Which bird has the keenest eyesight? And the eye is like an eagle's. The structure of the bird's retina

Nature has endowed birds with the most developed eyes among all living creatures. Eyes birds of prey may be equal in volume or greater than that of a person. All birds have excellent vision. A small bird, for example, a sparrow or tit, a hawk, an eagle or a falcon can be seen from a distance of more than a kilometer.

Vision is the main factor in the far and near orientation of birds. Unlike other vertebrates, among birds there is not a single species with reduced eyes. In terms of relative and absolute size, the eyes of birds are very large: in large raptors and owls they are equal in volume to the eye of an adult. Increasing the size of the eyes is beneficial because it allows you to get big sizes image on the retina and thereby distinguish its details more clearly. The relative sizes of the eyes, which differ among different species, are associated with the nature of food specialization and the method of hunting. In herbivorous geese and chickens, the mass of the eyes is approximately equal to the mass of the brain and constitutes 0.4-0.6% of the body weight; in birds of prey, the mass of the eyes is 2-3 times greater than the mass of the brain and constitutes 0.5-3% of the mass body, in owls that are active at dusk and at night, the mass of the eyes is equal to 1-5% of the body mass.

Some species that feed primarily on moving objects (daytime predators, herons, kingfishers, swallows) have two regions acute vision. Swifts have only one area of ​​acute vision, so their methods of catching prey in flight are less varied than those of swallows. A very mobile pupil prevents excessive “exposure” of the retina (during rapid turns in flight, etc.).

The structure of the eyes of birds.

The basic structures of the bird's eye are similar to those of other vertebrates. The outer layer of the eye at the front consists of a transparent cornea and two layers of sclera, a tough layer of collagen fibers. Inside the eye, the lens is divided into two main segments: anterior and posterior. The anterior chamber is filled with aqueous humor, and the posterior chamber contains the vitreous humor.

The lens is a transparent biconvex body with a hard outer and soft inner layer. It focuses light on the retina. The shape of the lens can be changed by the ciliary muscles, which are directly attached to it by means of zonular fibers. In addition to these muscles, some birds also have additional Crampton muscles that can change the shape of the cornea, thereby allowing a wider range of accommodation than in mammals. Such accommodation in divers waterfowl can be very fast. The iris is a colored muscular diaphragm in front of the lens that regulates the amount of light entering the eye. At the center of the iris is the pupil, a variable, circular opening through which light enters the eye.

The retina is a relatively smooth, curved, multilayered structure containing photosensitive rod and cone cells with associated neurons and blood vessels. Photoreceptor density is important in determining the maximum achievable visual acuity. Humans have about 200,000 receptors per mm2, the house sparrow has 400,000, and the common buzzard (bird of prey) has 1,000,000. Not all photoreceptors have an individual connection to the optic nerve; visual resolution is largely determined by the ratio of nerve ganglia to receptors. In birds, this figure is very high: the white wagtail has 100,000 ganglion cells per 120,000 photoreceptors.

Rods are more sensitive to light but do not provide color information, while the less photosensitive cones provide color vision. In diurnal birds, 80% of the receptors can be cones (up to 90% in some swifts), while in nocturnal owls the photoreceptors are represented almost exclusively by rods. Birds, like other vertebrates, with the exception of placental mammals, have double cones. In some species, such double cones can account for up to 50% of all receptors of this type.

Analysis of visual perception is carried out in the visual centers of the brain. Retinal ganglion cells respond to several stimuli: contours, color spots, directions of movement, etc. In birds, like other vertebrates, the retina has a region of sharpest vision with a depression in its center (the macula).

In the area of ​​the blind spot (the entry point of the optic nerve) there is a ridge - a folded formation rich in blood vessels, protruding into the vitreous body. Its main functions are supplying the vitreous body and inner layers of the retina with oxygen, as well as removing metabolic products. The eyes of reptiles also have a comb, but in birds it is larger and more complex. The mechanical strength of the eyes of birds is ensured by the thickening of the sclera and the appearance of bone plates in it. Many birds have well-developed movable eyelids and a developed nictitating membrane (third eyelid), which moves directly along the surface of the cornea, cleaning it.

Most birds have eyes located on the sides of their heads. The field of view of each eye is 150-170 degrees. The field of binocular vision is quite small and in many birds is only 20-30 degrees. Some birds of prey (such as owls) have eyes that move toward the beak, which increases the field of binocular vision. In some species with bulging eyes and a narrow head (some waders, ducks, etc.), the total field of view can be 360 ​​degrees, with narrow (5-10 degrees) fields of binocular vision formed in front of the beak (this makes it easier to grasp prey) and in the area the back of the head (this allows you to estimate the distance to an enemy approaching from behind). In birds with two areas of acute vision, they are usually located so that one of them projects into the area of ​​binocular vision, and the other into the area of ​​​​monocular vision.

Viewing angles.

All birds have excellent color vision, recognizing not only primary colors, but also their shades and combinations. Therefore, in the plumage of birds there are so often bright color spots that serve as species marks. Birds distinguish not only the movements of objects and their contours, but also details of shape, color, pattern, and surface textures. That is why visual perception is used by birds both to obtain a variety of information about the world around them, and how important tool during intraspecific and interspecific communication.

Birds rarely look up, because... It is more important for them to see everything that happens on earth. The structure of the bird's eyes reflects the correctness of this statement. The upper segment of the retina of birds sees better (sees the ground), and the lower segment sees worse (the lens builds an inverted image). Some birds see well both in the air and in the water (for example, the cormorant). This suggests the possibility of accommodation (changes in the refractive power of the optical system of the eye). The cormorant has the ability to change this characteristic by 4000 diopters.

Perception of contrast.

