Brooklin author/photographer Richard J. Leighton creates the popular “In the Right Place” posts online about life and nature in Maine. He shares a post the second Thursday of each month in The Ellsworth American.
The eyes have it
By Richard Leighton
While trying to find out why mature male wood ducks have red eyes, we realized that the more important story is the unusual vision of birds — the creatures that have the largest eyes relative to their size in the animal kingdom.
But first, the wood duck story: researchers theorize that adult male wood ducks may have red irises to enable females of their species to quickly identify sexually mature males, so that they don’t waste valuable time on dark-eyed juveniles.
Vision is critically important to female birds when selecting mates. But what happens with those females that the guide books show as being identical in appearance to males of their species? We must remember that those books are written only for humans.
Birds can see reflected ultraviolet light on feathers that humans cannot see and — you guessed it — research shows that the appearance of males often is different from females as seen by birds, but not by us.
The placement of birds’ eyes also affects their vision and varies by their behavior. Those with a pair of eyes in the front of their head, such as owls, have binocular vision; this enables them to quickly judge changing distances when approaching prey. Those with an eye on each side of the head, such as wood ducks that swim in open water, have a very wide field of vision to detect predators at a great distance.
The unusual placement of the eyes of ground-nesting American woodcocks reportedly gives them an extraordinary field of vision of 360 degrees horizontally and 180 vertically.
Finally, the mechanics of birds’ eyes can differ according to their behavior. A high-flying bald eagle reportedly can see its prey two miles away and, as the bird dives at high speed, it uses special muscles to continuously change the curvature of its eyes to keep the scurrying rabbit or swirling fish in pinpoint focus.