page_title: Bird vision
section_title: Diurnal birds of prey
hierarchical_section_title: Bird vision / Variations across bird groups / Diurnal birds of prey
caption_attribution_description: Closeup of the eye of a Red-tailed Hawk (Buteo jamaicensis). On an exhausted juvenile, near Whalers Cove, Pescadero, California.
context_page_description: Vision is the most important sense for birds, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". The avian eye resembles that of a reptile, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.
  Some bird groups have specific modifications to their visual system linked to their way of life. Birds of prey have a very high density of receptors and other adaptations that maximise visual acuity. The placement of their eyes gives them good binocular vision enabling accurate judgement of distances.
context_section_description: The visual ability of birds of prey is legendary, and the keenness of their eyesight is due to a variety of factors. Raptors have large eyes for their size, 1.4 times greater than the average for birds of the same weight, and the eye is tube-shaped to produce a larger retinal image. The resolving power of an eye depends both on the optics, large eyes with large apertures suffers less from diffraction and can have larger retinal images due to a long focal length, and on the density of receptor spacing. The retina has a large number of receptors per square millimeter, which determines the degree of visual acuity. The more receptors an animal has, the higher its ability to distinguish individual objects at a distance, especially when, as in raptors, each receptor is typically attached to a single ganglion. Many raptors have foveas with far more rods and cones than the human fovea (65,000/mm² in American kestrel, 38,000 in humans) and this provides these birds with spectacular long distance vision. It is proposed that the shape of the deep central fovea of raptors can create a telephoto optical system, increasing the size of the retinal image in the fovea and thereby increasing the spatial resolution. Behavioural studies show that some large eyed raptors (Wedge-tailed eagle, Old world vultures) have a 2 times higher spatial resolution than humans, but many medium and small sized raptors have comparable or lower spatial resolution.
  The forward-facing eyes of a bird of prey give binocular vision, which is assisted by a double fovea. The raptor's adaptations for optimum visual resolution (an American kestrel can see a 2–mm insect from the top of an 18–m tree) has a disadvantage in that its vision is poor in low light level, and it must roost at night. Raptors may have to pursue mobile prey in the lower part of their visual field, and therefore do not have the lower field myopia adaptation demonstrated by many other birds. Scavenging birds like vultures do not need such sharp vision, so a condor has only a single fovea with about 35,000 receptors mm². Vultures, however have high physiological activity of many important enzymes to suit their distant clarity of vision. Southern Caracara also only have a single fovea as this species forages on the ground for carrion and insects. However, they do have a higher degree of binocular overlap than other falcons, potentially to enable the caracara to manipulate objects, such as rocks, whilst foraging.
  Like other birds investigated raptors do also have coloured oil droplets in their cones. The generally brown, grey and white plumage of this group, and the absence of colour displays in courtship suggests that colour is relatively unimportant to these birds.
  In most raptors a prominent eye ridge and its feathers extends above and in front of the eye. This "eyebrow" gives birds of prey their distinctive stare. The ridge physically protects the eye from wind, dust, and debris and shields it from excessive glare. The osprey lacks this ridge, although the arrangement of the feathers above its eyes serves a similar function; it also possesses dark feathers in front of the eye which probably serve to reduce the glare from the water surface when the bird is hunting for its staple diet of fish.
caption: "Hawk-eyed" is a byword for visual acuity
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