Eagle eyes!

Has anyone ever called you eagle-eyed? Relative to humans, bald eagles have larger, sharper eyes that see further, collect more details, and produce stereoscopic vision to greatly improve depth perception.

A bald eagle's visual acuity begins with its eye size and shape. Dad's somewhat tubular eyes occupy over 50% of the volume of his skull, as compared with less than 5% in us spherically-eyed human types.  He can voluntarily adjust the curvature of his large cornea and lens (we're restricted to involuntary adjustments of our relatively smaller lenses only), which lets him rapidly change focus and increases the size of images reflected on to his retina. His two-lens system functions very similarly to a pair of binoculars, letting him see small objects in great detail at a distance and changing focus quickly as he flies - extremely important when hunting for squirrels, avoiding tree limbs, and locking talons with other eagles!

Eagle Eye Versus Human Eye. See end of blog for note.

The differences don't stop there. Our retinas would die without blood vessels, which supply oxygen but also scatter light and cause blind spots or shadows since light isn't reflected through them. But Dad's retinas don't have blood vessels! A pleated structure called the pecten supplies oxygen to Dad's eyes, preventing blood vessel-related light scattering and blind spots, and leaving more room to pack in light-sensitive rods and color-sensitive cones. In short, Dad's lack of retinal blood vessels enhances his visual acuity and ability to resolve detail. Large, razor-sharp images are reflected through his cornea and lens on to a much denser, clearer field of photosensors than our retinas have. How much denser? According to Keith Bildstein, the cone density of birds of prey is roughly 5 times greater than that of human beings. Dad sees farther than us, focuses better than we can, and collects more visual information about the world around him with his HD retinas!

Not enough visual acuity for you yet? Dad has two deep foveae compared to our single shallow one.  His more densely packed and deeper central foveae function in monocular vision, while his forward facing temporal foveae function in binocular vision. A densely cone-packed trench - Helen McDonald refers to it as a sort of smeared third fovea - connects Dad's central and temporal foveae, which helps him track moving objects as he switches from one mode of vision to the other, and lets him scan the horizon without moving his head. He can also fuse images from both foveae to produce a virtual reality-like stereoscopic image, which greatly improves his depth perception and ability to lock on objects. When Dad cocks his head sideways and looks skyward with one eye, he is using his monocular vision and his central foveae to scan for movement. When he looks forward intently, he is using his binocular vision and his temporal foveae to bring whatever he's looking at into sharp focus. Watch for those behaviors in the nest!

To create a 3-D'ish stereoscopic image, click on this image to enlarge it.
Cross your eyes until a third image forms between the left and the right image.

So given that Dad's visual system combines image detectors, tracking devices, binoculars, high-definition cameras, and virtual reality headsets, how far can he see? Accounts differ, but we've watched Mr. North respond to a squirrel in his nest from 550-600 feet away with no problem at all, and we've seen eagles at both nests respond to nest intruders long before they come into our view. Sources suggest that bald eagles can resolve:

• A 2-millimeter insect from 18 meters away
• A mouse from 446 feet away
• An ant from the top of a ten-story building
• The faces of basketball players from the back of a large arena
• A rabbit from over three miles away

As if that weren't enough, Dad also sees more colors than we do! In addition to perceiving red, green, and blue, a fourth cone allows him to see into the ultraviolet spectrum. Drops of oil in each cone selectively filter out certain colors, giving Dad greater sensitivity to different color shades and allowing him to see polarized light. These adaptations allow him to see the urine trails or prey, signal mate health, and may help him navigate as well. 

Helen McDonald states that Dr. Andy Bennett, a researcher in the field of avian vision, considers the difference between human vision and bird vision as that between black-and-white and color television. Bald eagle eyes see a world that is brighter, sharper, more colorful, and far more detailed than our own. I wish I could see like they do!

Did you know?

Bald eagles have three eyelids! Their clear nictitating membrane helps keep their eyes free of debris while they are flying. A specialized gland associated with the membrane produces an antibody (lysozyme) that helps keep their lens free of infection.

Despite these advantages, bald eagles have one big disadvantage - a blind spot directly in front of them. I was not able to find any information about the size of the spot, but it is most likely increased by their relatively prominent brows and large beaks. We've seen them hit one another - and in Dad's case, the tree trunk! - and this might also be another reason why collision-based trauma is a leading cause of death for bald and golden eagles. 

We can't see stereoscopically like bald eagles can unless we trick our eyes! You can learn more about stereoscopes here, or check out a video of images here!

Things that helped me learn and write about this topic
If you have a bird-lover in your life, I highly recommend the top two books

• Raptors: The Curious Nature of Diurnal Birds of Prey (Keith Bildstein)
• Falcon (Helen McDonald)
• These notes from an ornithology lecture on the avian eye
• This website on avian biological systems
• Wikipedia: Bird vision
• Wikipedia: Eagle eye
• Article: What if humans had eagle vision?
• Article: How do telescopes work?

Note: The image of the eagle's eye that I used can be found at several websites, including learner.org. However, it looks a little bit more like an owl's eye to me. Having said that, I'm not an expert and eagles and owls have fairly similar eyes, so I decided to use it.