Bird Eyes and Vision

Sight is the primary sense most birds use as they go about their daily business. Their color perception and visual acuity are far better than yours.

DATE:
September 19, 2020
CATEGORIES:
Author
Ivan Phillipsen

Blue Feet, Laser Focus

One of my standout memories from the Galapagos Islands is watching hundreds of Blue-footed Boobies, as they circled in the sky offshore and then shot into the ocean like a volley of arrows.

They were hunting fish, of course. Boobies and gannets plunge dive to catch fish below the surface. If they miss their targets, they will often chase fish underwater by flapping their wings, penguin-style.

I was told by our Ecuadorian guide that a booby’s eyes are specially adapted to adjust their focus instantaneously as they cross the boundary from air to water.

You can imagine the visual challenges in this situation. First, the bird needs to see a fish below the surface, focus on it, and accurately gauge its depth and position.

Then, when it dives at high speed, its eyes need to be protected from the impact with the water. If it has any hope of catching its fishy prey, the bird needs to see with sharp focus underwater. When you open your eyes underwater in the swimming pool, you can’t really focus on anything, can you? Well, that wouldn’t work for boobies. Most animals can see well in either air or water, not both. These seabirds, however, have evolved a solution to this.

In this article, we’ll talk about the adaptations of booby eyes and many other cool things about the vision of birds.

Blue-footed Boobies in the Galapagos Islands

Intro to Bird Vision

Birds and all the animals of Earth, including humans, live in a complex, physical world that they must navigate through if they hope to survive and maybe leave a few ungrateful kids behind as a genetic legacy.

Animals need to perceive features of the physical world. Because many of these features—like cliffs, quicksand, predators, and pointy sticks—can cause injuries or death. Others—like food and water—are necessities that animals need to stay alive.

Thanks to the long, brutal march of evolution by natural selection, animals have senses that serve as essential tools for survival. Senses helped our ancestors gather data about the environment, to avoid dangers and locate vital resources. And senses continue to serve us very well today.

Electromagnetic radiation, from the Sun or elsewhere, is a form of energy that animals find particularly useful as a source of data about the world. Waves of this energy, which you can also think of as photons, are zipping around all over the place in the form of visible light, ultraviolet light, microwaves, gamma rays, etc.

These waves/photons bounce off of objects or move through them. The sense of sight in animals allows them to detect some of these waves and therefore allows the objects themselves to be detected.

So sight is pretty dang helpful.

Birds, while they may not have the greatest sense of humor, have an excellent sense of sight. It can be argued that, of all the animals, birds are the best at seeing stuff.

For most species, sight is the primary sense they use as they go about their daily bird business.

As far as we know, birds have been primarily diurnal for their entire evolutionary history and eyesight has been important to them all along the way.

I point this out because mammals went through a long period of being nocturnal. During the time of the dinosaurs, mammals were small creatures of the night, with poor eyesight but an excellent sense of smell. When some mammals, like our primate ancestors, returned to a diurnal, daytime existence, it was again advantageous for them to see colors. So they re-evolved color vision and regained some visual acuity.

Meanwhile, birds have just been cruisin’ along, perfecting their eyesight the whole time. We mammals have yet to catch up to them.

Anatomy of a Bird's Eye

Let’s talk a bit about the anatomy of the avian eye.

If you’re familiar with the anatomy of human eyes, you’ll be able to picture a bird’s eye pretty well. There are many similarities, which reflect our shared ancestry with birds. The lineages that became birds and mammals split off from a common ancestor about 300 million years ago. That’s lots of evolutionary time, though, so some significant differences have also accumulated.

Of all animals, birds have the largest eyes relative to their body size. A bird’s eyes take up a lot of real estate in its head. The Common Ostrich has the largest eye of any land animal. Not in the relative sense, but in the absolute sense. So an ostrich eye is 5 times bigger than a human eye and is even bigger than an elephant’s eye. Each of this bird’s eyes is larger than its brain!

And the bigger your eyes, the better you can see. One reason is because a large eye can let in more light than a smaller eye. And a large eye can pack in more light-sensitive cells.

