A bird’s eye view — but not in the way you think
Looking down from a hot air balloon gives a panoramic vista often referred to as “a bird’s eye view” but that ‘view’ is not the subject of this article which is more about eagle-eyed vision.
For many years I was under the misapprehension that 20/20 vision meant perfect vision and only recently discovered that it simply implies ‘normal’ human vision. A person with 20/20 vision means that they can read the same line of characters on a Snellen chart, the one you see on the optician’s wall, at 20 metres as a normal human being can at the same distance. Most animals don’t come close to the 20/20 visual range. Dogs or cats, for example, can only see clearly from 20 feet what a human with otherwise normal vision can see from approximately 100 feet or 100/20 vision. (Contrary to widespread belief, dogs are not colour blind but they only register blue and yellow while the other colours are perceived as shades of grey.)
All birds of prey have excellent long-distance vision, but eagles stand out. They can see clearly about eight times as far as humans can, allowing them to spot and focus in on a rabbit or other animal at a distance of about two miles. This amazing visual acuity is the result of a combination of several factors — eye size, dimensions of the parts inside the eye, and the numbers of photoreceptor cells located inside the retina are all factors that strengthen visual acuity.
The eye of a golden eagle weighing 14 pounds is roughly the same size as that of a man weighing 200 pounds and the positioning of that eye in the head is quite different. Forward looking humans have an arc of vision of about 180 degrees. Eagles’ eyes are angled at 30 degrees away from the midline of the face, so they have a 340-degree field of vision. As the eagle descends from the sky to attack its prey, the muscles in the eyes continuously adjust the curvature of the eyeballs to maintain sharp focus and accurate perception throughout the approach and attack. They have better peripheral vision than an owl, but not quite as well as that of a woodcock, which has an impressive 360-degree field of view. The owl, being often a nocturnal hunter, relies even more on its hearing than its sight.
Cats have 285 degrees of vision while dogs have 250 degrees of lateral sight which is ideal for hunting and spotting predators in the wild. Horses and zebras have eyes on the side of their heads, which means that they can see predators behind them … but they also have a blind spot in right in front of them!
In fact, most animals have better peripheral vision than humans!
The bird’s retina is also much more complex than our own. Both species have retinas containing two types of photoreceptor cells — rods and cones. The rods detect shades of black and white at low light levels while the cones are responsible for colour vision. Human cone cells can detect three different colours — red, blue and green — to which many birds’ retinal cells add purple and ultraviolet light. Many small animal species that form the prey of these avian raptors mark their trail with drops of ultraviolet urine making their tracking and capture easy with UV vision. Their fur also reflects ultraviolet light making them easier to see in the deep grass.
It is not simply the characters of the photoreceptor cells but their density within the centre of the retina, the fovea, which makes such a difference. The small kestrel has 65,000 rods and cones per square millimetre of retina compared to the rather paltry 38,000 in humans while the Common Buzzard often has up to one million cones per square millimetre. Not much ‘common’ about that.
It is not just animal fur that reflects UV light. Many species of birds were first thought to be monomorphic meaning that there is little or no difference in the colouring of the sexes of the same species. The development of ultra-spectrometry has since proven that, under UV light, the plumage of apparently identical birds can be spectacularly different.
This not only applies to plumage but allows birds’ eyes to pick out food and even their own eggs even if camouflaged from the human eye. Many ground-nesting birds, such as plovers, rely on such camouflage to protect their clutch from egg thieves.
But let us return to the eagle. The anatomy of its retina has adapted to predatory habits. We humans have but one fovea — a small central concentration of cone photoreceptor cells — which gives the greatest sharpness of our vision — while the eagle has developed a second fovea. This temporal fovea can detect slow movement of prey while the central fovea focusses down to the object itself.
The eagles are certainly magnificent birds but even the humble gecko, found on Mediterranean villa walls, enjoys colour vision that is actually around 350 times better than humans, allowing them to spot their hidden prey more easily! Even the ubiquitous Trafalgar Square pigeons have better colour vision than most animals since they also see UV light. In fact, their vision is so good that they have been used to assist in Search and Rescue missions at sea.
To finish on one more bizarre fact concerning the eagle’s eye. Young fish eagles are unable to see fish below the surface of the water and rely on finding dead fish on the surface because the light refracted off the surface prevents the sight of fish below. But practice makes perfect and young eagles slowly discover that, by altering their angle of attack closer to the vertical, they can reduce this refraction to allow the sight of their prey beneath the water surface.
Clever birds, eagles.