COMMENTARY BY DR. DAVE SAMUEL
What land vertebrate has the largest eye? The obvious answer would be the elephant, but that isn’t correct. The answer is the ostrich. Ostriches are the largest birds in the world, so I guess it is not surprising that they have huge eyes. Their eyes are 5 centimeters long. Note you can’t see all of the ostrich eye from the outside, but if you look at the skull you will see huge eye sockets.
Ostrich eyes also have many photoreceptor cells and this, along with the size, allows them to see predators from very long distances.
This is just one of the many facts learned from a zoom meeting I attended this past Tuesday evening. It was hosted by the Mountaineer Chapter of the Audubon Society and the speaker was Dr. Ivan Schwab, professor emeritus of ophthalmology at the University of California, Davis. He received his undergraduate degree from WVU and his MD from the WVU School of Medicine. His talk was on the eyes of birds.
Being interested in eyes and birds runs in the Schwab family as his brother and my good friend, Dr. Larry Schwab, had a full and successful career in Ophthalmology and received many awards for his devotion to eye disease prevention and cure. A recent example was the 2020 International Blindness Prevention Award Dr. Schwab received from The American Academy of Ophthalmology. Even better for me was the fact that he was my eye doctor for many years here in Morgantown. Dr. Schwab is also a well-known bird photographer and bird watcher. An interest in birds seems to run in the family.
Dr. Schwab’s virtual presentation noted that for birds, their eyes outweigh the brain. Obviously, eyesight is very important for birds. A rule of thumb he noted was that the smaller the bird, the larger the size of the eye. Eye size is inverse to body size. Another rule of thumb is that size varies directly with the swiftness of the bird. Thus, hummingbirds have relatively large eyes.
Hummingbirds also have great color vision that is handy when visiting flowers. They see all the colors we do, plus they can process ultraviolet light and that means they can see other colors we can’t. If we stick our head out of a fast-moving car, in a short time we’d have to either close our eyes or put on goggles. Motorcyclists understand this. Fast flying birds have the same problem with dry eyes while flying. The hummingbird, and eagles and hawks and other fast fliers, have a translucent nictitating membrane (actually a flap of skin that acts as a third eyelid) that functions as a windshield wiper. It removes dust and helps keep the eye moist.
Every species of bird has modifications that help them see. For example, herons wade in the water with relatively long legs, looking down for small fish that they grab with long beaks. So, their eyes are high on their head so they can see well as they look down.
We’ve all seen swallows zipping around catching insects. My question is, how are swallows able to see and capture mosquitoes at dusk? It turns out that they have two areas on the retina that have a high concentration of cones. These are called fovea. This allows swallows to see flying mosquitoes as they swiftly dart about. Birds that dive from high altitudes to catch prey also have fovea. Eagles that dive to catch fish underwater use this special vision to track the swimming fish as they dive for them.
All birds have binocular vision only barn owls and a few other owl species have been shown to have binocular depth perception. The woodcock can see stereo vision in front of the head and behind the head. They can see 360 degrees without moving their head. It’s complicated, but it’s all about foraging for earthworms. Regardless, being able to see 360 degrees is one trait that this bowhunter would love to have.
Dr. Schwab had lots more for us on Tuesday evening, and it was one zoom meeting I was glad I attended. Hat’s off to our Audubon Chapter for making it happen.
Wear your mask and get vaccinated so we can all start to do things in person again.