Contrast is defined as the difference in brightness between two colors divided by the sum of their brightness. Contrast sensitivity is the inverse of the smallest contrast that can be detected. For example, a contrast sensitivity of 100 means that the smallest contrast that can be seen is 1%. Birds have relatively low contrast sensitivity compared to mammals. Humans can see contrasts of 0.5-1%, while most birds require 10% contrast to produce a response. The contrast sensitivity function describes the ability of animals to detect the contrast of patterns of different spatial frequencies.

Perception of movement.

Birds see fast movements better than people, for which flickering at a speed greater than 50 Hz is perceived as continuous movement. Therefore, a person cannot distinguish individual flashes of a fluorescent lamp oscillating at a frequency of 50 Hz. The hawk is capable of swiftly pursuing prey through the forest, avoiding branches and other obstacles at high speed; For a person, such a pursuit will look like a fog.

In addition, birds are able to detect slowly moving objects. The movement of the sun and stars across the sky is invisible to humans, but obvious to birds. This ability allows migratory birds to navigate during migrations.

To obtain a clear image during flight, birds keep their heads in the most stable position, compensating for external vibrations. This ability is especially important for birds of prey.

Perception of a magnetic field.

It is believed that the perception of the magnetic field by migratory birds depends on light. Birds turn their heads to determine the direction of the magnetic field. Based on studies of neural pathways, it has been suggested that birds are able to see a magnetic field. Right eye migratory bird contains light-sensitive cryptochrome proteins. Light excites these molecules, which release unpaired electrons that interact with the Earth's magnetic field, providing directional information.

It seems to us that animals see the world in much the same way as we do. In fact, their perception is very different from that of humans. Even in birds - warm-blooded terrestrial vertebrates, like us - the senses work differently than in humans.

Vision plays an important role in the life of birds. Someone who can fly needs to navigate the flight, notice food in time, often at a great distance, or a predator (which, perhaps, can also fly and is approaching quickly). So how does bird vision differ from human vision?

To begin with, we note that birds have very large eyes. So, in an ostrich their axial length is twice that of a human eye - 50 mm, almost like tennis balls! In herbivorous birds, the eyes make up 0.2–0.6% of the body weight, and in birds of prey, owls and other birds that look out for prey from afar, the mass of the eyes can be two to three times greater than the mass of the brain and reaches 3–4% of the body weight. for owls - up to 5%. For comparison: in an adult, the mass of the eyes is approximately 0.02% of the body mass, or 1% of the mass of the head. And, for example, in a starling, 15% of the mass of the head is in the eyes, in owls - up to a third.

Visual acuity in birds is much higher than in humans - 4–5 times, in some species, probably up to 8. Vultures feeding on carrion see the corpse of an ungulate animal 3–4 km away from them. Eagles notice prey from a distance of about 3 km, large species of falcons - from a distance of up to 1 km. And the kestrel falcon, flying at an altitude of 10–40 m, sees not only mice, but even insects in the grass.

What structural features of the eyes provide such visual acuity? One factor is size: larger eyes allow larger images to be captured on the retina. In addition, the bird's retina has a high density of photoreceptors. People in the zone of maximum density have 150,000–240,000 photoreceptors per mm2, the house sparrow has 400,000, and the common buzzard has up to a million. Besides, good resolution image is determined by the ratio of the number of nerve ganglia to receptors. (If multiple receptors are connected to a single ganglion, the resolution is reduced.) In birds this ratio is much higher than in humans. For example, in the white wagtail there are about 100,000 ganglion cells for every 120,000 photoreceptors.

Like mammals, birds' retinas have an area called the fovea, a depression in the middle of the macula. In the fovea, due to the high density of receptors, visual acuity is the highest. But it is interesting that 54% of bird species - raptors, kingfishers, hummingbirds, swallows, etc. - have another area with the highest visual acuity to improve lateral vision. It is more difficult for swifts to obtain food than for swallows, including because they have only one area of ​​acute vision: swifts only see well forward, and their methods of catching insects in flight are less varied.

The eyes of most birds are located quite far from each other. The field of view of each eye is 150–170°, but the overlap of the fields of both eyes (field of binocular vision) is only 20–30° in many birds. But a flying bird can see what is happening in front of it, from the sides, behind and even below (Fig. 1). For example, the large and bulging eyes of American woodcock Scolopax minor They are located high on a narrow head, and their field of vision reaches 360° in the horizontal plane and 180° in the vertical. The woodcock has a field of binocular vision not only in front, but also behind! Very useful quality: a feeding woodcock thrusts its beak into the soft ground, looking for earthworms, insects, their larvae and other suitable food, while also seeing what is happening around. Big eyes nightjars are slightly shifted back, their field of view is also about 360°. A wide field of view is characteristic of pigeons, ducks and many other birds.

And in herons and bitterns, the field of binocular vision is shifted downwards, under the beak: it is narrow in the horizontal plane, but extended vertically, up to 170°. Such a bird, when holding its beak horizontally, can see its own paws with binocular vision. And even raising its beak upward (as a bittern does when waiting for prey in the reeds and camouflaging itself with vertical stripes on its plumage), it is able to look down, notice small animals swimming in the water and catch them with precise throws. After all, binocular vision allows you to determine the distance to objects.

For many birds, it is more important not to have a large field of view, but rather to have good binocular vision with both eyes at once. These are primarily birds of prey and owls, as they need to judge the distance to their prey. Their eyes are close-set, and the intersection of their visual fields is quite wide. In this case, the narrow overall field of view is compensated by neck mobility. Of all bird species, owls have the best developed binocular vision, and they can turn their heads 270°.

