Anomalous binocular vision in African Harrier-Hawks

An animal's visual field is the three-dimensional space around its head from which it can extract visual information at any instant¹. Bird visual fields vary markedly between species, and this variation is likely to be driven primarily by foraging ecology^1,2,3. The binocular visual field is the region in which the visual fields of the two eyes overlap; thus, objects in the binocular field are imaged by both eyes simultaneously. The binocular field plays a pivotal role in the detection of symmetrical optic flow-fields, providing almost instantaneous information on the direction of travel and the time to contact a target towards which the head or feet is travelling; thus, information from the binocular field is crucial in guiding key foraging behaviours^2,3. Here, we demonstrate an unusual visual field and binocular extent above the head in African Harrier-Hawks, also known as Gymnogenes (Polyboroides typus) compared to 18 other members of the Accipitridae^4,5. We argue that the observed visual field can be attributed to the unusual and specific foraging behaviour of African Harrier-Hawks.