Sex ratio of Western Bluebirds Sialia mexicana is mediated by phenology and clutch size

Mothers may produce more of one sex to maximize their fitness if there are differences in the cost of producing each sex or there are differences in their relative reproductive value. We tested if breeding date, clutch size and drought conditions influenced offspring sex ratios in Western Bluebirds (Sialia mexicana) from 1997 to 2017. We found that hatch dates late in the breeding season were associated with the production of more females. When clutch size was taken into account, small clutches yielded significantly more females late in the breeding season compared to the early and middle parts of the breeding season that produced significantly more males. Large clutches early in the season tended to produce more females, although this was not significant. Drought severity was not correlated with sex ratio adjustment.