Data from: Spatial distribution of nests constrains the strength of sexual selection in a warbler

In socially monogamous species, extra-pair paternity may increase the strength of inter-sexual selection by allowing males with preferred phenotypes to monopolize matings. Several studies have found relationships between male signals and extra-pair mating, but many others fail to explain variation in extra-pair mating success. A greater appreciation for the role that ecological contingencies play in structuring behavioral processes may help to reconcile contradictory results. We studied extra-pair mating in a spatial context in the common yellowthroat (Geothlypis trichas), a territorial wood warbler. Over the course of six years, we observed 158 breeding attempts by 99 males, resulting in a total of 369 nests and 520 sampled nestlings. The spatial distribution of territories varied greatly, with males having between 0 and 10 close neighbors and between 3 and 39 neighboring nestlings close enough to represent extra-pair siring opportunities. Both within-pair and extra-pair reproductive success increased with breeding density, but the opportunity for sexual selection and strength of selection varied with density. Total variance in reproductive success was highest at low density and was mostly explained by variation in within-pair success. In contrast, at high density, both within-pair and extra-pair success contributed substantially to variance in reproductive success. The relationships between plumage and extra-pair mating also varied by density; plumage was under strong sexual selection via extra-pair mating success at high density but no selection was detected at low density. Thus, ecological factors that structure social interactions can drive patterns of sexual selection by facilitating or constraining the expression of mating preferences.

在社会一夫一妻制（socially monogamous）物种中，额外配对父权（extra-pair paternity）可通过使携带偏好表型的雄性独占交配机会，增强性间选择的强度。已有多项研究发现雄性信号与额外配对交配存在关联，但诸多研究未能解释额外配对交配成功率的变异。若能更深入理解生态偶发因素在塑造行为过程中的作用，或有助于调和各项研究间的矛盾结论。本研究以具有领域性的林莺——普通黄喉地莺（Geothlypis trichas）为研究对象，在空间情境下探究其额外配对交配行为。六年的研究周期内，我们共记录到99只雄性个体的158次繁殖尝试，总计获得369个鸟巢以及520只待测雏鸟。领域的空间分布差异极大，雄性个体的近邻数量介于0至10只之间，同时存在3至39只邻近雏鸟，足以构成额外配对父权的潜在机会。配对内与额外配对的繁殖成功率均随繁殖密度上升而提高，但性选择机会与选择强度却随密度发生变化。繁殖成功率的总方差在低密度条件下最高，且主要由配对内繁殖成功率的变异所解释。与之相反，在高密度条件下，配对内与额外配对的繁殖成功率均对繁殖成功率的方差产生显著贡献。羽色与额外配对交配之间的关联同样随密度发生变化：在高密度条件下，羽色通过额外配对交配成功率受到强烈的性选择作用，但在低密度条件下未检测到此类选择。由此可见，塑造社会互动模式的生态因素，可通过促进或制约交配偏好的表达，进而调控性选择的作用模式。