Social Environment Affects Beak Color in Captive Male House Sparrows Passer Domesticus

Abstract Black beak color in male House Sparrows (Passer domesticus) is a secondary sexual characteristic whose expression is directly correlated with testosterone levels. To experimentally determine if social environment affects black beak color and by implication testosterone levels, we housed wild-caught male sparrows either individually (n = 10), “Solo” males, or in the company of two other males (n = 4 “Group”s of 3 males), “Group” males, from 26 May – 7 July 2006. We predicted that “Solo” males would lose beak color faster than would “Group” males. We used digital images to monitor beak color once a week starting on 26 May. Beak blackness decreased more quickly and to a greater degree in “Solo” than in “Group” males. These results show that social environment in captivity affected beak color and suggest that male House Sparrows may require physical interactions to maintain breeding season levels of testosterone.