Common Ostrich portrait
The Common Ostrich has ginormous eyes.

Light enters a bird’s eye through the transparent cornea and then passes through the lens. These structures both have a curved, convex shape that helps focus light waves. A lot of the eye’s resolving power comes from the cornea. Birds have a small group of muscles encircling the cornea and another group around the lens. By contracting or relaxing, these tiny muscles change, independently, the shape of the cornea and/or the lens to achieve focus. Humans aren’t able to change the shape or their corneas. We adjust our focus using only the lens.

Compared to humans, some diving birds such as ducks and cormorants have stronger muscles around the lens and the lens itself is relatively flexible. These birds have up to 10 times the focusing power of humans.

Let’s return to the Blue-footed Booby that we started this article with. Boobies and gannets are cousins to the cormorants. These guys all belong in the Suliformes order of birds. As diving birds, they must make instantaneous adjustments to switch from seeing and focusing in air to seeing in water.

Underwater, the focusing power of the cornea is lost, so the lens has to take over. Cormorants—and most likely boobies—change the shape of their lenses to be almost spherical underwater. And their irises open up underwater, dilating to let in more light. These adjustments create the optical conditions for good focus—or at least good enough focus—while pursuing fish beneath the waves.

Another thing that bird eyes have that human eyes don’t is a third eyelid that crosses the eye in a horizontal direction. This is called the nictitating membrane. It protects the eyes from debris and keeps them moistened.

Many diving birds have transparent nictitating membranes, which act like built-in swim goggles. These membranes also protect the eyes of birds from damage while diving at high speeds. This is the case for birds like Peregrine Falcons and the boobies we’ve been talking about.

Mammals can have a third eyelid, too. Camels, polar bears, and your house cat have nictitating membranes. And our ancestors had them millions of years ago. But now humans have only a sad vestige, in the form of that little bump of pink tissue in the corner of your eye. It’s called a semilunar fold.

Nictitating membrane on a Masked Lapwing
Nictitating membrane on a Masked Lapwing. Photos by Toby Hudson.

Okay, so after light waves enter the eye and get focused by the lens, they strike the retina.

The retina is a thin layer of photoreceptor cells lining the inside wall of the eye, at the back end. Light enters the eye and strikes those cells, which then produce signals that are sent to the brain via the optic nerve.

The photoreceptor cells in birds are called rods and cones. Humans have these, too. Rods are associated with black-and-white vision whereas cones give us color vision.

In the center of the retina, there is an area where the photoreceptor cells are packed in at their densest. This area is called the fovea, and it provides the highest resolution images. Unlike humans, some birds like raptors, kingfishers, and hummingbirds have a second fovea. One fovea provides acute forward-facing vision, the other helps make out images to the side of the bird’s head.

Color Vision in Birds

One of the coolest things about bird retinas is that they have four different types of cone cells, instead of only 3 as in humans. That fourth cone is sensitive to ultraviolet wavelengths of light, which humans can’t see.

The so-called tetrachromatic color space seen by birds is much larger than the range of about one million colors that humans can see. Having four types of cones is actually a primitive characteristic, shared by amphibians, as well as birds and other reptiles. Mammals lost their fourth type of cone when they went through that nocturnal phase.

So get this: Birds can see colors out there that not only can humans not see, we can’t even imagine them!

If we could see through a bird’s eyes, we’d have to come up with names for these exotic new colors. Names like, I don’t know, Grorange or Ultra-Blurple.

So what advantages do birds have by being able to see this expanded color palette?

For starters, colors in the ultraviolet range are reflected by many flowers, fruits, and berries. So birds that rely on these plant products for food can be more efficient, more successful in their foraging.

Some raptors might benefit from this ability, too. Research in the mid-90s suggested that Eurasian Kestrels, which are small falcons, could find their rodent prey by looking for trails of vole urine, which supposedly reflect UV light. More recent research has cast some doubt on this, but the idea of fluorescent vole pee is intriguing, no?

There’s no doubt, however, that many birds have patches of feathers that strongly reflect UV light. This has been proven. The colors of these feathers can be attractive to potential mates.

