Pigeons make slow, divergent eye movements during flight and large, convergent eye movements when landing

As animals move through the world, images of surfaces and edges in the environment move across the retina, a visual signal known as optic flow.¹ Optic flow is beneficial for several processes, perhaps most notably navigation,² but it poses a conundrum. When faced with optic flow, the optomotor body, head, and eye movements reflexively minimize optic flow to maintain a stable retinal image.³ This retinal image stabilization is essential for normal visual function.^4,5 How then can an animal move through the world yet maintain a stable retinal image? Birds are known for their remarkable head stability and also exhibit a variety of eye movements, including rapid and stabilizing vestibular and optokinetic eye movements.^6,7 However, it remains unknown if they move their eyes during flight.^8,9 We developed an onboard camera system to separately track eye movements and record optic flow from homing pigeons (Columba livia domestica; hereafter pigeons) released several kilometers from their home loft. During flight, the eyes of pigeons exhibit slow, temporal eye movements consistent with a divergent optokinetic response. These eye movements matched optic flow along the horizon generated by forward flight. Head-restrained pigeons exhibited binocular divergent eye movements in response to computer-generated visual stimuli simulating self-translation. These results are consistent with pigeons stabilizing the horizon in the lateral visual fields during flight. By contrast, during landing, pigeons exhibited large, convergent binocular eye movements directed toward the perch, allowing for stereopsis. In summary, both optokinetic and convergent eye movements appear important for avian flight control.