Intrasexual competition and the mating system in primarily monogamous birds: the case of the song sparrow

Abstract

This study focuses on the effect of intrasexual competition in each sex and its role in the occurrence of polygyny in a primarily monogamous song sparrow, Melospiza melodia, population. Polygyny resulted after males annexed neighbouring territories and after floating females settled on the territories of incubating females. Polygynous males raised more young than monogamous males. Females in polygynous groups often raised young without the aid of a male and consequently raised fewer young than monogamous or male-aided females. Two-year-old males were most likely to become polygynous, but the reverse was found for females, suggesting that age-related differences in competitive ability influenced the observed pattern of matings. Added supplemental food on 15 territories resulted in a reduction in foraging time, an increase in body mass and a greater portion of the time budget spent in surveillance for floaters by both males and females. Floaters settled on control territories more often than expected, even though reproductive success was four times higher on experimental territories, as expected if a territory owner's ability to engage in territory defence affects the likelihood of polygyny. Models that emphasize the role of female choice over that of female competition for resources that limit reproduction predict that females settle where they are most likely to maximize their reproductive success. Monogamy is probably the most frequent mating association in this population because males are usually unable to defend the territory of more than one female, and because territorial females are usually able to prevent settlement by floating females.