We humans often take for granted that all other creatures see the world as we do. But they don’t. Their eyes are arranged differently on their heads and not all of them see the colors that we do. So looking specifically at birds, what do we know? For most birds the eyes are much larger than ours in relation to their heads. Relative to the size of their body, the eyes of most birds are almost twice as big as most mammals.
Some birds are able to see very clearly over a great distance. These are the raptors or birds of prey. This is a necessity since their food is creatures that move about. One of the reasons the hawks, owls and eagles can see so well is due to the presence of something called fovea. The fovea is just a slight depression on the retina on the back of the eye. In the raptor family there are two fovea, while we humans only have one on each retina (the same is true for almost half of all bird species). These tiny ‘pits’ lack blood vessels and have a greater number of photoreceptors. This gives the bird the sharpest possible image.
We all know the phrase, “Eagle-eyed”, but you could also say ‘Kestrel-eyed’ although that doesn’t roll off the tongue as easily. Scientists have determined that a Kestrel can observe or detect a 2 millimeter (less than an inch) insect from a distance of 59 feet. One naturalist decided to see how close he would have to be to see the same size insect. He got to 13 feet before he saw it.
So we know something about the acuity of birds, but what about seeing all the colors we see? One of the most wonderful aspects of bird watching is the seemingly unlimited range of colors that coat their feathers. Surely birds must be able to see what we see, or why else would they have evolved with so many variations? Surprisingly, one of the ways their colors help them is to blend into the surroundings. You would assume, brown or gray or even black birds would do this most easily; taking advantages of the shadows, but even the most colorful birds, like the Andean Cock-of-the-Rock can blend into the forest and be invisible to our eyes. When these birds are in courtship mode, the males ‘dance’ in full sun and their red body feathers almost glow, but out of the sun they lose their brilliance. Do the females see the same dramatic colors we see?
Yes and No. We humans see only see three colors; blue, green and red and various mixes of those colors.. Birds can see blue, green, red and UV – which stands for ultra-violet. By definition, in nature UV light means having no color. That means birds can see a whole array of colors that are invisible to us.
One scientific experiment in 2007 helped to confirm this amazing fact. Using a spectrophotometer (an instrument that measures the intensity of light in a part of the spectrum), they looked at the feathers of 166 North American songbird species, focusing on those that didn’t appear to have differences of appearance between genders. From a human perspective 92% of those species there was no visible difference in the genders. The study showed that those same birds when examined with the spectrograph actually had colors we humans could not see. But where it really matters is for the birds that can then identify genders of their own species. As an example, for the Yellow-breasted Chat (the largest warbler found in the U.S.), both male and female have stunning yellow breast feathers. But what we cannot see are the ultraviolet feathers on the male’s breast. This allows potential mates and rivals to know his gender. For birds, beauty is definitely in the eye of the beholder.
This information gives us an entirely new way of looking at the visitors that come to our bird feeding stations. Even though we may not be able to see all the colors that the do, they still brighten our lives, especially in the winter months when those of us in the northern regions have so little color in our yards and neighborhoods.
By Kate Crowley