Matthew ch6 v23 (King James Bible)
Getting up close - eye-to-eye close - to a bird of prey is an unnerving and thrilling experience. Even in the field, through binoculars or a telescope, that unblinking stare fires back a warning. 'I am on a short, fierce fuse' it says, 'you have been warned.' With a few exceptions raptors lead a solitary, highly territorial life. Being eyeballed by one it is easy to understand why. They are definitely not for trifling with.
Having a large bird of prey perched on my wrist sends the adrenalin up another notch. A leather gauntlet feels barely adequate to protect against talons capable of generating enough force to crush bone. I try not to stare, for fear of provoking her, but there is a world behind those eyes that is incomprehensible and irresistible. It is like looking at a Mark Rothko painting; there is a yawning primitiveness that beckons me to step through the canvas.
|Golden Eagle. Her heavy brows protect her eyes against wind |
and dust, giving a characteristic penetrating stare.
The vision of raptors is legendary, proverbial. 'Eagle-eyed' we say, or 'hawk-eyed'. The udjat, or Eye of Horus the falcon god of the sky and hunting, was one of the most powerful symbols of the ancient Egyptians, the all-seeing eye worn on amulets as a source of royal protection. So just how well can a bird of prey see? It is a question that is almost impossible to answer without making comparisons with our own eyesight.
It is an unwritten law of nature that almost everything is a compromise. Things evolve to be just good enough to shorten the odds of survival, no more. Resources are directed at what is important, at the expense of things that are less so. Nature is not interested in perfection, only pragmatism. Take the golden eagle for example, what can we say about her? To start with she hunts during the daytime and from a great height. Even more than those murderous talons, eyesight is the sharpest weapon in her arsenal. The light receptors on her retinas are packed so densely that she has a resolution six to eight times greater than humans. She also perceives rapidity of movement that would leave us shrugging. A television screen appears solid enough to our slow eyes but try taking a photograph of it. In the eyes of a diurnal raptor it would flicker on and off distractingly, just as it does to the camera.
|High speed killer. Aerial strikes by a Saker Falcon at 200mph |
require pinpoint accuracy. The slightest error spells disaster.
So she has high resolution, high speed imaging, then where's the catch? The receptors in her eyes don't function well in low light. Visual performance tails off sharply at dawn and dusk, forcing her to roost. The receptors also lack the pigmented oil droplets found in many other birds, so her colour perception is mediocre at best and she cannot see in the ultraviolet spectrum like many other birds (or us). Her plumage provides a clue here; birds of prey are not brightly coloured as a family. Why waste energy on what they cannot see? Colour is unimportant when zeroing in for the kill, so colour discernment is sacrificed for what really matters - needlepoint precision.