Current avian migration patterns in temperate regions have been developed during the glacial retreat and subsequent colonization of the ice‐free areas during the Holocene. This process resulted in a geographic gradient of greater seasonality as latitude increased that favoured migration‐related morphological and physiological (co)adaptations. Most evidence of avian morphological adaptations to migration comes from the analysis of variation in the length and shape of the wings, but the existence of intra‐feather structural adjustments has been greatly overlooked despite their potential to be under natural selection. To shed some light on this question, we used data from European robins Erithacus rubecula overwintering in Campo de Gibraltar (Southern Iberia), where sedentary robins coexist during winter with conspecifics showing a broad range of breeding origins and, hence, migration distances. We explicitly explored how wing length and shape, as well as several functional (bending stiffness), developmental (feather growth rate) and structural (size and complexity of feather components) characteristics of flight feathers, varied in relation to migration distance, which was estimated from the hydrogen stable isotope ratios of the summer‐produced tail feathers. Our results revealed that migration distance not only favoured longer and more concave wings, but also promoted primaries with a thicker dorsoventral rachis and shorter barb lengths, which, in turn, conferred more bending stiffness to these feathers. We suggest that these intra‐feather structural adjustments could be an additional, largely unnoticed, adaptation within the avian migratory syndrome that might have the potential to evolve relatively quickly to facilitate the occupation of seasonal environments.

中文翻译：

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古北雀雀飞羽迁移的机械和结构适应

目前温带地区的鸟类迁徙模式是在全新世冰川退缩和随后无冰地区的殖民过程中发展起来的。随着纬度的增加，这一过程导致更大的季节性地理梯度，这有利于与迁移相关的形态和生理（共）适应。大多数鸟类形态适应迁徙的证据来自对翅膀长度和形状变化的分析，但尽管它们有可能受到自然选择的影响，但羽毛内部结构调整的存在却被大大忽视了。为了阐明这个问题，我们使用了在坎波德直布罗陀（伊比利亚南部）越冬的欧洲知更鸟 Erithacus rubecula 的数据，久坐的知更鸟在冬天与同种共存，显示出广泛的繁殖起源，因此，迁徙距离。我们明确地探索了飞行羽毛的翅膀长度和形状，以及几个功能（弯曲刚度）、发育（羽毛生长率）和结构（羽毛成分的大小和复杂性）特征，是如何随着迁移距离而变化的，这是估计的来自夏季产生的尾羽的氢稳定同位素比率。我们的结果表明，迁移距离不仅有利于更长和更凹的翅膀，而且还促进了具有更厚的背腹轴和更短的倒钩长度的初级羽毛，这反过来又赋予了这些羽毛更多的弯曲刚度。我们建议这些羽内结构调整可能是额外的，