Vestibular gaze stabilization: different behavioral strategies for arboreal and terrestrial avians

In birds, it is thought that head movements play a major role in the reflexive stabilization of gaze and vision. In this study, we investigated the contributions of the eye and head to gaze stabilization during rotations under both head-fixed [vestibuloocular (VOR)] and head-free conditions in two avian species: pigeons and quails. These two species differ both in ocular anatomy (the pigeon has 2 distinct foveal regions), as well as in behavioral repertoires. Pigeons are arboreal, fly extended distances, and can navigate. Quails are primarily engrossed in terrestrial niches and fly only short distances. Unlike the head-fixed VOR gains that were under-compensatory for both species, gaze gains under head-free conditions were completely compensatory at high frequencies. This compensation was achieved primarily with head movements in pigeons, but with combined head and eye-in-head contributions in the quail. In contrast, eye-in-head motion, which was significantly reduced for head-free compared with head-fixed conditions, contributed very little to overall gaze stability in pigeons. These results suggest that disparity between the stabilization strategies employed by these two birds may be attributed to differences in species-specific behavior and anatomy.