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 instant. Bird visual fields vary markedly between species, and this variation is likely to be driven primarily by foraging ecology. 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. 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. We argue that the observed visual field can be attributed to the unusual and specific foraging behaviour of African Harrier-Hawks.