Why are house sparrows predominantly monogamous? A test of hypotheses

Abstract

Observational and experimental evidence collected in a 4-year study of house sparrows, Passer domesticus, was used to test hypotheses about the evolution of monogamy. Reproductive success increased for males and decreased for females with increasing harem size. This rejects hypotheses that state that monogamy is advantageous for both sexes. Only 9·6% of 197 control males were polygynous. However, after removal of 30 males, between 36·7 and 73·3% of potential bigamous males acquired an additional female; 43–65% of the experimental widows mated with already mated males while only 23·3% rejected polygynous bonds. This contradicts the prediction, derived from the hypothesis based upon the polygynythreshold model, that widowed females prefer to remate with unmated males. Of 86 males, 27·9%, more than the percentage of polygynous males, remated after a first breeding attempt with a female that had not bred in the colony that season. Hence, the limitation of the polygyny level cannot be explained by a lack of females. When nestboxes were put closer together, males held more of them, but they did not increase their harem size. This suggests that the limitation of nest sites is not a proximate determinant of monogamy. Resident females were very aggressive towards a caged female placed close to their nest. The way in which the aggression rate varied with breeding phase and intruder persistence favours the hypothesis that females protect their monogamous status. However, further direct tests are required to confirm that monogamy in this population is maintained by aggression between females.