To focus the eyes on an object during rapid movement (either its own, or the object’s, or total), good accommodation of the lens is needed, that is, the ability to quickly and strongly change its curvature. Birds' eyes are equipped with a special muscle that changes the shape of the lens more effectively than in mammals. This ability is especially developed in birds that catch prey underwater - cormorants and kingfishers. Cormorants have an accommodation capacity of 40–50 diopters, and humans have 14–15 diopters, although some species, such as chickens and pigeons, have only 8–12 diopters. Diving birds are also helped to see under water by the transparent third eyelid that covers the eye - a kind of goggles for scuba diving.

Everyone has probably noticed how brightly colored many birds are. Some species - redpolls, linnets, robins - are generally dimly colored, but have areas of bright plumage. Others develop brightly colored body parts during the mating season, for example, male frigatebirds inflate a red throat sac, and puffins have a bright orange beak. Thus, even from the coloring of birds it is clear that they have well-developed color vision, unlike most mammals, among which there are no such elegant creatures. Among mammals, primates are the best at distinguishing colors, but birds are ahead of even them, including humans. This is due to some structural features of the eyes.

There are two main types of photoreceptors in the retina of mammals and birds - rods and cones. Rods provide night vision; they dominate the eyes of owls. Cones are responsible for daytime vision and color discrimination. Primates have three types (they perceive red, green and blue colors), other mammals have only two. Birds have four types of cones with different visual pigments - red, green, blue and violet/ultraviolet. And the more varieties of cones, the more shades the eye can distinguish (Fig. 2).

Unlike mammals, each cone of birds contains another drop of colored oil. These drops play the role of filters - they cut off part of the spectrum perceived by a specific cone, thereby reducing the overlap of reactions between cones containing different pigments, and increasing the number of colors that birds can distinguish. Six types of oil droplets were identified in the cones; Five of them are mixtures of carotenoids that absorb waves of varying lengths and intensities, and the sixth type lacks pigments. The exact composition and color of the droplets varies from species to species, perhaps fine-tuning vision to best suit its environment and feeding behavior.

The fourth type of cones allows many birds to distinguish ultraviolet color, invisible to humans. The list of species for which this ability has been experimentally proven has grown significantly over the past 35 years. These are, for example, ratites, waders, gulls, auks, trogons, parrots and passerines. Experiments have shown that the areas of plumage displayed by birds during courtship often have ultraviolet coloration. To the human eye, about 60% of bird species are not sexually dimorphic, meaning males and females are indistinguishable in appearance, but the birds themselves may not think so. Of course, it is impossible to show people how birds see each other, but you can roughly imagine this from photographs where ultraviolet areas are tinted with a conventional color (Fig. 3).

The ability to see ultraviolet color helps birds find food. Fruits and berries have been shown to reflect ultraviolet rays, making them more visible to many birds. And kestrels may see the paths of voles: they are marked with urine and excrement, which reflect ultraviolet radiation and thereby become visible to the bird of prey.

However, having the most better perception color among terrestrial vertebrates, birds lose it at dusk. To distinguish colors, birds need 5–20 times more light than humans.

But that is not all. Birds have other abilities that are not available to us. So, they see fast movements much better than people. We do not notice flickering at a speed greater than 50 Hz (for example, the glow of a fluorescent lamp seems continuous to us). Temporary O e visual resolution in birds is much higher: they can notice more than 100 changes per second, for example, in the pied flycatcher - 146 Hz (Jannika E. Boström et al. Ultra-Rapid Vision in Birds // PLoS ONE, 2016, 11(3): e0151099, doi: 10.1371/journal.pone.0151099). This makes it easier for small birds to hunt insects, but perhaps makes life in captivity unbearable: the lamps in the room, which, according to humans, are normally luminous, blink disgustingly for the bird. Birds are also able to see very slow movement - for example, the movement of the sun and stars across the sky, inaccessible to our naked eye. It is assumed that this helps them navigate during flights.

Colors and shades unknown to us; all-round view; switching modes from “binoculars” to “magnifying glass”; the fastest movements are clearly visible, as if in slow motion... It is difficult for us to even imagine how birds perceive the world. One can only admire their capabilities!

Vision is extremely important in the life of birds. There may be birds without a voice, but there are no birds without eyes, blind. There are no birds with underdeveloped eyes either. And there are many species of birds whose eyes are more developed than other animals of similar size. A buzzard, for example, has an eye volume approximately equal to the volume of a human eye, while a golden eagle has an eye significantly larger than a human eye. But the golden eagle is 30-40 times its weight less than a person. The weight of an owl's eyes is one-third the weight of its head.

Birds' visual acuity is amazing. The peregrine falcon sees small birds, the size of a turtledove, from a distance of more than one kilometer. Birds without a sense of smell can search for their prey by hearing or sight. The vulture spots its prey in the mountains - a fallen ungulate, sometimes from a height of two or three kilometers.

As you know, in birds the head rotates freely on the neck up to 180 and even 270 degrees. They take advantage of it. Owls especially like to turn their heads and look around. Owls cannot move their eyes from right to left; their eyeballs are tightly wedged in their sockets. And besides, their eyes, unlike other birds, are directed forward. Therefore, in the forest you sometimes see such a strange picture at first glance: an owl sits on a tree with its back to the observer, and its head is turned upside down so that its beak is directly in line with the middle of its back, and the bird’s gaze is directed straight back. This is convenient for the owl. She can, without making the slightest noise and without wasting time turning, calmly examine everything that is happening around her. Well, can a flying duck look back, especially if there is danger behind? Turning her head, the slightest distraction from the flight can mean death for her. And even a running bird cannot look back.

What to do then?

