Archive for February, 2012

Not so Eagle-Eyed

I have read stories of eagles, particularly in the open spaces of the mountain west, being killed by flying into power lines.  But if eagles have such wonderful vision, why don’t they see the power lines and avoid them?

The explanation is that the power line appears as a pencil thin 2-dimensional line not as a 3-dimensional object.  It is almost like looking thru a picture window than has a single strand of a spider web in one corner.   You never focus on the thin strand.  Instead, you easily disregard it as you gaze at the vast panorama image until it is too late.  In the wide open spaces of the west a tiny line is easily overlooked.

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At the EagleFest children often ask about how well can an eagle see.

The terms “eagle-eye” or “hawk-eye” are often used to describe exceptional vision.  There is much truth (and science) to the belief that raptors have much better vision than humans.  A red-tailed hawk is said to be able to spot a mouse at a distance of one-half mile or more.  I can’t do that, even with binoculars.

All birds have good vision.  They have to in order to survive.  Birds do not have a good sense of smell, so they must rely on sight to find food and to spot a potential predator or other dangers.  Not only are their eyes large compared to their overall body size they have greater image gathering ability.

It’s impossible to know for sure what the world looks like to an eagle, but we know from studying the anatomy of their eyes that their view must be enlarged and magnified compared to our view. Eagle eyes are the same size (weight) as human eyes (though a full grown adult Bald Eagle weighs no more than about 14 pounds!) But an eagle eye has a much different shape from ours. The back is flatter and larger than the back of our eye, giving an eagle a much larger image than we can see. And its retina has many more concentrated rod and cone cells-the cells that send sight information to the brain. Some animals, including humans, have a special area on their retina called the fovea where there is an enormous concentration of these vision cells. In a human, the fovea has 200,000 cones per millimeter, giving us wonderful vision. In the central fovea of an eagle there are about a MILLION cones per millimeter. That’s about the same number of visual cells jammed into a square millimeter as the finest computer monitor has on its entire screen when set at its highest resolution. 


While there is no official eye exam for birds, they do have superior vision to human.  When you go birding, you can be sure that they will see you before you see them.

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Only one real surprise at the EagleFest.  The mild weather attracted a lot of people on Saturday (about 4,000 attendees)  However, it also kept many eagles farther north.   Last year there were about 110 Bald Eagles between Croton Point Park and the Bear Mountain Bridge.   This year, maybe 20-25.  

I was the trip leader for the first BirdWalk of the EagleFest and we were fortunate to spot two Bald Eagles.  The mid-morning walk was most productive yielding 8 Bald Eagles, 1 N. Harrier and a handful of E. Bluebirds.

My biggest surprise of the day was the following explanation as to how Croton Point was the birthplace of the modern field guide. 

In 1931 Roger Tory Peterson, who was studying art in New York City, was an active member of the Bronx Bird Club.  The club made a field trip to Croton Point.  During their trip Roger identified a flock of Pippits as they flew overhead.  That greatly impressed another member of the club, Bill Vogt, who asked “How did you know what they were?”  In those days birders still identified most birds by shotgun, shooting them and then examining them closely while holding their dead bodies.  Roger explained his technique of looking for key field marks as a good way to ID birds without shooting them.  Vogt, who was a successful drama critic and novelist, was amazed and encouraged Peterson to put his technique into book form.  “I can get it published for you” he told Roger.

Peterson went to work and three years later the first modern Field Guide to Birds was ready.  Vogt had to contact several publishers before Houghton-Mifflin agreed to publish the book.  You have to remember that 1934 was the height of the great depression so publishing any book was a risk.   Amazingly, the first press run of 2,000 copies sold out in less than a week.  Additional copies were printed immediately and Peterson Field Guide to Birds has been continuously in print ever since (almost 70 years), selling millions of copies.   The price of the first edition was $2.75.  I checked the internet and found you can still buy a copy for $600-$700.

And, if you believe this version, the idea for the first modern field guide, the guide credited with making birding accessible to the general public, was first hatched at Croton Point Park.

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