Adaptive Seasonal Variation in the Sex Ratio of Kestrel Broods

In contrast with the situation in mammals, sex ratio variations of offspring in birds have rarely been documented, and never been shown to be adaptively tuned to systematic differences in the prospects for daughters and sons. The sex ratio (% males) in broods of European kestrels, Falco tInnunculus L., declined with progressive date of birth. This decline enhanced reproductive prospects of the broods since the probability of breeding as yearling declined with birth date for male offspring, but not for females. The sex ratio bias of the brood was produced by non-random sex segregation at meiosis: by altering the within-clutch sequence of sexes (first male then female eggs in early clutches, the reverse in late clutches) laying kestrels assigned the sex with the better long-term reproductive prospects to the initial eggs of their clutch, which suffer least mortality in the nest.