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/.
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