Environmental- and parental condition-related variation in sex ratio of kestrel broods

Variation of brood sex ratio was studied in a Finnish population of Eurasia n Kestrels Falco tinnunculus breeding in an unpredictably variable environm ent. From those young that survived until 2-4 weeks of age, blood was colle cted and their sex determined from polymorphic DNA profiles produced by hyb ridisation with a human minisatellite probe. The sex ratio was male-biased during a year of food (vole) scarcity. Furthermore, in broods without morta lity, contrasting seasonal trends in sex ratios emerged. In this subsample, the proportion of males increased with later laying date during years of l ow and moderate food supply, whereas the opposite was true in a year of rel atively high food supply. These trends may indicate circumstances that Favo ur the raising of different sex. The proportion of males in the brood was a lso negatively correlated with body condition of both male and female paren ts, reflecting an adaptive condition-dependent sex-ratio adjustment, or alt ernatively the inability of the parents to meet the requirements of the mor e energetically expensive female offspring. We discuss the limitations that unpredictable conditions during brood raising can impose on adaptive sex-r atio manipulation, particularly in species with sexual size dimorphism and consequent differences in the cost of raising the two sexes.