Data from: Sons do not take advantage of a head start: parity in herring gull offspring sex ratios despite greater initial investment in males

Skewed adult sex ratios sometimes occur in populations of free-living animals yet the proximate mechanisms, timing of sex-biases, and the selective agents contributing to skew remain a source of debate with contradictory evidence from different systems. We investigated potential mechanisms contributing to sex biases in a population of herring gulls with an apparent female skew in the adult population. Theory predicts that skewed adult sex ratios will adaptively lead to skewed offspring sex ratios to restore balance in the effective breeding population. Parents may also adaptively bias offspring sex ratios to increase their own fitness in response to environmental factors. Therefore, we expected to detect skewed sex ratios either at hatching or at fledging as parents invest differentially in offspring of different sexes. We sampled complete clutches (n = 336 chicks) at hatching to quantify potential skews in sex ratios by position in the hatch order, time of season, year, and nesting context (nest density), finding no departure from equal sex ratios at hatching related to any of these factors. Further, we sampled 258 chicks at near-fledging to investigate potential sex biases in survival at the chick stage. Again, no biases in sex ratios were recorded. Male offspring were favored in this population via greater maternal investment in eggs carrying male embryos and greater parental provisioning of male offspring which reached greater sizes by fledging. Despite the advantages realized by male offspring, females were equally as likely to fledge as males. Thus, biased adult sex ratios apparently arise in the post-fledging and pre-recruitment stage in our population.自由生活的野生动物种群有时会出现成年个体性比偏倚的现象，但导致该偏倚的近因机制、性偏倚的发生时序，以及相关选择因子，始终是学界争论的热点，不同研究体系间的相关证据亦存在矛盾。本研究针对一个成年种群呈现雌性偏倚的银鸥（herring gulls）种群，探究了可能导致性偏倚的潜在机制。现有理论预测，成年性比偏倚会通过适应性调整产生后代性比偏倚，以恢复有效繁殖种群的性比平衡。此外，亲本也可能根据环境因素适应性地调整后代性比，以提升自身的适合度。因此，我们推测在孵化期或离巢期，亲本对不同性别的后代存在差异化投资，进而可检测到性比偏倚。我们对孵化期的完整产卵窝（共336只雏鸟）进行了采样，以量化孵化序位、季节时段、年份以及巢境（巢密度）对性比偏倚的影响，结果未发现上述任一因素导致孵化期性比偏离1:1的均等比例。进一步，我们在雏鸟接近离巢时采样了258只个体，以探究雏鸟阶段存活过程中可能存在的性偏倚，本次采样同样未记录到性比偏倚现象。该种群的雄性后代具备发育优势：携带雄性胚胎的卵会获得更多的母本投资，且亲代对雄性后代的育雏投入更高，使得雄性后代在离巢时体型更大。尽管雄性后代具备上述生存优势，但雌性后代与雄性后代的离巢概率并无差异。由此可见，本研究种群的成年性比偏倚，显然产生于离巢后至种群补充前的阶段。