Abstract
Western Gull (Larus occidentalis) chicks on Santa Barbara Island, California, had a sex ratio at hatching of 1.12 M/F (n = 609); the sex ratio of chicks ≥35 days of age was 0.89 (n = 189). The sex ratio at hatching and fledging did not vary significantly from 1.0 or from each other, but the data suggest that male mortality before fledging exceeded that of females. Depressed growth rates of male chicks hatched third may be responsible for these higher male mortality rates. We found no evidence for seasonal or hatching-order effects on sex ratios at hatching. We suggest that postfledging differences in mortality between the sexes are in part responsible for the skewed sex ratio (0.67 males/female) observed in the adult breeding population.
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