Data from: The hawk-eyed songbird: retinal morphology, eye shape, and visual fields of an aerial insectivore

Swallows are a unique group of songbirds because they are active-pursuit predators that execute all aspects of hunting prey in flight: search, detection, pursuit, and capture. We show that swallows have evolved a visual system that is unlike that of any other studied songbird. Swallows have a bifoveate retina that provides sharp lateral and frontal vision, an unusually long eye that enhances spatial resolution, a large posterior blind area, and a narrow binocular field. We also show that swallows and diurnal raptors (hawks and falcons) have converged on a similar visual configuration but that, interestingly, predatory songbirds that ambush prey (flycatchers) have not converged on the same suite of traits. Despite the commonly held belief that predators rely on binocular vision, the temporal (frontally projecting) fovea present in swallows—but not present in other songbirds—is likely not involved in binocular vision. Instead, swallows have four nonoverlapping foveae in a 100° arc around the beak, which can improve the tracking of frontally located aerial prey that are engaging in evasive maneuvers. Overall, vision in pursuit predators reflects the complex sensory demands of hunting in the air at high speeds and emphasizes the importance of acute frontal vision in predators.燕类是一类独特的鸣禽，因属于主动追击型捕食者，可在飞行中完成捕猎的全部环节：搜寻、侦测、追击与捕获猎物。我们的研究表明，燕类演化出了一套与所有已研究鸣禽均不相同的视觉系统：燕类拥有双中央凹视网膜（bifoveate retina），可提供清晰的侧方与前方视觉；其眼部异乎寻常地修长，可提升空间分辨率；同时存在较大的后部盲区，以及狭窄的双眼视觉视野。此外，我们还发现燕类与昼行性猛禽（鹰、隼）演化出了相似的视觉构型；但有趣的是，以伏击方式捕猎的鸣禽捕食者（霸鹟）并未演化出相同的一系列特征组合。尽管学界普遍认为捕食者依赖双眼视觉，但燕类拥有的颞侧（前方突出）中央凹（fovea）——其他鸣禽并无此结构——大概率并不参与双眼视觉活动。与之相反，燕类在喙部周围100°的弧度范围内拥有四个互不重叠的中央凹，这一结构可提升对位于前方、正在进行规避动作的飞行猎物的追踪效率。综上，主动追击型捕食者的视觉系统，反映了高速空中捕猎所带来的复杂感官需求，同时凸显了敏锐前方视觉在捕食者生存中的重要性。