We can all appreciate the aerial superiority of a raptor – their flying agility is honed to a peak of refinement in order to hunt and catch their prey. What most people don’t realise is just how developed their senses have become in order for them to function in such spectacular ways. I am personally fascinated by a raptor’s structure and behaviour and in particular by its vision. We have all heard the phrase “eyes like a hawk”, but how well can they actually see, and do they even live in the same visual world as us?

American Bald Eagle showing third eyelid or 'nictitating membrane'.

Birds of prey have the most highly evolved eyesight of all living organisms. The eyes of a raptor are large, occupying about two thirds of the skull and almost touching each other inside the head. They are protected and supported by a ring of bony plates embedded around the rim of each eyeball and by a supra orbital ridge forming a tough bone and cartilage eyebrow. This ridge serves to shadow the eye from strong sunlight, and also gives a raptor its fierce appearance. As well as an upper and lower eyelid, which close to meet mid-eye, raptors have a third eyelid, or nictitating membrane. The membrane cleans, moistens, and protects the eye and the hawk seldom uses its other eyelids during the daytime. The third eyelids may also flicker constantly across the eye, or be closed on impact to protect the eye from its prey, sharp feathers or thorns.

Aerial view of raptor head showing range of vision

The retinal surface of a raptor eye is more tightly packed with sensory cells than any other vertebrate and especially with colour-sensitive cones in the two retinal fovea or focal points.

Harris Hawk eyes

They have a wider spectrum of colours than a human eye, and many can see into the ultraviolet range; plumage patterns that to us appear drab may appear quite colourful to birds. Kestrels can even detect areas of vole trails by the ultraviolet urine stains. In European Buzzards the retinal cones are packed more densely than in humans and they can probably resolve details eight times better than us. While we only have one part of the visual field in focus, buzzards have two focal spots or foveae in each eye and these are joined by a horizontal strip of extra sensitive area. One of these foveae, the central fovea, faces outward and is adapted for detecting small movements. When a raptor cocks its head slightly to examine the sky, it is using one of its central foveae. The other, the temporal fovea, faces forward and is best at resolving detail, in conjunction with binocular vision. The horizontal strip is used for scanning. Even outside the foveae a buzzard’s visual acuity is about twice ours. Most diurnal raptors have about 45-55 degrees of binocular vision and a total field for each eye of about 170 degrees. There is a blind spot behind the head of about 70 degrees.

Raptor vision versus human vision

Another important point where raptors are at advantage over us is their superior resolution of movement in relation to time. Their so-called flicker-fusion frequency (the time interval required to separate two visual impressions) is about 50% better than ours. This easiest way to explain flicker-fusion frequency (FFF) is to use the television as an example. The television doesn’t show a whole picture; it is just a fast-moving dot which tricks the human eye into thinking it is a picture. The television flickers 25 pictures a second which the human eye translates as one moving object because it can only see up to 20 events per second. For many raptors their FFF may be important during rapid manoeuvring in forest or judging changes in direction or speed of their prey. A dragonfly, for example has an FFF of up to 300 events per second with a high-speed brain and reactions to match. A Hobby trying to catch a dragonfly needs similar fast reactions otherwise it wouldn’t stand a chance.

Illustration credits:
Overhead diagram – Fox, N (1995) Understanding the Bird of Prey Surrey: Hancock House Publishers Ltd.
Raptor vs. human vision diagram – Newton, I (1990) Birds of Prey NSW: Golden Press Pty Ltd.