Research has suggested that, for at least some species, females may prefer males with feathers that shine brightly in the UV part of the spectrum. The plumage of a male in UV light might be a signal of his overall health or fitness—useful info to females looking for a mate. There’s some evidence for this phenomenon in Blue Tits, Pied Flycatchers, Budgerigars, and Bluethroats.

Male Bluethroat Bird
A male Bluethroat may attract mates with feathers that reflect UV light. Photo by Bouke-ten-Cate.

So as colorful as many birds seem to us, they probably look even more dazzling to each other. And, interestingly, this means that some species have differences between males and females that we can’t see.

For example, there’s an antbird species living in the Amazon Basin called the Black-spotted Bare-eye. To us, males and females of this species are pretty much indistinguishable. But, it turns out that the crown feathers on the male fluoresce brightly under UV light.

This info comes from a 2005 research study on 139 monochromatic bird species—that is, species where there aren’t obvious color differences between the sexes. Once the researchers took UV into account, it turned out that over 90% of these 139 bird species were actually dichromatic. These birds have and can see differences between males and females that humans are completely blind to. That’s so cool!

This continues to be an active area of research. For example, a very recent study of wild Broad-tailed Hummingbirds in Colorado showed that these birds can see what we call nonspectral colors that include an ultraviolet component. These hummers quickly learned to associate artificially generated nonspectral colors with sugar water in a feeder. Again, these are colors that humans can’t see.

Another feature of some bird eyes, related to color vision, is microscopic oil droplets in the cones cells of the retina. This oil absorbs certain wavelengths of light and narrows the band of wavelengths that each of the four types of cone can respond to. Many seabirds like terns, gulls, and albatrosses have these oil droplets. This gives them the ability to see colors more accurately and seems to give them better sight in hazy conditions. Somehow.

The Visual Acuity of Bird Eyes

Besides being able to perceive colors that might, if we could see them, melt our human brains, birds have another superpower when it comes to their sense of sight.

Many of them have visual acuity that far exceeds ours. In other words, their eyesight is really sharp.

Birds can discern finer details than we can, often at greater distances.

Think about the standard eye chart. You know, the one with a big ‘E’ at the top and increasingly smaller letters below. It measures visual acuity. A human has 20/20 vision if they can read the letters on the 8th line down, while standing 20 feet from the chart.

Some birds, like eagles, are said to have the equivalent of 20/5 vision. That means that an eagle could make out details of an object 20 feet away that a normal human could only discern at 5 feet away. So, you could say that such an eagle would have visual acuity that is four times better than a normal human. They can spot small prey animals far below them when they’re flying or perched up at the top of a tree.

And this kind of acuity might also be how a crow can zero in on a discarded cheeseburger wrapper from way over on the far side of a Walmart parking lot.

You can think of birds as having between 2 and 8 times the visual acuity of humans. It depends on the bird species you’re talking about.

Not only do birds see more details in the spatial dimension, many of them are better than humans at perceiving patterns in the temporal, or time, dimension. By that, I mean they can detect fast movements that would only be a blur to us.

For example, humans can’t detect movements that occur at a rate faster than 50 times a second. Fluorescent light bulbs flicker at about 60 cycles a second. So their light looks continuous to us. But at least some birds can detect movements of over 100 cycles a second. So they would see that fluorescent light as flickering. More like an obnoxious strobe light.

It makes sense that birds would need such amazing eyesight. As flying creatures, they need to maneuver at high speed through a three-dimensional space that can be filled with obstacles like tree branches. Many birds need to see and catch small, fast-flying insects or other flying prey. There are so many ways that birds use their awesome eyesight.

But how do they do this? What about their anatomy makes their vision so good?

Earlier, I mentioned that some birds have higher focusing power than mammals, because of the anatomy of their lenses and associated muscles. That’s one thing.

But perhaps the key feature responsible for the incredible eyesight of birds is the density of photoreceptor cells in their retinas. Remember the rods and cones we were talking about a few minutes ago? Well, the number of cone cells per square millimeter can be up to 1 million for some raptors. The House Sparrow has 400,000 per square millimeter. What density do humans have? About 200,000 cones per square millimeter, at most.