Before answering this question, let's look at how the eyes are located on the bird's head. With the exception of owls, birds' eyes are not located in front of the head, but on the sides, and birds see more to the side than to the front. Therefore, the overall field of view of birds is very large. Passerine birds and pigeons can, without moving their eyes or moving their heads, immediately cover up to 300 degrees with their vision, only one sixth of the circle remains outside the visible. Enviable horizons! Let me remind you that a person’s total field of vision is only 150 degrees.

There are also “happier” birds. In nightjars, the temporal edge of the eye is turned slightly backward and its field of vision is 360 degrees. This means that the nightjar can, without turning its head, completely freely notice what is happening in front of it, to the side and behind. Advantageous position for this bird! After all, the nightjar catches its prey, small insects, in the air. If he chases only what he notices in front, he will not be satisfied. The nightjar's flight is dexterous and nimble. What should he, having noticed prey flashing from the side or even from behind, immediately turn around and grab it with his wide mouth. To do this, you must first of all notice this prey, that is, see it both in front and behind during the flight.

But one nightjar is so lucky. Woodcock can also see what is happening behind. When feeding, it thrusts its beak into the soft ground and searches for food there by touch, forgetting, one might say, about everything around it. It’s completely inappropriate for him to look around. The lateral (and even slightly backward) position of the eyes allows him to notice the approaching danger without turning his head, without unnecessarily removing his beak from the feeding area of ​​the soil.

Not all birds need such a wide field of view. Predators have no use for it. Birds of prey tend to feed quite big catch, notice it in advance and, rushing towards it, must keep it vigilantly in their field of vision all the time. The predator's eyes are directed forward, the overall field of vision is not so large (in the kestrel, for example, 160 degrees), but their binocular vision is better developed. But, of course, binocular vision is best developed in owls. But owls are also inferior to humans in this regard.

The bird of prey does not see what is happening behind it, and it does not need to. She needs only forward and partially lateral vision. And if it is necessary to consider what is happening behind, the predator turns its head, like the owl, back, aiming its binocular vision at the object of interest to it.

The duck in this respect is the direct opposite of the hawk. It is useful for her to see what is happening behind her, and to see, so to speak, in passing, without turning her head. Here she passes greasy silt through her beak on the shore of a reservoir. There's not much to see here. Let better eyes keep an eye on what's happening behind them. A duck also needs to see from behind during flight. What if there is a predator behind? And the duck can actually notice it without turning its head. That's what a 360 degree field of view means!

In addition to the position of the eyes, the direction of the sharpest vision of each eye is of great importance in birds. This direction depends on the anatomical structure of the eyes of different species of birds and is never the same for them. The most acute visual perception in birds is usually directed laterally, beyond the limits of binocular vision, which allows a flying bird to have fields of clear vision to the right and left, but dependent on each other.

A comparison of swallows and swifts is indicative in this regard. Both feed on homogeneous food in the air - aerial plankton, and the eyes of these birds are structured differently. The swift looks mainly forward. Another thing is the swallow. Her keen visual perception is directed mainly to the side, and she perfectly notices every midge that flashes past her, whether it flies in front or from the side. The swallow's flying apparatus is such that it can immediately make a turn and grab a glimpse of prey. The swallow's flight speed is not that high, and it makes turns on the spot very easily. A swift cannot make a turn on the spot; it flies too quickly. Due to the peculiarities of its vision, the swift simply will not notice the midge that is behind, it only catches what is in front. Which method of hunting is “more profitable”? As long as there is a lot of air plankton in the air, it makes absolutely no difference. But when there is less food in the air, the swift is the first to find itself in a difficult situation. The fact that he “plows” his beak in the air in a straight line is no longer enough for him. Possible food to the right and left of it is hidden due to the peculiarities of vision. The swallow excellently gets out of the situation, turning behind every midge that flashes from the side. Moreover, she can even, flying along a sun-warmed rock or the wall of a house, scare off insects with her wing and immediately grab them. Therefore, the swift cannot stay with us for long until autumn, but the swallow can. Birds don't look up much. For them, the main thing is what happens on earth. This also affects the structure of their eyes. In the retina of diurnal birds, its upper segment, the one that perceives rays coming from the ground, is more saturated with so-called bipolar cells and ganglia; simply put, it sees better, while the lower segment, reflecting the sky, is depleted of these formations . So the bird, if it needs to take a closer look at what is happening in the sky (say, whether a predator is flying), throws its head on its back and looks up in this position.

What do the bird’s eyes reflect, do they have “expression”? The hawk has light yellow eyes, they leave an unpleasant impression, it seems that the hawk has an evil character. However, this is not a matter of character at all, it’s just that this predator’s iris is yellow, and its eyes do not express anything at all. The eyes of old cormorants glow deeply green tone and they also don’t express anything. All this is the external design of the eyes, not related to how the bird behaves.

Some bird species need to see well in different environments. Merganser, for example, and cormorant see well in the air and no worse in water. This requires an increased ability to accommodate. Indeed, a cormorant is capable of changing the refractive power of the eye by 40-50 diopters, while a person is only able to change it by 14-15 diopters. But owls have a very insignificant ability to accommodate, some 2-4 diopters. As a result, they apparently cannot see anything in their immediate vicinity.

The question is sometimes asked whether birds have color vision. The answer to this question suggests itself. Why then do birds bright colors, why the colorful and often very original colors? Observations show that many details of a bird's plumage have a signaling value for them and are perfectly perceived by them. Another thing is whether birds see colors exactly as humans see them. This remains unclear. But, apparently, the bird’s eyes do not have any special differences in this regard. Birds can sometimes be trained for colors, for example.

We humans are confident that our visual system is perfect. It allows us to perceive space in three dimensions, notice objects at a distance and move freely. We have the ability to accurately recognize other people and guess their facial emotions. In fact, we are such “visual” creatures that it is difficult for us to imagine the sensory worlds of animals with other abilities that are not available to us - for example, a bat, a nocturnal hunter that detects small insects based on the echoes of high-frequency sounds it makes.

