Fitness correlates of song repertoire size in free-living song sparrows

Birdsong delights listeners and intrigues evolutionary ecologists. Female birds are thought to preferentially mate with males with more complex or extravagant songs. But why should females prefer these males? What information does a male’s song convey?

Jane M. Reid and fellow researchers studied a population of song sparrows inhabiting Mandarte Island, British Columbia, Canada, where males sing elaborate repertoires of songs. They found that male sparrows with larger song repertoire sizes contributed more offspring and grand-offspring to the breeding population on Mandarte. This was because these males lived longer and reared more hatched chicks to independence from parental care, not because the females who mated to males with larger repertoires laid or hatched more eggs.

Furthermore, they discovered that independent offspring of males with larger repertoires were more likely to survive to breed and then to leave more grand-offspring than independent offspring of males with small repertoires. This effect of paternal repertoire size on offspring performance was stronger in sons than in daughters.

Although they do not yet know whether these patterns reflect genetic or environmental effects on males or their offspring, their results suggest that female song sparrows would leave more descendants in both current and subsequent generations by pairing with males with large song repertoires.