Data to estimate brood sex ratio at fledging (BSR, see equation S1 in Supporting Information). Variables: BROOD - brood ID, MALE - number of male chicks in the brood, FEMALE - number of female chicks in the brood, MAHATCH - hatching date (number of days since 1 March), AGE - brood age at capture. BSR was estimated at the middle of the four periods: 49, 72, 95, 117.5 days since 1 March.