It is quite natural that our knowledge of color vision is based mainly on own experience: It is easy for researchers to conduct experiments with subjects willing to answer things like which color mixtures look the same and which look different. Despite the fact that neuroscientists, by recording the discharge of neurons, confirmed the information obtained for a number of species of living beings, still until the beginning of the 70s. In the last century, we were unaware that many non-mammalian vertebrates see colors in a part of the spectrum that is invisible to humans - in the near ultraviolet (UV).

The discovery of ultraviolet vision began with studies of insect behavior by the eminent Englishman Sir John Lubbock, Lord Avebury, friend and neighbor of Charles Darwin, member of parliament, banker, archaeologist and naturalist. In the early 1880s. Lubbock noticed that in the presence of UV radiation, ants move their larvae to darker areas or those illuminated by longer wavelengths of light. Then in the mid-1900s. Austrian naturalist Karl von Frisch proved that bees and ants not only see ultraviolet as a separate color, but also use it as a kind of celestial compass.

Many insects also perceive ultraviolet light; According to research over the past 35 years, birds, lizards, turtles and many fish have UV receptors in the retina. Why then are mammals not like everyone else? What causes the impoverishment of their color perception? The search for an answer has revealed a fascinating evolutionary history and led to new understanding of the extremely rich visual world of birds.

How did color vision develop?

To better understand the essence of the discoveries, it is first worth getting acquainted with some basic principles of color vision. First of all, it is necessary to abandon one common misconception.

Indeed, as we were taught in school, objects absorb light with certain wavelengths and reflect the rest, and the colors we perceive are related to the wavelengths of the reflected light. However, color is not a property of light or objects that reflect it, but a sensation born in the brain.

Color vision in vertebrates is due to the presence of cones in the retina, a layer of nerve cells that transmit visual signals to the brain. Each cone contains a pigment consisting of a type of opsin protein bound to a molecule of a substance called retinal, which is closely related to vitamin A. When the pigment absorbs light (more precisely, individual bundles of energy called photons), the energy it receives makes the retinal change their shape, which triggers a cascade of molecular transformations that activate the cones, and after them the retinal neurons, one type of which sends impulses along the optic nerve, transmitting information about the perceived light to the brain.

The stronger the light, the more photons are absorbed by the visual pigments, the stronger the activation of each cone, and the brighter the perceived light appears. However, the information coming from a single cone is limited: it cannot tell the brain what the wavelength of the light that triggered it is. Light wavelengths of different wavelengths are absorbed differently, and each visual pigment has a specific spectrum that shows how light absorption varies with wavelength. The visual pigment can equally absorb light of two different wavelengths, and although the photons of light will carry different energies, the cone will not be able to distinguish between them, since both cause a change in the shape of the retinal and thus trigger the same molecular cascade leading to activation. The cone can only read absorbed photons; it cannot distinguish one wavelength of light from another. Therefore, the cone can be activated equally by strong light of a relatively poorly absorbed wavelength and by dim light of a well-absorbed wavelength.

In order for the brain to see color, it must compare the responses of several classes of cones containing a variety of visual pigments. Having more than two types of cones in the retina allows for better color discrimination. Opsins, which distinguish some cones from others, have provided us with a good opportunity to study the evolution of color vision. Researchers can determine the evolutionary relationship of opsins in various classes cones and in all kinds of species, by studying the sequence of nucleotide bases (the “alphabet” of DNA) in the genes encoding these proteins. The result is family tree, indicating that opsins are very ancient proteins that existed before the appearance of the main groups of animals that inhabit the Earth today. We can trace four lineages in the development of vertebrate cone pigments, named descriptively for the region of the spectrum to which they are most sensitive: long-wavelength, mid-wavelength, short-wavelength and ultraviolet.

HUMAN COLOR VISION

Humans and some primates see colors through the interaction of three types of cones in the retina. Each type contains a different pigment that is sensitive to a specific range of light wavelengths. Three types of cones have the greatest sensitivity - about 560, 530 and 424 nm.

The two thin vertical lines on the graph indicate the different wavelengths of light absorbed equally by pigment 560. Although photons from light rays with a wavelength of 500 nm (blue-green light) carry more energy than photons with a wavelength of 610 nm (orange light), both cause the same pigment reaction and, accordingly, the same activation cones. Thus, a single cone cannot tell the brain the wavelength of light it absorbs. To distinguish one wavelength from another, the brain must compare signals from cones with different visual pigments.

In addition to cones, all major groups of vertebrates also have rods in their retinas, which contain the visual pigment rhodopsin and provide the ability to see in very low light. Rhodopsin is similar in structure and spectral absorption characteristics to cone pigments, which are most sensitive to wavelengths in the middle of the visual spectrum. It evolved from such pigments hundreds of millions of years ago.

Birds possess four cone pigments with different spectral characteristics, one from each lineage. Mammals, on the other hand, usually have only two such pigments: one of them is especially sensitive to violet light, and the other to long-wavelength light. Why were the animals deprived? Probably the fact is that in the early stages of development, during the Mesozoic period (from 245 to 65 million years ago), they were small animals, leading secretive night look life. As their eyes got used to seeing in the dark, everything higher value acquired highly sensitive rods, and the role of color vision decreased. Thus, the animals have lost two of the four cone pigments that their ancestors possessed and which were preserved in most reptiles and birds.