More photoreceptor cells results in higher resolution, higher visual acuity.

It’s like with digital cameras, where older models, circa 2005, could take only 5 megapixel photos, but cameras today can easily take 40 megapixel photos. The difference is in the resolving power of the sensors. Having more cones in your retina is like having a more powerful camera sensor.

As we established earlier, having large eyes is super helpful too. Diurnal raptors like hawks and eagles have huge eyes for their size. The Wedge-tailed Eagle of Australia is thought to have the highest visual acuity of any land animal, given its big eyes and the density of cones cells in its retina.

Wedge-tailed Eagle
Scientists think that the Wedge-tailed Eagle of Australia may have the highest visual acuity of any land animal.

Now, let’s not forget about owls and other birds of the night. Besides owls, nocturnal birds include nightjars, potoos, night herons, oilbirds, kiwis, and more. Kiwis are adorable, but they have lousy eyesight. But that’s no problem, really, because they have keen senses of smell and touch, which serve them well as they grub around for worms in the dark. Most other nocturnal birds have enormous eyes to gather as many photons as possible, and they can have exceptional night vision.

Owls are the best known example. They have relatively massive eyes, stuck in a fixed position in their eye sockets, in their skulls. To look around, they have to move their heads, rather than their eyes. That’s why owls have those famously flexible necks that can rotate their heads in almost 360 degrees.

The retina of a large owl can be larger than the retina in your eye. And owl retinas are packed with rod cells. Recall that cone cells are adapted for color vision whereas rod cells are best for black-and-white vision. Well, owls have almost all rods in their retinas. So, they have superb night vision, but their color vision kind of sucks.

Like many mammals, owls and other nocturnal birds have a layer of shiny tissue behind the retina called the tapetum lucidum. This layer reflects more light onto the photoreceptor cells and improves night vision. This causes the eyes of these animals to shine when lit up by a car’s headlights.

In case you’re wondering: humans don’t have a tapetum lucidum. That’s why my eyes don’t shine in the light of my neighbor’s flashlight when he catches me rummaging through his trash cans.

Binocular vs monocular

One more thing to talk about is binocular versus monocular vision in birds.

Picture a chicken or a pigeon. The eyes of these birds are positioned on the sides of the head. Each eye is looking out at the world to the side of the bird’s body. Each eye sees a different image. This is monocular vision. The advantage here is that the bird has a wide field of view. It can see a large portion of its surroundings.

You’ll often see birds with monocular vision moving their heads around and switching from one eye to the other as they inspect something. This is how they must gauge the three-dimensionality, the depth of their environment.

Compare that to humans. Our eyes are both looking forward. Humans have superb binocular vision. We see the same image, or at most of it, with both eyes. This makes it easier to perceive depth. This is why predatory birds with binocular vision—such as hawks, eagles, and owls—have such great depth perception.

Our Blue-footed Boobies also have some level of binocular eyesight. This is how they can pinpoint small fish from the air and get a good sense of their target’s depth.

Often, the trade-off for having great binocular vision is having a narrower field of view. Humans, for example, have a field of view that is 180 degrees.

An interesting and extreme example of binocular and monocular vision can be seen in shorebirds in the genus Scolopax, composed of the eight species of woodcock.

Eurasian Woodcock
This Eurasian Woodcock will see you trying to sneak up on it.

The eyes of a woodcock are set way back on the bird’s head. To me, this makes woodcocks look kind of alien, almost insect-like. But these weirdos have an amazing ability: they can see in all directions at once, all 360 degrees! To the sides, they have a super wide field of monocular vision. But to both the front and the back, they have a narrow band of binocular vision. This is an amazing adaptation that allows these birds to see predators like weasels or bobcats, no matter which direction they might come from.

It’s hard to imagine what it would be like to see the world through the eyes of a woodcock. If only we could, maybe we’d finally learn to set aside our differences to live in peace and harmony.

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