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. 

Sex_Ratio_WEBLs.csv: Sex ratios of individual Western Bluebird nestlings from 1997 to 2017. 

Columns:

Year: Year data was collected

Nestling ID: Individual identification number of each nestling

Nest Box: Nest box in which nestlings were raised

Elevation (ft): Elevation of the nest box in feet

Brood Clutch Size: Size of the brood within a nest box

Hatch Date: Julian date that the first egg hatched in a brood

Sex: Sex of each nestling (1 = male, 0 = female)

Watershed: Watershed where each box was located

Canyon: Canyon where each box was located