When the dinosaurs became extinct 65 million years ago, mammals were given new opportunities to specialize, and their diversity began to increase rapidly. Representatives of one group, which included the ancestors of humans and other living primates, switched to a diurnal lifestyle, climbed trees, and fruits became an important part of their diet. The colors of flowers and fruits often make them stand out from foliage, but mammals, with their single cone pigment for long-wavelength light, would not be able to distinguish contrasting colors in the green, yellow and red parts of the spectrum. However, evolution had already prepared a tool that helped primates cope with the problem.

Occasionally, during the formation of eggs and sperm during cell division, due to unequal exchange of chromosome sections, gametes with chromosomes containing additional copies of one or more genes arise. If such additional copies are preserved in subsequent generations, then natural selection can fix beneficial mutations that arise in them. According to Jeremy Nathans ( Jeremy Nathans) and David Hogness ( David Hogness) from Stanford University, something similar happened over the past 40 million years in the visual system of the ancestors of primates. Unequal exchange of DNA in germ cells and subsequent mutation of an additional copy of the gene encoding a pigment sensitive to long-wave light led to the appearance of a second pigment, the region of maximum sensitivity of which was shifted. Thus, this branch of primates differs from other mammals in that it has not two, but three cone pigments and trichromatic color vision.

Although the new acquisition significantly improved the visual system, it still did not give us the quintessential perception of the world around us. Our sense of color bears traces of correction of an evolutionary error; it lacks one more pigment before tetrachromatic visual system birds, many reptiles and fish.

We are genetically deficient in yet another way. Both of our genes for pigments sensitive to the long-wavelength part of the spectrum lie on the X chromosome. Since males have only one, a mutation in any of these genes can make it difficult for an individual to distinguish between red and green colors. Females are less likely to suffer from this disorder because if a gene is damaged on one X chromosome, the pigment can still be produced according to instructions contained in a healthy gene on the other X chromosome.

OVERVIEW: EVOLUTIONARY HISTORY
Color vision in vertebrates depends on cells in the retina called cones. Birds, lizards, turtles and many fish have four types of cones, but most mammals have only two.
The ancestors of mammals had full set cones, however, they lost half during that period of their evolution when they were predominantly nocturnal and color vision was not of much importance to them.
The ancestors of primates, which includes humans, again acquired a third type of cones due to a mutation in one of the two existing ones.
Most mammals, however, have only two types of cones, making their color perception quite limited compared to the visual world of birds.

Avian supremacy

Analyzing DNA modern species animals, researchers were able to peer back into time and determine how cone pigments changed during the evolution of vertebrates. The results show that early in their development they had four types of cones (colored triangles), each containing a different visual pigment. Mammals, at a certain stage of evolution, lost two of the four types of cones, which was probably due to their nocturnal lifestyle: in low light, cones are not needed. Birds and most reptiles, on the contrary, have retained four cone pigments with different absorption spectra. After the dinosaurs went extinct, the diversity of mammals began to increase rapidly, and one of the lines of evolution that led to today's primates is African monkeys and humans - again acquired the third type of cones due to duplication and subsequent mutation of the gene for one of the remaining pigments. Therefore, we, unlike most mammals, have three types of cones (instead of two) and trichromatic vision, which, of course, has become some progress, but cannot be compared with the rich visual world of birds.

Early in their evolution, mammals lost more than just their cone pigments. Each cone of a bird's or reptile's eye contains a colored drop of fat, but mammals have nothing similar. These clumps, which contain high concentrations of substances called carotenoids, are arranged in such a way that light must pass through them before hitting the stack of membranes in the outer segment of the cone, where the visual pigment is located. Fat droplets act as filters, not transmitting short-wavelength light and thereby narrowing the absorption spectra of visual pigments. This mechanism reduces the degree of overlap between the spectral sensitivity zones of pigments and increases the number of colors that a bird can theoretically distinguish.

IMPORTANT ROLE OF FAT DROPS IN CONES

The cones of birds and many other vertebrates have retained several features lost to mammals. The most important of these for color vision is the presence of colored droplets of fat. Bird cones contain red, yellow, almost colorless and transparent droplets. In a micrograph of a chickadee's retina, yellow and red spots are clearly visible; Several colorless drops are circled in black. All droplets, except transparent ones, serve as filters that do not transmit light with short wavelengths.
This filtering narrows the areas of spectral sensitivity of three of the four types of cones and shifts them to the part of the spectrum with longer wavelengths (graph). By cutting off some of the wavelengths that cones respond to, the fat droplets allow birds to distinguish more colors. Ozone in upper layers The atmosphere absorbs light with a wavelength shorter than 300 nm, so birds' UV vision only works in the near ultraviolet - in the range from 300 to 400 nm.

Testing color vision in birds

The presence of four types of cones containing different visual pigments strongly suggests that birds have color vision. However, such a statement requires a clear demonstration of their abilities. Moreover, during the experiments, other parameters (for example, brightness) that birds could use should be excluded. Although researchers have conducted similar experiments before, they have only begun to study the role of UV cones in the last 20 years. My former student Byron K. Butler and I decided to use color matching to understand how the four types of cones contribute to vision.

To understand how the comparison occurs various shades, first let's look at our own color vision. Yellow light activates both types of cones that are sensitive to long-wavelength light. Moreover, it is possible to select a combination of red and green that excites the same two types of cones to the same extent, and the eye will see such a combination as yellow (as well as pure yellow light). In other words, two physically different lights can be the same color (confirming that the perception of color originates in the brain). Our brain distinguishes colors in this part of the spectrum by comparing the signal from two types of cones that are sensitive to long-wavelength light.

Armed with knowledge physical properties four types of cones and fat droplets, Butler and I were able to calculate which combination of red and green would be the same shade as the yellow we had chosen in the birds' perceptions. Because the visual pigments of humans and birds are not identical, the given color range is different from what a human would perceive if we asked him to make the same comparison. If the birds respond to colors as we hypothesize, this will confirm our measurements of the properties of visual pigments and fat droplets and allow us to continue our research to determine whether and how UV cones are involved in color vision.

