Data from: Contributions of feather microstructure to eider down insulation properties

Insulation is an essential component of nest structure that helps provide incubation requirements for birds. Many species of waterfowl breed in high latitudes where rapid heat loss can necessitate a high energetic input from parents and use down feathers to line their nests. Common eider Somateria mollissima nest down has exceptional insulating properties but the microstructural mechanisms behind the feather properties have not been thoroughly examined.

Here, we hypothesized that insulating properties of nest down are correlated to down feather (plumule) microstructure. We tested the thermal efficiency (fill power) and cohesion of plumules from nests of two Icelandic colonies of wild common eiders and compared them to properties of plumules of wild greylag goose Anser anser. We then used electron microscopy to examine the morphological basis of feather insulating properties. We found that greylag goose down has higher fill power (i.e. traps more air) but much lower cohesion (i.e. less prone to stick together) compared to common eider down. These differences were related to interspecific variation in feather microstructure. Down cohesion increased with the number of barbule microstructures (prongs) that create strong points of contact among feathers. Eider down feathers also had longer barbules than greylag goose down feathers, likely increasing their air-trapping capacity. Feather properties of these two species might reflect the demands of their contrasting evolutionary history. In greylag goose, a temperate, terrestrial species, plumule microstructure may optimize heat trapping. In common eiders, a diving duck that nests in arctic and subarctic waters, plumule structure may have evolved to maximize cohesion over thermal insulation, which would both reduce buoyancy during their foraging dives and enable nest down to withstand strong arctic winds.

羽绒保温层是鸟巢结构的关键组成部分，可为鸟类的孵化过程提供必要的温度保障。许多水禽物种在高纬度地区繁殖，此处热量流失速度极快，亲鸟需要投入大量能量维持体温，并借助羽绒羽毛铺设巢穴。普通绒鸭（Somateria mollissima）的巢用羽绒具备优异的保温性能，但其羽毛特性背后的微观结构机制尚未得到充分研究。

本研究假设巢用羽绒的保温性能与羽绒（plumule）的微观结构存在关联。我们对冰岛两个野生普通绒鸭种群的巢用羽绒的热效率（蓬松度，fill power）与粘结性进行了测试，并与野生灰雁（Anser anser）的羽绒特性进行对比。随后通过电子显微镜分析羽毛保温性能的形态学基础。我们发现，相较于普通绒鸭羽绒，灰雁羽绒的蓬松度更高（即空气截留能力更强），但粘结性显著更低（即更不易粘连成团）。上述差异与物种间羽毛微观结构的变异相关。羽绒的粘结性随羽小枝微观结构（prongs，即强化羽毛间接触点的结构）的数量增加而提升。普通绒鸭的羽绒羽小枝长度也长于灰雁，这可能进一步提升了其空气截留能力。这两个物种的羽毛特性或与其截然不同的演化历程相契合：作为温带陆地物种的灰雁，其羽绒微观结构可优化热量截留能力；而作为在北极及亚北极水域筑巢的潜水鸭类，普通绒鸭的羽绒结构则可能演化出相较于保温性能更优先的高粘结性，这既能够降低其觅食潜水时的浮力，同时也可使巢用羽绒抵御北极强风的侵袭。