Seasonal shifts in sex ratios of fledgling American kestrels (Falco sparverius paulus): The Early Bird Hypothesis

We document a seasonal shift in the sex ratios of broods produced by reside nt southeastern American kestrels (Falco sparverius paulus) breeding in nes t boxes in Florida. Early in the breeding season, most biased broods were b iased towards males, whereas later in the season, most biased broods were b iased towards females. Computer-simulated broods subjected to sex-biased eg g and/or nestling mortality demonstrate that it is possible that differenti al mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipu lating the primary sex ratio of the broods. We present evidence that this s ex ratio shift is adaptive: for males we detected breeding as yearlings, al l had fledged early the previous season. No such relationship between seaso n and the probability of breeding as a yearling was found for females. We p ropose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition amo ng post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non -migratory population of the Eurasian kestrel (F. tinnunculus) and non-migr atory American kestrels breeding in Florida (F.s. paulus) exhibit this seas onal shift in sex ratios, whereas migratory American kestrels (F. s. sparve rius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.