Bartlow Andrew W, Jankowski Mark D, Hathcock Charles D, Ryti Randall T, Reneau Steven L, Fair Jeanne M
Los Alamos National Laboratory, Biosecurity and Public Health, Mailstop M888, Los Alamos, NM 87545, USA.
Laboratory Services and Applied Science Division, U.S. Environmental Protection Agency, 1200 6 Avenue, Suite 900, Seattle, WA 98101, USA.
Ibis (Lond 1859). 2021 Feb 5;163(3):977-989. doi: 10.1111/ibi.12935.
Mothers may produce more of one sex to maximize their fitness if there are differences in the cost of producing each sex or there are differences in their relative reproductive value. Breeding date and clutch size are known to influence offspring sex ratios in birds through sex differences in dispersal, social behaviours, differential mortality, and available food resources. We tested if breeding date, clutch size and drought conditions influenced offspring sex ratios in a sexually size-monomorphic species, the Western Bluebird, by interrogating a 21-year dataset. After controlling for differential mortality, we found that hatch dates late in the breeding season were associated with the production of more females, suggesting that the value of producing males declines as the breeding season progresses. When clutch size was taken into account, small clutches yielded significantly more females late in the breeding season compared to the early and middle parts of the breeding season that produced significantly more males. Large clutches early in the season tended to produce more females, although this was not significant. Drought severity was not correlated with sex ratio adjustment. We propose and discuss several explanations for these patterns, including male offspring, but not female offspring, acting as helpers, increased female nestling provisioning late in the breeding season, differences in food abundance, and egg-laying order. Future work will help to uncover the mechanisms leading to these patterns. Identifying patterns and mechanisms of sex ratio skew from long-term datasets is important for informing predictions regarding life-history trade-offs in wildlife populations.
如果生产每种性别的成本存在差异，或者它们的相对繁殖价值存在差异，母亲可能会生产更多的某一种性别以实现其适应性最大化。已知繁殖日期和窝卵数会通过扩散、社会行为、死亡率差异和可用食物资源方面的性别差异来影响鸟类的后代性别比例。我们通过分析一个长达21年的数据集，来测试繁殖日期、窝卵数和干旱条件是否会影响一种性别大小单态物种——西部蓝鸲的后代性别比例。在控制了死亡率差异之后，我们发现繁殖季节后期的孵化日期与更多雌性后代的产生有关，这表明随着繁殖季节的推进，生产雄性的价值会下降。当考虑窝卵数时，与繁殖季节早期和中期相比，繁殖季节后期小窝卵数产生的雌性显著更多，而早期和中期产生的雄性显著更多。季节早期的大窝卵数往往会产生更多雌性，尽管这并不显著。干旱严重程度与性别比例调整无关。我们提出并讨论了对这些模式的几种解释，包括雄性后代（而非雌性后代）充当帮手、繁殖季节后期增加对雌性雏鸟的育雏、食物丰度差异和产卵顺序。未来的研究将有助于揭示导致这些模式的机制。从长期数据集中识别性别比例偏差的模式和机制，对于为野生动物种群生活史权衡的预测提供信息非常重要。
