Avian trans-Saharan migrants travelling long distances and crossing ecological barriers experience different constraints in terms of time, energy and safety than short-/medium-distance migrants without barrier-crossings. As such, natural selection shapes the aerodynamic properties of these groups differently. Yet, to the best of our knowledge, we lack information on whether natural selection has contributed to reducing energetic flight costs through generally lower body mass in trans-Saharan migrants. To fill parts of this gap, we investigated this eco-morphological pattern in 5,410 individuals of 22 Palearctic songbird species ranging from short-/medium-distance to trans-Saharan migrants. We used individual size-independent scaled lean body mass values based on wing length as a measure of body size and, for the first time, precisely determined lean body mass values by direct measurements via quantitative magnetic resonance technology. Scaled lean body mass for a given body size was significantly higher in short-/medium-distance migrants than in trans-Saharan migrants. Although scaled lean body mass significantly decreased with increasing migration distance in short-/medium-distance migrants, no such effect was found in trans-Saharan migrants. Our results thus show an eco-morphological pattern relating species' lean body mass not only to migration distance but also to migration group. This suggests that selective effects of the presence/absence of ecological barriers and/or of a threshold level for migration distance on migrant birds may be more important than the linear continuum of migration distance per se.

中文翻译：

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鸟类轻量级：跨撒哈拉移民的瘦体重低于短/中距离移民。

与没有穿越障碍的短/中距离移民相比，长距离旅行和穿越生态障碍的跨撒哈拉鸟类在时间、能源和安全方面受到不同的限制。因此，自然选择不同地塑造了这些群体的空气动力学特性。然而，据我们所知，我们缺乏关于自然选择是否通过跨撒哈拉移民普遍较低的体重来降低精力充沛的飞行成本的信息。为了填补这一空白，我们在 22 种古北鸣禽物种的 5,410 只个体中研究了这种生态形态模式，范围从短/中距离到跨撒哈拉迁徙。我们使用基于翅膀长度的与个体尺寸无关的缩放瘦体重值作为体型的度量，并且第一次，通过定量磁共振技术直接测量精确确定瘦体重值。对于给定的体型，短/中距离移民的比例瘦体重明显高于跨撒哈拉移民。尽管在短/中距离移民中随着迁移距离的增加，缩放的瘦体重显着降低，但在跨撒哈拉移民中没有发现这种影响。因此，我们的结果显示了一种生态形态模式，将物种的瘦体重不仅与迁徙距离有关，而且与迁徙群体有关。这表明存在/不存在生态障碍和/或迁徙距离阈值水平对迁徙鸟类的选择性影响可能比迁徙距离的线性连续体本身更重要。