Sex-Specific Recruitment and Brood Sex Ratios of Eurasian Kestrels in a Seasonally and Annually Fluctuating Northern Environment

Timing of birth and food availability may select for biased offspring sex ratios when they differentially affect the reproductive value of male and female young. Here we show that early hatching date enhances more the probability of male Eurasian kestrels (Falco tinnunculus) to breed as one-year-old than that of females in a Finnish population. This rarely documented phenomenon has been previously observed in a kestrel population in the Netherlands. As kestrels in the Finnish population are migratory, our results refute the hypothesis that early-fledged males would have an advantage for early breeding only in resident populations. Contrary to the predictions, the Finnish population showed no change in brood sex ratio during the breeding season in a long-term data from 8years. As far as we know, this is the first demonstration that biased sex allocation may not occur even when it would appear to be adaptive. This result is different from the Dutch kestrel population, in which the season began with a bias towards males and ended with a bias in favour of females. We suggest that high inter-annual variation in food abundance in Finland might reduce selection for a sex ratio trend.