The Early Bird Hypothesis predicts that males fledged early in the breeding season have an advantage over their later-fledged counterparts during competition for breeding sites. We tested this hypothesis by examining the sex ratio of 1,025 fledglings from 265 broods of American Kestrels (Falco sparverius) breeding in nest boxes in Wisconsin during the period 1968–1997. We found a seasonal shift in the sex ratio: the sex ratio of fledglings was biased toward males early in the breeding season, but became increasingly biased toward females as the season progressed. Our results provide support for the Early Bird Hypothesis and suggest that the steepness of this trend may decrease with increasing latitude.