We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition among 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-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.