the area that it occupies on the retinal surface, and on the number of receptors that connect to a single neuron and function as a single receptor. Density depends both on the size of an individual receptor, i.e. The number of functional receptors per unit area of retina – the receptor density – varies from one region of the retina to another. Although the spiral path is longer than the straight path, a mathematical model for an ‘ideal falcon’ shows that the falcon could reach the prey more quickly along the spiral path because the speed advantage of a straight head more than compensates for the longer path. Raptors could resolve this conflict by diving along a logarithmic spiral path with their head straight and one eye looking sideways at the prey, rather than following the straight path to the prey with their head turned sideways. Having their most acute vision towards the side causes a conflict in raptors such as falcons, which dive at prey from great distances at high speeds: at a speed of 70 m s −1, turning their head sideways to view the prey straight ahead with high visual acuity may increase aerodynamic drag by a factor of 2 or more and slow the raptor down. This dependence of head position on distance suggests that raptors use their more acute sideways vision to look at distant objects and sacrifice acuity for stereoscopic binocular vision to look at close objects. At distances of 40 m or more, raptors looked sideways at the object 80 % or more of the time. At a distances closer than 8 m, they spent more time looking at the object straight, but as the distance increased to 21 m, they spent more time looking at it sideways. The proportion of time that the raptors spent looking straight or sideways at an object depended on how far away the object was. The movements occurred approximately every 2 s on average in hawks and falcons, and approximately every 5 s in bald eagles. Since raptors do not rotate their eyes noticeably in the sockets, these movements presumably cause the image of the object to fall on the shallow and deep foveae. Several species of raptors in this study repeatedly moved their heads among three positions while looking at an object: straight, with the head axis pointing towards the object or sideways to the right or left, with the head axis pointing approximately 40 ° to the side of the object. The anatomy of the foveae suggests that the deep fovea has the higher acuity.
The line of sight of the deep fovea points forwards and approximately 45 ° to the right or left of the head axis, while that of the shallow fovea also points forwards but approximately 15 ° to the right or left of the head axis. Raptors – falcons, hawks and eagles in this study – have two regions of the retina in each eye that are specialized for acute vision: the deep fovea and the shallow fovea.
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