For our experiments we chose Australian budgies (Melopsittacus undulatus). We trained the birds to associate a food reward with a yellow light. Our subjects sat on a perch from which they could see a pair of light stimuli located a meter away from them. One of them was just yellow color, and the other arose as a result of various combinations of red and green. During the test, the bird flew to the light source where it expected to find food. If it headed towards the yellow stimulus, then the feeder with grain was opened for a short period of time, and the bird had the opportunity to have a light snack. Another color did not promise her any reward. We varied the combination of red and green in an irregular sequence and alternated the location of both stimuli to prevent the parrots from associating food with the right or left side. We also varied the light intensity of the sample stimulus so that luminance could not serve as a cue.

We tried many combinations of red and green, but the birds easily chose the yellow sample and received grains as a reward. But when the parrots saw light that was approximately 90% red and 10% green (and according to our calculations, this proportion should be the same shade as yellow), they became confused and made a random choice.

Confident that we could predict when colors matched in birds' perceptions, we attempted to similarly demonstrate that UV cones contribute to tetrachromatic color vision. In the experiment, we trained birds to obtain food where there was a violet stimulus and studied their ability to distinguish this wavelength from a mixture of blue light and light of different wavelengths in the near-UV range. We found that winged participants could clearly distinguish natural violet light from most imitations. However, their selection dropped to random levels when mixing 92% blue and 8% UV - the very proportion that, according to our calculations, should make the color scheme indistinguishable from violet. This result means that light in the UV range is perceived by birds as an independent color and that UV cones contribute to tetrachromatic vision.

Beyond Human Perception

Our experiments showed that birds use all four types of cones for color vision. However, it is virtually impossible for humans to understand how they perceive color. Birds not only see in the near ultraviolet, but can also distinguish colors that we cannot even imagine. As an analogy, our trichromatic vision is a triangle, but their tetrachromatic vision requires an additional dimension and forms a tetrahedron, or three-sided pyramid. The space above the base of the tetrahedron contains all the variety of colors that lie beyond the limits of human perception.

How can winged creatures benefit from such a wealth of color information? In many species, males are much brighter colored than females, and when it became known that birds perceive UV light, experts began to study the influence of ultraviolet colors, invisible to humans, on the choice of sexual partners in birds. In a series of experiments Muir Eaton ( Muir Eaton) from the University of Minnesota studied 139 species of birds in which both sexes look the same, according to humans. Based on measurements of the wavelength of light reflected from the plumage, he concluded that in more than 90% of cases, the bird's eye sees a difference between males and females, which ornithologists had not previously realized.

This video clearly illustrates what budgerigars look like in ultraviolet color. We can only imagine how parrots themselves see themselves, but one of the consequences of having vision in the ultraviolet spectrum is budgies is greater reproductive success in birds of a natural green color; given a choice, female parrots prefer males with a larger area of ​​plumage reflecting the UV spectrum.

Introducing the ultraviolet world

Despite the fact that no one knows what the surrounding reality looks like for birds, photographs of thunbergia flowers allow us to at least remotely imagine how much UV light could change the world we see. For us, there is a small black circle in the center of the flower (on the left). However, a camera equipped to shoot in UV light alone "sees" a completely different picture, including a much wider dark spot in the center (right)

Franziska Hausmann ( Franziska Hausmann) studied males of 108 Australian bird species and found that colors with a UV component are most often found in decorative plumage, which is used in courtship displays. Interesting data were obtained by scientific groups from England, Sweden and France while studying blue tits ( Parus caeruleus), Eurasian relatives of North American chickadees, and common starlings ( Sturnus vulgaris). It turned out that females prefer those gentlemen whose plumage reflects more UV rays. The fact is that the reflection of UV light depends on the submicroscopic structure of the feathers, and therefore can serve as a useful indicator of health status. Amber Keyser of the University of Georgia and Jeffrey Heal of Auburn University found that those male blue guiraki, or blue greatbills, Guiraca caerulea), which have plumage that is more saturated, bright blue color, shifted to the UV region, turn out to be larger, control larger territories rich in prey, and feed their offspring more often than other individuals.

Video showing the plumage of a caique and an owl in the ultraviolet spectrum.

The presence of UV receptors may give an animal an advantage in obtaining food. Dietrich Burkhardt of the University of Regensburg in Germany noticed that the waxy surfaces of many fruits and berries reflect UV rays, making them more visible. He discovered that kestrels were able to see the paths of voles. These small rodents create odorous trails marked with urine and excrement that reflect ultraviolet light and become visible to the kestrel's UV receptors, especially in the spring when the marks are not hidden by vegetation.

People unfamiliar with such intriguing discoveries often ask me, “What gives birds ultraviolet vision?” They consider this feature to be some kind of quirk of nature, without which any self-respecting bird could live quite happily. We are trapped by our own feelings and, understanding the importance of vision and afraid of losing it, we still cannot imagine a picture of the visible world that is more picturesque than our own. It is humbling to realize that evolutionary perfection is deceptive and elusive, and that the world is not quite as we imagine it to be when viewed through the lens of human self-importance.

A VIRTUAL LOOK INTO THE VISUAL WORLD OF BIRDS

The space of human color vision can be depicted as a triangle. The colors of the spectrum that we see are located along the thick black curve inside it, and the entire variety of other shades obtained by mixing is located below this line. To represent a bird's color vision, we need to add another dimension, and the result is a three-dimensional body, a tetrahedron. All colors that do not activate UV receptors lie at its base. However, since the fat droplets in the cones increase the number of colors that birds can distinguish, the spectrum they perceive does not form a figure reminiscent of a shark's fin, but is located along the very edges of the triangular base. Colors, in the perception of which UV receptors are involved, fill the space above the base. For example, the red, green, and blue plumage of the painted bunting (Passerina ciris) reflects varying amounts of ultraviolet light in addition to the colors we see.

