What is it that makes an eagle’s vision so impressive?
Think of your five primary senses: sight, hearing, taste, smell and touch. All are important, but you’d probably agree that none are more important than sight. Well the same goes for birds of prey, like eagles and hawks. Humans and birds of prey are probably the two animals on the planet that rely the most on their vision. So we thought we’d compare both to see if human vision is any match for eagles.
Clarity is key
Visual acuity is the ability to pick out details of an object separately and without blur. Basically, it’s how clear an object appears. Several characteristics contribute to good visual acuity, such as the overall size of the eye, the dimensions of the parts inside and the number of receptors on the retina. Humans do pretty well when it comes to visual acuity. As you probably already know, “normal” vision for a human is 20/20. Dogs and cats see around 20/100, which means what a dog or cat can see clearly from 20 feet, a person with normal vision could see from 100 feet.
Eagles and other birds of prey are the kings of the animal kingdom when it comes to visual acuity. Their retinas are not only packed with light-detecting cells called cones, but they also have a much deeper fovea—a cone-rich structure in the back of the eye—which acts like a telephoto lens on a camera. These characteristics are believed to give eagles visual acuity of 20/5, or 20/4; an important characteristic for a high flying predator in search of mice hundreds of feet below.
How do you measure an eagle’s vision anyway?
An optometrist’s alphabet eye chart is obviously of no use. One common scientific setup involves training the birds to fly down a long tunnel with two TV screens at the end. One screen displays a striped pattern, and the birds get a treat when they land on it. Scientists test the bird’s acuity by changing the width of the stripes and determining from what distance the eagles begin to veer in the correct direction. Voila!
There’s no need for HD TV
We humans seem to think that we live in a bright, colourful world. But if we saw colours the same way an eagle sees them then we would see the world with even more brilliance; with bright colours rendered in an unbelievable array of shades.
Eagles can see colours more vividly than we do; they can pick out more shades and can even see ultraviolet light. This ability to see UV light possibly gives them an ability to spot the traces left by prey. The urine of mice and other small prey is visible in the ultraviolet range, so they would stand out against the uniform colour of a field.
Eyes behind your head
We have eyes positioned at the front of our head. This gives humans binocular vision, so with the exception of the images on the periphery of our vision, we’re always seeing and looking at things with both eyes (close one eye at a time, and you’ll notice what portion of your vision is being seen with just one eye). Using both eyes gives us the ability to see how far an object is (depth of field) and how fast something is moving. The downside to this is we have poor peripheral vision—a human can only see 180 degrees.
Eagles are a predatory bird, and therefore its eyes are rotated towards the front of the head. But their eyes are still angled 30 degrees away from the midline of the face, so they have a 340 degree field of vision. They have better peripheral vision than an owl, but not quite as good as that of a woodcock, which has an impressive 360 degree field of vision. The woodcock actually has better binocular vision to the rear than to the front.
Focusing near and far
Generally speaking the human eye is designed to focus primarily on distance objects. In order for us to see things up close, tiny muscles are needed to change the shape of our lens which enables us to focus on objects just a few inches from our face. This ability to see near and far is called accommodation. Unfortunately, as all humans age, these muscles become less and less effective, which is why so many of us need reading glasses.
Eagles have us beat in this category as well. Not only do they have a lens that changes shape—just like us. Their cornea—the clear, outer portion of the eye—also has the ability to change shape to better focus on near and far objects.
The search for eagle eyes
Birds of prey like eagles and hawks clearly have excellent vision. So you might be wondering; can laser vision correction help you achieve 20/5 vision. Unfortunately it can’t. The dimensions and characteristics of the human cornea, lens, retina, etc., means that there’s a limit to our visual acuity. Studies have shown that the best vision a human can achieve is 20/10, possibly 20/8. LASIK can on occasion help someone achieve 20/10 vision, but it’s rare. However, LASIK can get you to 20/20 and a fair number of patients exceed expectations and achieve 20/15 vision—which is still very impressive, especially if you’ve had to wear glasses for most of your life. It may not be enough to help you spot a mouse from a mile away, but we can leave that to the eagles.