Relative abundance of males to females affects behaviour, condition and immune function in a captive population of dark‐eyed juncos Junco hyemalis

Relative numbers of males and females in breeding groups may vary from expected values owing to a variety of factors. To determine how sex ratio might influence individual phenotypes in a captive population of dark‐eyed juncos Junco hyemalis during the breeding season, we established three treatment groups: a male‐biased (2:1), equal (1:1), and female‐biased group (1:2). Within‐group density (birds/m 2 ) was constant across groups. We assessed the frequency of flight chases (a proxy for social instability), measured changes in body mass and pectoral muscle condition, assayed plasma levels of testosterone (T) and compared cell‐mediated immunity of individuals. We found significantly more chases in the male‐biased group than in the female‐biased group. Birds in the male‐biased group lost more mass and displayed poorer pectoral‐muscle condition than birds in the equal group. Cell‐mediated immune responses were reduced in individuals in the male‐biased group in comparison to the female‐biased group. Plasma T levels in both sexes did not vary with sex ratio. Collectively, these results suggest that during the breeding season, social instability is greater in male‐biased populations, and instability may lead to decreased general health and vigour.