Data from: Convergent evolution in social swallows (Aves: Hirundinidae)

Behavioral shifts can initiate morphological evolution by pushing lineages into new adaptive zones. This has primarily been examined in ecological behaviors, such as foraging, but social behaviors may also alter morphology. Swallows and martins (Hirundinidae) are aerial insectivores that exhibit a range of social behaviors, from solitary to colonial breeding and foraging. Using a well-resolved phylogenetic tree, a database of social behaviors, and morphological measurements, we ask how shifts from solitary to social breeding and foraging have affected morphological evolution in the Hirundinidae. Using a threshold model of discrete state evolution, we find that shifts in both breeding and foraging social behavior are common across the phylogeny of swallows. Solitary swallows have highly variable morphology, while social swallows show much less absolute variance in all morphological traits. Metrics of convergence based on both the trajectory of social lineages through morphospace and the overall morphological distance between social species scaled by their phylogenetic distance indicate strong convergence in social swallows, especially socially foraging swallows. Smaller physical traits generally observed in social species suggest that social species benefit from a distinctive flight style, likely increasing maneuverability and foraging success and reducing in-flight collisions within large flocks. These results highlight the importance of sociality in species evolution, a link that had previously been examined only in eusocial insects and primates.

行为转变可通过将演化支推入全新的适应性生态位，进而启动形态演化进程。此前此类研究多聚焦于觅食等生态行为，但社交行为同样可能改变物种形态。燕科（Hirundinidae）鸟类为食虫性空中捕食者，其社交行为跨度广泛，涵盖从单独繁殖、觅食到集群繁殖、觅食的全部类型。本研究借助高分辨率系统发育树、社交行为数据库与形态测量数据集，探究燕科鸟类从单独繁殖/觅食转向社交繁殖/觅食的行为转变对其形态演化造成的影响。借助离散状态演化阈值模型开展分析后，我们发现燕科系统发育树中，繁殖与觅食相关的社交行为转变均广泛存在。单独栖息的燕类形态变异程度极高，而社交性燕类的所有形态性状的绝对方差均显著更低。基于社交性演化支在形态空间中的演化轨迹，以及经系统发育距离标准化的社交性物种间整体形态距离所测算的趋同指标显示，社交性燕类存在显著的形态趋同现象，其中以社交觅食类燕类尤为突出。社交性燕类普遍具备更小巧的躯体性状，这表明它们得益于独特的飞行模式：该模式或可提升飞行机动性与觅食成功率，并减少大种群内的空中碰撞风险。本研究结果凸显了社交性在物种演化中的重要意义——此前该演化关联仅在真社会性昆虫与灵长类中得到过探究。