Seeing danger? Visual constraints and collision risk in the Bearded Vulture Gypaetus barbatus

Abstract

The Bearded Vulture, Gypaetus barbatus is highly vulnerable to collisions with human-made aerial infrastructures. Yet, its visual sensory system remains poorly understood, limiting the development of species-specific collision mitigation measures. To address this gap, we characterized its visual ecology across four complementary dimensions: visual fields, spatial resolution, contrast sensitivity and spectral sensitivity of cone visual rhodopsins. Bearded vultures have no blind area in the direction of travel during foraging flight, indicating that frontal visual-field limitations are unlikely to contribute to their risk of collisions. Their axial length predicts high spatial visual resolution, suggesting that insufficient spatial resolution is unlikely to be a primary cause of collisions. By contrast, Bearded Vultures display very low peak contrast sensitivity compared to other birds, which may limit their ability to detect objects when contrast with the background, or within the objects themselves, is low. Functional characterization of the four cone visual pigments indicated sensitivity spanning approximately 350-640 nm, with a potential chromatic-discrimination range from 400 to 602 nm. Together, these results identify that contrast sensitivity, rather than visual-field configuration or spatial resolution, is a key perceptual constraint to consider in collision mitigation of the Bearded Vulture. Visual markers should therefore maximize both internal contrast and brightness contrast against the surrounding environment. More broadly, our multidisciplinary approach highlights how birds differ substantially in how they perceive anthropogenic structures and illustrates the importance of incorporating species-specific sensory ecology into conservation efforts, especially in the design of collision-mitigation strategies.