which evolve to exploit the pre-existingfemale response bias.