Why do some birds have red eyes? Birds with red eyes have a lot of tiny blood vessels in their eyes.
Human eyes come in one of six colors. Our Eyes can be amber, blue, brown, gray, green, hazel, or pink. Pink eyes are actually a result of albinism. If someone is born albino, they have no pigment in their skin of eyes, which means the blood vessels in the eyes are visible, making their eyes pink.
Bird eyes have a far greater range of eye color than we do. Their eyes can be black, brown, red, orange, yellow, blue, green, white, silver, gold, and other shades as well. As well as this, eye color can vary within a species and some birds can even change the color of their eyes with the season. There are multiple reasons why birds might have different colored eyes. Let’s have a look at some of the reasons.
One reason could be thermoregulation. Red eyes in birds is caused by a large concentration of small blood vessels in the eyes. When birds are hot, they have a few ways of losing the excess heat. They can open their beaks to let heat out. They can move the muscles in their necks, which releases heat. They can lose heat through their legs. Some birds can also lose heat through their eyes. The blood vessels close to the surface of the eyes, that give them the red color, make it very easy for the birds to lose heat through their eyes.
Some birds have colored eyes to help them see further. At least, this is a theory that has never actually been tested. It is thought that some birds that hunt and need to see very far, such as eagles, have an eye color that absorbs light and reduces glare. If there is less glare, they can see far more easily and see further, being able to catch more food.
Another theory is that bird eye color helps with camouflage. Owls are predators that are generally nocturnal, and they tend to have dark eyes. This makes it much easier for them to blend in to their surroundings and not be seen. However, owls that are awake and hunt during the day tend to have yellow or orange eyes. This can be broken down because owls that hunt in the dawn or dusk have orange eyes and owls that hunt during the day have yellow eyes. That would give more proof to the camouflage theory because the light in the dawn and dusk is more orange and the light during the day is more yellow. Birds that are preyed upon also often have darker eyes. If they usually survive by hiding away or nesting in dark places, they will have darker eyes.
Another reason for the different colored eyes is not just for camouflage. It is possible that different colored eyes are more sensitive to different colors of light. In the same way that eagle’s eyes could reduce the glare, owl’s eyes could help them see in the dawn, or during the daylight, or even at night.
Most of the research, though, seems to suggest that the huge variety of colors in bird’s eyes is down to cosmetics. Birds often have brilliantly colored feathers in order to attract a mate. Their eyes could be just another part of this process. This can be shown because the eyes of the Brown Pelican change from brown to brilliant blue during the mating season. If this was not connected to finding a mate, it is unlikely that it is a process that would have been selected by evolution. Of course, just because something happens, doesn’t mean there is an evolutionary reason for it, but the chances are good. The eye colors of some birds also change as they get older. When they are born, their eyes are one color and once they reach maturity, their eyes are another color. This could be to show other birds when a bird is mature enough to mate. Woodpeckers are a good example of this.
There is one other difference between our eyes and the eyes of birds, and most other animals. We have colored irises, but we also have a lot of white in our eyes. This white part is called the sclera and it is the protective layer that encloses the eyeball. Ours is white, but in most animals it is dark or the same color as the iris. The reason for this, it is thought, is that it hides the eyes of these animals more effectively. Any white in the eyes would show up when an animal was hiding or hunting. So, why do we have whites? We have evolved to be social, and we use our eyes to communicate. With white sclera, it is very easy to see where we are looking. Animals don’t want to show where or what they are looking at. Interestingly, animals that hunt in groups tend to have white sclera and animals that hunt individually or hide, don’t. Which shows they play a big part in communication. And this what I learned today.
Photo by Rajukhan Pathan: https://www.pexels.com/photo/colorful-bird-with-green-shiny-plumage-6543040/
Sources
https://frontiersinzoology.biomedcentral.com/articles/10.1186/s12983-017-0243-8
https://nationalzoo.si.edu/animals/news/how-do-birds-handle-heat
https://centerofthewest.org/2021/10/10/birds-eye-color-and-eyelashes
https://www.audubon.org/news/bird-eyes-come-amazing-array-colors-why-mystery
https://web.stanford.edu/group/stanfordbirds/text/essays/Eye_Color.html
https://www.sciencefocus.com/nature/do-any-mammals-other-than-humans-have-whites-of-the-eyes