For birds, the eyes have it. Vision is their predominant sense.
A snowy owl sitting on a flat snowy field in North Dakota is about to demonstrate that.
The bird is estimated by its would-be trappers to be 600 feet away. The trappers are part of Operation Snowstorm, an ongoing project to capture and equip these owls with GPS devices that record movement via satellite. This is winter last year.
The two men have a spotting scope, a wire box trap and a brown mouse. They watch the owl watch them for about five minutes before it flies in, hops into the trap and eats the mouse.
The owl recognized the mouse as food from 600 feet.
The owl, like all raptors, has extreme visual resolution. Detail is its key to the dining room.
Our eyes have elements in the retina known as rods and cones. They absorb light energy that is sent to the brain, where it becomes an image. Our retinas have about 130 million light-sensitive cone cells per square inch. These determine acuity.
Certain raptors have five times as many cells per square inch, allowing them, say, to identify a mouse in a cage at 600 feet — extreme resolution.
That’s what allows the red-tailed hawk to hunt atop the light standard along I-35 or soar above, watching the grass below for rodent movement.
Birds also have pliable lenses in their eyes that can be focused as needed, near or far. They can change the curvature of the lens. If you and I could do that we wouldn’t need glasses.
Birds are believed to do much better with color than we do. We have cones sensitive to red, blue and green, from which we somehow make out yellow. Birds have cones sensitive to five colors, vastly expanding the range of color they see.
Some birds see ultraviolet light. Chickadees have ultraviolet plumage colors. It’s one way those birds recognize sexual differences. All chickadees might look alike to us, but not to themselves.
Birds obviously have better visual reaction time than we do. They keep things in focus at rapidly changing distances.
Birds flying into thick shrubs or the deep foliage of a tree must see, at high speed, every possible collision. A Cooper’s hawk focuses on its fleeing prey as it sees clearly every branch and leaf in its path. Its dodging prey sees them, too.
The robin in your yard stops and cocks its head, first to the left, then to the right. Robins, like many other bird species, have monocular vision, eyes set to the side of the head.
An image is created one eye at a time. Cocking its head allows it meld images, thus seeing in three dimensions what it might want to eat. (It is not cocking its head to listen.)
Birds with eyes on the sides of their heads, like bitterns or woodcocks, monocular in function, can see front, rear, up, and down at the same time, as much as 340 degrees of vision for certain species. It’s a safety feature in a world of predators.
An owl, eyes pointed straight ahead, has binocular vision with a field of view limited to no more than 70 degrees. He has excellent three-dimensional vision, but it comes with a price.
Understand that all of this is based on interpretation of bird action and reaction, on observation and testing. No one has ever looked through the eyes of a bird.
We really don’t know what their world looks like.
Read Jim Williams’ birding blog at startribune.com/wingnut.