My journey home for the holidays used to mean a cross-country flight. Since I moved back to the Midwest, it means a long drive over some of the most beautiful rural highways in Wisconsin, Minnesota, and Iowa. Long ago, my dad entertained us on similar car trips by watching for raptors all across the Hawkeye State.
Red-tails on power lines, kestrels on road signs, harriers soaring over fields, eagles near the river, and owls on fence posts captured our attention. If the traffic was light and a field driveway was near, Dad would “flip a Louie,” get out his camera with the long lens (nicknamed “Big Bertha,”) and go back to see if he could get a good shot. We’d hold our breath and try not to wiggle the car. Sometimes the raptor was cooperative, and posed for a while, blinking in the sun. Sometimes they took off just as Dad was about to push the trigger on the motor drive.
My trip home for the holidays this year included many raptor sightings, and while I didn’t do any U-turns to photograph them, I admired their calm grace and their high hunting posts in roadside trees. It is hard to believe that birds of prey could be looking for mice from way up there, but I know they can see between four and eight times as well as us humans. According to one source, “If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building.”
How do they do that?
Accommodation is one trick birds use to focus on objects at a variety of distances. This simply means that tiny muscles around the eye alter the curve of the lens so that it can focus on objects that are far or near. It’s like using the focus wheel on binoculars. Humans share this adaptation for focusing. You may have noticed the time-delayed focus as you stare at an object to see it better and your muscles automatically curve the lens after a second. The amazing thing about raptors is that they can change the shaped of their cornea as well as their lens. This gives them an even more precise focus on the world.
I experienced the lack of accommodation when my holiday journey detoured to the eye doctor’s for my annual checkup. As I sat in the waiting room waiting for the drops to finish dilating my eyes, I noticed that I was having increasing difficulty reading the magazine. I asked, and Dr. Landis explained, that the numbing drops don’t just stop the muscles around my pupils from working, they also stop my accommodation muscles from working.
While squinting in the bright snow and frustrated at blurred vision on the way home from town, I was still able to spot a rough-legged hawk soaring over the corn stubble. Their feathered or “rough” legs are well suited for both summers in the Arctic, and the relatively mild winters of Iowa. Hunting on the wing presents some challenges, though, and I can’t imagine being able to see details on the ground from such a height. Raptors and other birds not only use muscles to focus their eyes, they also have more light-receptor cells to focus the images on.
Humans have a fovea, or focal point, at the center of our retina with 200,000 light-receiving cone cells per millimeter. This provides for our good color vision. Eagles (and other raptors), on the other hand, have about a million cones per millimeter in their central fovea. That gives them much higher resolution vision. Not only is their fovea packed more densely with cones, it is also deeper than ours, so it may act like a telephoto lens and give them extra magnification in the center of their field of view.
Moreover, raptors actually have TWO fovea. In addition to their central concentration of cones, they have a lateral fovea that allows them to keep the horizon and the ground in focus simultaneously.
All this extra visual resolution gives hawks and eagles somewhere between 20/5 and 20/2 vision. At best, the physical properties of human eyeballs limit us to 20/10 or 20/8 vision. In other words, what a normal person could see at eight feet, an exceptional person could see at twenty feet. Or, what a normal person could see at TWO feet, a hawk could see at TWENTY feet.
Scientists consider bird vision to be the finest in the animal kingdom, and raptors are at the top of their class. But raptors don’t lead the class in every respect. Smaller birds may see faster than raptors, and receive more colors. Migratory birds may be able to see polarized light and the Earth’s magnetic field. While owls are raptors, and do have excellent eyesight, their eyes are tuned for night vision.
Over the holiday, eagles and hawks soared past our windows on a regular basis. Each time, several sets of human eyes focused eagerly on the majestic visitor, still registering barely more than a blur of flight. It is fun to imagine just how differently various creatures can see the world. Wouldn’t it be fun if we could truly be in the hawk eye state?
For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.
Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.