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On the Park - April 2006
RAPTOR VISION 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?
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.
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.
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:
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