Male-Skewed Sex Ratio in Saltmarsh Sparrow Nestlings

[摘要] Abstract. 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 initiate...

[学科分类] 动物科学

[关键词] Ammodramus caudacutus;costs of nestling care;nestling morphology;risk-return tradeoffs;secondary sex ratio;unbalanced populations;biased sex ratio.