Eye size across avian lineages covaries with participation in a specialized foraging behaviour.

Foraging ecology and visual ability are often strongly related across animal lineages, as many organisms identify food sources by sight. Birds particularly rely upon vision to seek out prey or other food items, leading to the correlated evolution of eye size and foraging behaviour. Here, we focus on a specialized foraging tactic termed "disturbance foraging," whereby a responding species exploits prey items flushed by a disturbing species. Using global databases of disturbance-responder species and eye size measurements from museum specimens, we tested the prediction that relative eye size accounting for body mass allometry (a proxy for visual acuity and sensitivity) would be larger in disturbance foragers that require enhanced visual performance to locate escaping prey (N = 463) compared to other species (N = 2,840). As predicted, disturbance foragers possessed larger relative eye sizes. Residual eye size was correlated with a gradient in avian foraging behaviour, such that species with the smallest and largest relative eye sizes were near-sighted and far-sighted non-disturbance foragers, respectively, while disturbance foragers had intermediate eye sizes. Birds appeared to invest similarly in acuity and sensitivity in relation to foraging behaviour as measured by their respective anatomical proxies (residual axial length [AL] and cornea diameter [CD]), although there was partial evidence that some species groups invested more in acuity based upon the eye shape ratio (CD/AL). These patterns imply that even highly specialized behavioural tactics may evolve in concert with their respectively linked neurological and sensory systems.