The Science of Birds - Vision in Birds

Episode Date: September 22, 2020

Episode: 7SummaryIt can be argued that, of all the animals, birds are the best at seeing stuff. Most species have an excellent sense of sight.In this episode, I’ll first introduce you to the anatomy of a bird’s eye.Then, we’ll look into (see what I did there?) how birds perceive color and their visual acuity.And last we’ll talk about the difference between monocular and binocular vision in birds.Research CitationsWild hummingbirds and ultraviolet colors (Stoddard et al. 2020. PNAS)Seemingly monochromatic birds are more colorful under UV light (Eaton. 2005. PNAS)‍Link to this episode on the Science of Birds websiteSupport the show

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 missed their targets, they will often chase fish underwater by flapping their wings, penguin style. I was told by our Ecuadorian guide that Abubi'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, then focus

on it and accurately gauge its depth and position. Then, when it dives down at high speed, its eyes need to be protected from the impact with the water. If it has had to be protected, it has any hope of catching its fishy prey, the bird needs to be able 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 today's episode, we'll talk about the adaptations of booby eyes and many other cool things about the vision of birds.

Hello and welcome. This is the Science of Birds. I'm your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode is all about the sense of sight in birds. The main topics here, in order are eye anatomy, color vision, visual acuity, and binocular versus monocular vision. Okay, let's dive in. 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 be able 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 a sensual tools for survival. Senses helped our ancestors gather data about the environment to avoid dangers and locate vital resources. And of course, 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 slash 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, do have an excellent sense of sight. It can be argued that, of all the animals,

birds are the best at seeing stuff. Birds are, by and large, very visual animals. 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 cruising along, perfecting their eyesight the whole time. We mammals have yet to catch up to them. 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 a lot of evolutionary times. 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 actually has the largest eye of any land animal, not in the relative sense,

but in the absolute sense. So an ostrich eye is five 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. 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 eyes resolving power actually 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 of 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. Now let's return to the blue-footed booby that we started the episode with. Boobies are cousins to the cormorants. These guys all belong in the suleformis 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 semi-luner fold. 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 phobia. and it provides the highest resolution images. Unlike humans, some birds like raptors, kingfishers, and hummingbirds have a second fovea in each eye.

One phobia provides acute forward-facing vision, the other helps make out images to the side of the bird's head. One of the coolest things about bird retinas is that they have four different types of cone cells instead of only three as in humans. That fourth cone is sensitive to ultraviolet wavelengths of light, which humans can't see. The so-called tetra-chromatic 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 of their evolutionary history. 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, grange 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 rodent prey, voles, by looking for trails of vol urine, which supposedly reflect UV light. More recent research has cast some doubt on this, but the idea of fluorescent volpe 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 ultraviolet 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 blue throats. 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 a type of ant bird living in the Amazon basin called the black-spotted bear 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 UV light was taken 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 are called non-spectral colors that include an ultraviolet component. These hummers quickly learn to associate artificially generated non-spectral 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 the presence of microscopic oil droplets in the cone cells of the retina. This oil absorbs certain wavelengths of light

and actually 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. 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. In general, 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 eighth line down while standing 20 feet from the chart. Some birds, like eagles, are said to have the equivalent of 25 vision. That means that an eagle could make out details of an object 20 feet away, that a normal human could only discern at five 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. In general, you can think of birds as having between two and eight 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 steady and

continuous to us. But at least some birds can detect movements of more than 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, due to the anatomy of their lenses and associated muscles. That's one thing. But perhaps the main 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 one 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 best. More photoreceptor cells means 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.

Diornal 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 cone cells in its retina. Now let's not forget about owls and other birds of the night. Besides owls, nocturnal birds include night jars, potus, night herons, oil birds, kiwis, and more. Kiwis are adorable, but they have terrible 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 big eyes to gather as many photons as possible, and they can have exceptional night vision. Owls are the best known example, of course. They have relatively massive eyes which are 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 tapidum lucidum. Tepidum lucidum. This layer reflects more light onto the photoreceptor cells and improves night vision. This is what 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 tepidum 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.

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 of 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 really good binocular vision, where the same image is seen, or at least part of it is seen, by both eyes.

This makes it easier to perceive depth. This is why predatory birds with binocular vision have such great depth perception. This is the case for hawks, eagles, and owls. Our blue-footed boobies also have some level of binocular eyesight. This is how they are able to pinpoint small fish from the air and get a good sense of their target's depth. Often, the trade-off for having really great binocular vision is having a narrower field of view. Humans, for example, have a field of view that is about 180 degrees.

A really 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. 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. So 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 which allows these birds to see predators like weasels or bobcats, no matter which direction they might be coming 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. Okay, to briefly summarize what we've learned about the amazing eyesight of birds. They can see many more colors than humans can, and that's because they have four types of cone cells in their retinas instead of only three. Some of the colors they see are in the ultraviolet range of the spectrum. birds also have higher visual acuity or sharpness than us. That's because their retinas are jam-packed with cones at a much higher density than in human retinas.

And lastly, many birds have primarily monocular vision with eyes that face out to the sides, but raptors, owls, and some other birds have great binocular vision and can perceive depth as well as or better than humans. Thank you so much for listening and learning. with me today. With each episode, I hope to make the Science of Birds podcast better and better. I'd love to hear any thoughts or comments you have about the show. Just let me know by sending me an email to Ivan at Scienceofbirds.com. Ivan at Scienceofbirds.com. And hey, if you love birds and you want to learn more about their biology, please subscribe to the podcast to hear more content like this. You can also

see the show notes for this episode, which is episode 7, on the Science of Birds website, scienceofbirds.com. On the website, you can subscribe to our email newsletter, which would be super duber. You can do that again at scienceofbirds.com. I'm Ivan Philipson, and I'll catch you next time. Peace.