Cooperative Fish and Wildlife Research Units Program: Pennsylvania
Education, Research and Technical Assistance for Managing Our Natural Resources

Sex ratios of adult birds are frequently skewed, but in few species is the sex ratio of offspring also skewed. Adult females, however, should modify their offspring’s sex ratio and invest more in the sex with the greatest future reproductive potential. We investigated the relationship between nestlings’ morphology, timing of reproduction, and sex ratios in broods of the Saltmarsh Sparrow (Ammodramus caudacutus), a species with extreme levels of multiple paternity: males provide no parental care and sex ratios of adults may be highly skewed. By molecular methods, we sexed 157 nestlings from 52 Saltmarsh Sparrow nests in Connecticut over 2 years. Their sex ratio was significantly skewed toward males (59%) and did not vary with brood size, but this imbalance is insufficient to fully explain the reported skew in adults’ sex ratio. Just prior to fledging, male nestlings were larger than female nestlings, which may suggest that they are more costly to produce. Survivorship is greater in nests initiated immediately after high spring tides, but we found no relationship between sex-ratio bias and the risk of tidal flooding. Female Saltmarsh Sparrows consistently produce male-skewed broods, regardless of their nests’ risk of flooding, suggesting that male nestlings are a consistently better investment. The Saltmarsh Sparrow appears to be an exception to the pattern typical of birds, in which the population of adults is skewed toward males but the sex ratio of offspring is balanced. These results may help to refine future age-specific population projections for this species of high conservation concern.