Data from: Sex-specific winter distribution in a sexually dimorphic shorebird is explained by resource partitioning

Sexual size dimorphism (SSD) implies correlated differences in energetic requirements and feeding opportunities, such that sexes will face different trade-offs in habitat selection. In seasonal migrants, this could result in a differential spatial distribution across the wintering range. To identify the ecological causes of sexual spatial segregation, we studied a sexually dimorphic shorebird, the bar-tailed godwit Limosa lapponica, in which females have a larger body and a longer bill than males. With respect to the trade-offs that these migratory shorebirds experience in their choice of wintering area, northern and colder wintering sites have the benefit of being closer to the Arctic breeding grounds. According to Bergmann's rule, the larger females should incur lower energetic costs per unit of body mass over males, helping them to winter in the cold. However, as the sexes have rather different bill lengths, differences in sex-specific wintering sites could also be due to the vertical distribution of their buried prey, that is, resource partitioning. Here, in a comparison between six main intertidal wintering areas across the entire winter range of the lapponica subspecies in northwest Europe, we show that the percentage of females between sites was not correlated with the cost of wintering, but was positively correlated with the biomass in the bottom layer and negatively with the biomass in the top layer. We conclude that resource partitioning, rather than relative expenditure advantages, best explains the differential spatial distribution of male and female bar-tailed godwits across northwest Europe.

性大小二态性（Sexual size dimorphism, SSD）指两性间能量需求与觅食机会存在相关性差异，致使二者在栖息地选择上面临不同的权衡策略。对于季节性迁徙物种而言，该特征可能导致其在越冬范围内呈现空间分布分化。为阐明性空间隔离的生态学成因，本研究以具有性大小二态性的鸻鹬类鸟类——斑尾塍鹬（Limosa lapponica）为研究对象，该物种雌性的体型与喙长均大于雄性。迁徙鸻鹬类在越冬地选择上存在权衡：纬度更靠北、气候更寒冷的越冬位点，相较于北极繁殖地的距离更近，具备这一区位优势。根据贝格曼法则，体型更大的雌性个体单位体质量的能量消耗低于雄性，使其能够在寒冷环境中越冬。然而，由于两性喙长差异显著，二者越冬位点的分化也可能与其地下猎物的垂直分布有关，即资源分隔。本研究针对欧洲西北部斑尾塍鹬（Limosa lapponica）lapponica亚种的全部越冬分布区，对6处主要潮间带越冬栖息地开展对比分析，结果显示：不同位点的雌性占比与越冬成本无显著相关性，但与底层沉积物中的猎物生物量呈正相关，与表层沉积物中的猎物生物量呈负相关。综上，相较于相对能量消耗优势，资源分隔更能解释欧洲西北部斑尾塍鹬雌雄个体间的空间分布分化。