To graphically imagine what colors the female cardinal sees when she looks at her partner, we must go out of the plane of the triangle into the volume of the tetrahedron. The colors reflected from small areas of the plumage are represented by clusters of dots: bright red for the breast and neck, darker red for the tail, green for the back and blue for the head. (We cannot, of course, show the colors that a bird sees, since no human being is capable of perceiving them.) The more UV in a color, the higher the points are located above the base. The dots in each cluster form a cloud because the wavelength of the reflected light varies within the same area, and we humans can see this too when we look at the red areas on the chest and throat.

Proof of UV vision in birds

Do birds see ultraviolet as an independent color? In his experiment, the author proved the truth of this statement. Researchers trained budgies to distinguish violet light from a combination of blue and UV light. When the combination contained only about 8% UV, the birds could no longer distinguish it from the control pure color and often made mistakes. Their choice dropped to a random level at the point (arrow) at which the colors should have matched according to the author’s calculations, based on measurements of the characteristics of visual pigments and fat droplets in the cones of birds’ eyes.

Timothy H. Goldsmith is a professor of molecular and cellular biology at Yale University and a member of the American Academy of Arts and Sciences. For 50 years he studied the vision of crustaceans, insects and birds. He is also interested in the evolution of human mind and behavior. Author of the book Biology, Evolution, and Human Nature.

ADDITIONAL LITERATURE
1. The Visual Ecology of Avian Photoreceptors. N.S. Hart in Progress in Retinal and Eye Research, Vol. 20, No. 5, pages 675–703; September 2001.
2. Ultraviolet Signals in Birds Are Special. Franziska Hausmann, Kathryn E. Arnold, N. Justin Marshall and Ian P. F. Owens in Proceedings of the Royal Society B, Vol. 270, No. 1510, pages 61–67; January 7, 2003.
3. Color Vision of the Budgerigar (Melop-sittacus undulatus): Hue Matches, Tetrachromacy, and Intensity Discrimination. Timothy H. Goldsmith and Byron K. Butler in Journal of Comparative Physiology A, Vol. 191, No. 10, pages 933–951; October 2005.

Vision is the main receptor for far and near orientation in birds. Unlike other vertebrates, there is not a single species among them with reduced eyes. The eyes are very large in relative and absolute size: in large raptors and owls they are equal in volume to the eye of an adult. Increasing the absolute size of the eyes is beneficial because it allows one to obtain larger image sizes on the retina and thereby more clearly distinguish its details. The relative sizes of the eyes, which differ among different species, are associated with the nature of food specialization and hunting methods. In predominantly herbivorous geese and chickens, the mass of the eyes is approximately equal to the mass of the brain and constitutes 0.4-0.6% of the body mass; in birds of prey that catch mobile prey and look out for it at long distances, the mass of the eyes is 2-3 times greater than the mass of the brain and makes up 0.5-3% of body weight; in owls active at dusk and at night, the eye mass is equal to 1-5% of body weight (Nikitenko M.F.).

In different species, per 1 mm2 of the retina there are from 50 thousand to 300 thousand photoreceptors - rods and cones, and in the field of acute vision - up to 500 thousand - 1 million. With different combinations of rods and cones, this allows either to distinguish many details of an object, or its contours in low light. The main analysis of visual perceptions is carried out in the visual centers of the brain; retinal ganglion cells respond to several stimuli: contours, spots of color, directions of movement, etc. In birds, like other vertebrates, the retina has a region of sharpest vision with a depression (fovea) in its center.

Some species that feed primarily on moving objects have two areas of acute vision: diurnal predators, herons, kingfishers, swallows; Swifts have only one area of ​​acute vision, and therefore their methods of catching prey in flight are less varied than those of swallows. The cones contain oil drops - colored (red, orange, blue, etc.) or colorless. They probably act as light filters that increase the contrast of the image. A very mobile pupil prevents excessive illumination of the retina (during rapid turns in flight, etc.).

Accommodation (focusing the eye) is carried out by changing the shape of the lens and its simultaneous movement, as well as by some changing the curvature of the cornea. In the area of ​​the blind spot (the entry point of the optic nerve) there is a ridge - a folded formation rich in blood vessels, protruding into the vitreous body (Fig. 60, 13). Its main function is to supply the vitreous body and inner layers of the retina with oxygen and remove metabolic products. The comb is also present in the eyes of reptiles, but in birds, apparently due to the large size of the eyes, it is much larger and more complex. The mechanical strength of the large eyes of birds is ensured by the thickening of the sclera and the appearance of bone plates in it. Movable eyelids are well developed, and in some birds they bear eyelashes. A nictitating membrane (third eyelid) is developed, moving directly along the surface of the cornea, cleaning it.

Most birds have eyes located on the sides of their heads. The field of vision of each eye is 150-170*, but the field of binocular vision is small and in many birds is only 20-30*. In owls and some birds of prey, the eyes shift toward the beak and the field of binocular vision increases. In some species with bulging eyes and a narrow head (some waders, ducks, etc.), the total field of view can be 360 ​​*, while narrow (5-10 *) fields of binocular vision are formed in front of the beak (makes it easier to grasp prey) and in the back of the head (allows you to estimate the distance to an enemy approaching from behind). In birds with two areas of acute vision, they are usually located so that one of them projects into the area of ​​binocular vision, and the other into the area of ​​​​monocular vision (