Data from: Interspecific variation in the structural properties of flight feathers in birds indicates adaptation to flight requirements and habitat

1. The functional significance of intra- and interspecific structural variations in the flight feathers of birds is poorly understood. Here, a phylogenetic comparative analysis of four structural features (rachis width, barb and barbule density and porosity) of proximal and distal primary feathers of 137 European bird species was conducted. 2. Flight type (flapping and soaring, flapping and gliding, continuous flapping or passerine type), habitat (terrestrial, riparian or aquatic), wing characteristics (wing area, S and aspect ratio, AR) and moult strategy were all found to affect feather structure to some extent. Species characterized by low wing-beat frequency flight (soaring and gliding) have broader feather rachises (shafts) and feather vanes with lower barb density than birds associated with more active flapping modes of flight. However, the effect of flying mode on rachis width disappeared after controlling for S and AR, suggesting that rachis width is primarily determined by wing morphology. 3. Rachis width and feather vane density are likely related to differences in force distribution across the wingspan during different flight modes. An increase in shaft diameter, barb density and porosity from the proximal to distal wing feathers was found and was highest in species with flapping flight indicating that aerodynamic forces are more biased towards the distal feathers in flapping flyers than in soarers and gliders. 4. Habitat affected barb and barbule density, which was greatest in aquatic species, and within this group, barb density was greater in divers than non-divers, suggesting that the need for water repellency and resistance to water penetration may influence feather structure. However, we found little support for the importance of porosity in water repellency and water penetration, because porosity was similar in aquatic, riparian and terrestrial species and among the aquatic birds (divers and non-divers). We also found that barb density was affected by moult pattern. 5. Our results have broad implications for the understanding of the selection pressures driving flight feather functional morphology. Specifically, the large sample size relative to any previous studies has emphasized that the morphology of flight feathers is the result of a suite of selection pressures. As well as routine flight needs, constraints during moulting, habitat (particularly aquatic) and migratory requirements also affect flight feather morphology. Identifying the exact nature of these trade-offs will perhaps inform the reconstruction of the flying modes of extinct birds.

1. 鸟类飞羽的种内与种间结构变异的功能意义迄今仍鲜有研究。本研究针对137种欧洲鸟类的近端与远端初级飞羽（primary feathers）的四项结构特征——羽轴（rachis）宽度、羽枝（barb）与羽小枝（barbule）密度以及孔隙度（porosity）——开展了系统发育比较分析。
2. 研究发现，飞行类型（振翅翱翔、振翅滑翔、持续振翅或雀形目飞行模式）、栖息生境（陆生、河岸生或水生）、翼部特征（翼面积S与展弦比（aspect ratio，AR））以及换羽策略均会在不同程度上影响飞羽结构。振翅频率较低的飞行类群（如翱翔与滑翔类），其羽轴更宽，且羽片的羽枝密度低于更依赖主动振翅飞行的鸟类。不过，在控制翼面积与展弦比变量后，飞行模式对羽轴宽度的影响不再显著，这表明羽轴宽度主要由翼形态决定。
3. 羽轴宽度与羽片密度或与不同飞行模式下翼展的力分布差异相关。研究发现，从近端翼羽到远端翼羽，羽轴直径、羽枝密度与孔隙度均呈递增趋势，且该趋势在振翅飞行类群中最为显著，这表明相较于翱翔与滑翔类鸟类，振翅飞行鸟类的空气动力更偏向于远端翼羽。
4. 栖息生境会影响羽枝与羽小枝密度，水生鸟类的该两项指标最高；其中潜水鸟类的羽枝密度高于非潜水水生鸟类，这表明鸟类对疏水性与抗水渗透能力的需求可能会影响飞羽结构。然而，我们并未发现孔隙度在疏水性与抗水渗透中发挥重要作用的证据，因为水生、河岸生与陆生鸟类的孔隙度并无显著差异，且水生鸟类（潜水与非潜水类群）间的孔隙度也无明显区别。此外，羽枝密度还会受到换羽模式的影响。
5. 本研究结果对于理解驱动飞羽功能形态演化的选择压力具有广泛意义。具体而言，相较于此前所有相关研究，本研究的样本量更大，这进一步证实了飞羽形态是一系列选择压力共同作用的结果。除常规飞行需求外，换羽期的约束、栖息生境（尤其是水生生境）以及迁徙需求同样会影响飞羽形态。明确这些权衡取舍的具体机制，或可为重建已灭绝鸟类的飞行模式提供参考。