How much do you know about bird vision?
Do birds see well? Why do they have different colored eyes? And do birds see in color?
So many questions, lots of answers.
Bird Vision
Let’s start with the basics. Birds have terrific vision. It’s better than human vision, according to most experts. Vision is the most important sense for birds since good eyesight is essential for safe flight. And birds have a number of adaptations which give them visual acuity superior to that of other vertebrate groups.
Birds with eyes on the sides of their heads have a wide visual field which is useful for detecting predators. Birds with eyes on the front of their heads, such as owls, have binocular vision. This allows them to estimate distances when hunting. Their eyes tend to be rotated toward the front of the head so that the visual fields of each eye overlap to some degree.
Most birds have binocular vision. Small birds that are likely to be prey for raptors tend to have their eyes set on the sides of the head, allowing them to watch for danger in all directions. At the opposite extreme from the owls are the woodcocks, mud-probers with eyes set high and back on the head, out of the way of vegetation and splattering mud and in a position to look out for predators. In fact, the woodcock has better binocular vision to the rear than to the front! (This info comes from an article by Paul R.Ehrlich, David S. Dobkin, and Darryl Wheye, 1988.)
Shorebirds, waterfowl, pigeons, and other birds that have minimal binocular vision seem to depend on differences in apparent motion between close and distant objects for much of their depth perception. When a bird's eye is moving, closer objects appear to move at a faster rate than do distant objects-- a phenomenon familiar from the way roadside telephone poles seen from the window of a moving car appear to pass more rapidly than the distant landscape. Presumably to enhance this distance-measuring method, shorebirds, and waterfowl often bob their heads up and down, and pigeons move theirs back and forth while walking. (Ibid) So that’s why they do that!
The size and structure of a bird’s eyes also are important. Compared with mammals, birds have relatively large eyes. In simple terms, a bigger eye means better vision, and excellent vision is essential for avoiding collisions in flight or for capturing fast-moving or camouflaged prey. Visual acuity in birds is also enhanced by the structure of the retina itself, which has tightly packed receptors and possesses other adaptations for producing a fine-grained image. (Ibid.)
I don’t know about you, but I find this stuff endlessly fascinating. Now let’s consider some published info about eye color which I’ve gleaned from the Hilton Pond Center for Piedmont Natural History in South Carolina.
Eye Color
A bird's iris includes millions of pigment cells. These, in turn, contain a variety of compounds--especially melanin--and other substances such as colored oils and guanine. All these substances work in combination through reflection, refraction, and scattering to give each bird its unique eye color. In albino birds, pigment cells lack pigments entirely and the underlying blood vessels make the iris appear pink. The diversity of visual pigments found in birds' eyes, and the presence of an array of brightly colored oil droplets inside the cones, suggest that avian color perception may surpass our own.
Most young birds have irises that are different in hue and clarity than adults of the same species. We suspect the change in iris color over time provides evidence of a bird's age — important, for example when a mature female's reproductive potential is enhanced by her selection of an older and therefore healthier and more "worldly wise" father for her offspring. (I’m going to avoid delving into the unique feature of the nictating membrane because this will turn into a term paper if I don’t keep editing.) Let’s take a look at some close-ups of bird eyes featured on the Hilton Pond website from back in 2003.
Birds See Color — and More
Do birds see color? You bet they do. They not only see in color, they are able to perceive familiar colors as well as parts of the ultraviolet spectrum that are invisible to human eyes. The discovery that birds see the UV spectrum has been confirmed only in recent years. That 1988 study I cited earlier in this diary contained this sentence: "There is also evidence that some birds' eyes are sensitive to ultraviolet light.” Much more is known now about birds and UV light, but I’m getting ahead of myself. First, let’s talk color perception. (Most of this discussion comes from Tim Burkhead in Audubon Magazine, May/June 2013 and Melissa Mayntz in The Spruce in April 2017.)
The cells in the eye responsible for color detection – the cones, discussed earlier – are positioned in the retina, and birds have four types of cones rather than the three humans have. These cones are defined by the color of the light they absorb: red, green, and blue. The exact number of cones varies in each bird species but is typically higher than humans and other mammals.
However good we may think our color vision is, it is rather poor compared to birds. Birds have four single-cone types: red, green, blue, and — wait for it --ultraviolet (UV). Not only do birds have more types of cones, they have more of them. What’s more, birds’ cone cells contain the aforementioned colored oil droplet, which may allow them to distinguish even more colors.
Dirurnal birds that are active during the day have the best color sense. Perceiving different colors is less crucial for nocturnal birds, and many birds that are most active at night have a greater number of rod cells in their eyes instead, which allows them to capture more light and see better in low light conditions, though they may not see colors as clearly. This makes total sense, right? It also follows the logic of visual acuity and eye placement discussed earlier. (Donald Trump wouldn’t understand any of this, but you and I do, of course.)
The ability to see ultraviolet light changes the perception birds have of many objects, even though humans may not see those differences. Some berries and other fruits have waxy coatings that reflect UV light, making them stand out vibrantly against green foliage. Birds can see the fruit much more clearly, making foraging much easier. Some insects also reflect UV light, and certain flowers will as well, giving birds a distinct advantage for finding those food sources.
Raptors also use UV light to track prey, since the urine splashes and trails that voles, mice, and other rodents use to mark their territory are brilliantly visible in ultraviolet light. And, perhaps most important of all, bird species that may not appear dimorphic to humans may actually look very different in UV light. We may not be able to tell genders apart in some birds but seen under UV light, birds have no difficulty telling the difference and can use those UV markings to help select mates, defend a territory or tell individual birds apart.
Wrapping up, this seems like a good place to mention that though we humans don’t see UV light, we can put UV light to work for birds we want to help in the form of window decals that can prevent bird strikes. I can’t recall how many times over the years here at Dawn Chorus someone has mentioned in the comments either having a bird hit a window or finding a bird on the ground that was a bird strike victim. Most often these have been hummingbirds. At most birding supply stores and at various places online (including Amazon) you can purchase inexpensive Window Alert Decals.
They’re pretty unobtrusive because we hardly see them, but they’re lifesavers for birds. There are various brands, but this is one I've seen appearing several places online.
So, this is all likely more than you ever wanted to know about bird vision. There’s actually much more information out there including the fact that birds use their left and right eyes for different tasks, that birds can fly while asleep with one eye open, and much, much more. But this certainly is more than enough for one Dawn Chorus.
As always, consider this an open thread and let us all know what’s been going on in your birding world.