Articles: The Avian Respiratory System; Raptor Vision, Parts I & II – some of the graphics in these articles are adapted from textbooks and reference articles.
The Avian Respiratory System
The avian respiratory system, like all others, delivers oxygen from the air to the tissues and removes carbon dioxide, and plays a role in maintaining normal body temperature. It is different from that of other vertebrates, however, in several important ways. Birds have relatively small, rigid lungs and possess nine (or more) air sacs in the body cavity. In addition, their skeletal system has pockets, or sacs, of air. The whole design is geared to reducing weight and increasing buoyancy and oxygenation – adaptations that, like nearly all avian characteristics, make flight more efficient.
Immature Golden Eagle Nares
Respiratory Structure: Lungs and Air Sacs This interconnected system permits a constant unidirectional flow of air through the lungs, so that the air in the lungs is constantly refreshed and has a high oxygen content. In contrast, in mammals, air flow is bidirectional, moving into and out of the lungs. Air entering a mammal’s lungs on an in-breath is mixed with air that has been in the lungs for a while and has less oxygen.
The bird’s flexible air sacs act like bellows, moving air through almost completely rigid lungs, where gas exchange takes place. The sacs do not directly participate in gas exchange, but make it more efficient, which make possible the high metabolic rates found in birds.
In addition, some of the air sacs integrate with the skeleton, forming air pockets in otherwise dense bone. There are two humeral air sacs, and several other smaller air sacs within other bones. These pneumaticized bones serve to lighten the skeletal structure, giving birds buoyancy in flight. (For those curious about the presence of marrow and the production of red blood cells, as in mammals – the bird femur, a large bone, is marrow rich, and there are other smaller marrow bones throughout the body, to take care of the production of red blood cells.)
Most birds have nine main air sacs: one interclavicular sac; two cervical sacs; two anterior thoracic sacs; two posterior thoracic sacs; two abdominal sacs.
Typical Avian Air Sac System
How Birds Breathe
Birds breathe through the mouth and the nostrils (nares). Air entering during inspiration passes into the trachea (or windpipe), which usually extends the length of the neck. Some birds, such as cranes, have an exceptionally long trachea that coils within the hollowed keel of the breastbone. This arrangement gives additional resonance to their loud calls.
The trachea splits into two primary bronchi (or tubes) at the syrinx. The syrinx is unique to birds and is their “voice box” (in mammals, sounds are produced in the larynx. The process by which the syrinx produces sounds is not covered here.) The primary bronchi enter the lungs and are then called mesobronchi. Branching off from the mesobronchi are smaller tubes called ventrobronchi. The ventrobrochi, in turn, lead into the still smaller parabronchi, whose walls contain hundreds of tiny branching and interconnecting “air capillaries” surrounded by a profuse network of blood capillaries.
During inhalation, air enters first into the posterior air sacs and lungs and, simultaneously, air moves out of the lungs and into the anterior air sacs.
During exhalation, the air sacs diminish in volume as air moves (1) from the posterior air sacs through the lungs and (2) from the anterior air sacs out of the body via the trachea.
Here’s how it works: Air flows through the avian lungs and air sacs during respiration in the following manner:
1 – On first inhalation, air flows through the trachea and bronchi, primarily into the posterior (rear) air sacs.
2 – On exhalation, air moves from the posterior air sacs into the lungs.
3 – With the second inhalation, air moves from the lungs into the anterior (front) air sacs.
4 – With the second exhalation, air moves from the anterior air sacs back into the trachea and out.
Birds do not have a diaphragm. Instead, the entire skeleto-musculature system works to move air in and out of the respiratory system.
During inspiration: The sternum moves forward and downward while the vertebral ribs move cranially (toward the head) to expand the sternal ribs and the thoraco-abdominal cavity. This expands the posterior and anterior air sacs (see Item 1 above) and lowers the pressure, causing air to move into those air sacs.
Air from the trachea and bronchi moves into the posterior air sacs and simultaneously, air from the lungs moves into the anterior air sacs.
During expiration: The sternum moves backward and upward and the vertebral ribs move caudally (toward the tail) to retract the sternal ribs and reduce the volume of the thoraco-abdominal cavity. This reduces the volume of the anterior and posterior air sacs, causing air to move out of those sacs.
Air from the posterior sacs moves into the lungs and simultaneously, air from the anterior sacs moves into the trachea and out of the body.
Complicated! But not beyond grasp! Note that it takes two respiratory cycles to move one “packet” of air completely through the bird (see Items 1, 2, 3, and 4 above). The advantage is that air moves unidirectionally through the system, providing more oxygen, that is, energy, for powered flight.
Avian Respiratory System: Illustration from Fox, Nick. Understanding the Bird of Prey, 1995
March 16, 2013
Raptor Vision I: How Hawks See
The Art of the Eagle Eye
What do we mean when we say someone is “Hawk-eyed” or “an Eagle Eye”?
“A Golden Eagle could read The New York Times across a football field!”
“A Red-tailed Hawk can see a mouse a quarter of a mile away!”
Tall talk? Probably. But raptors’ vision is indeed extraordinary and vital to their success. And while the image of an eagle in spectacles reading the newspaper is ridiculous, it illustrates perfectly the problems we run into when we interpret the bird’s capacities in light of our own. We are limited in our understanding of other animals by the way in which we ourselves perceive the world.
Still, we have little choice. To explore how another species perceives anything, we have to tread a fine line between comparing their experiences with our own and winkling out hints of their strange, even alien, lives.
Eagle Eyes: “An eagle could read the newspaper across a football field!”
Without keen eye-sight, birds would find high-speed flight and successful day-time hunting impossible. Raptors need exquisite precision in both detail resolution and movement detection in order to feed themselves. Golden Eagles soar hundreds of feet over the earth, spot their prey – sometimes a rabbit, sometimes an animal as large as a small deer – and fly it down in a powerful and fast flight. How do they do it? Leaving aside the mechanics of flight for now, how can they see what they’re doing?As we tackle the specific mechanism of the eagle’s eye, we’ll explore some aspects of avian vision in general. Just remember that birds’ eyesight is so different from ours and our inter-species communication is so limited, we’ll never really see what they see.
Birds’ Eye View
Birds share qualities of the eye with both reptiles and mammals. Birds and reptiles have the capacity to change the shape of the lens rapidly and to a greater extent than mammals. And like reptiles, most birds’ eyes are not round, but flattened, which makes the eye shallow and gives a broad visual field. Raptors and some other birds needing heightened acuity, however, have a globular or even tubular shape of eye, which increases the depth of the orb, which in turn increases acuity but reduces the field. Birds and reptiles have more receptor cones in the retina than mammals, and some, diurnal raptors included, possess dual focal points or foveae. (See Part 2 for receptors and foveae.)
Like mammals, on the other hand, the birds’ lens is more forward in the orb, which increases the size of the image on the retina and this, too, increases acuity.
Differences in Eye Structure Reflect Differences in Life Styles
External Eye Structure
Relative to body size, birds have the largest eyes among vertebrates. The ostrich’s eye is twice the size of the human’s in a head only a third the size. In some birds, the eyes take up more space than the brain. Birds’ eyes are so large, they fill most of the cranial cavity, and eye movement is limited. Owls, with eyes to the front, have to turn the entire head to get side objects in view. The relatively numerous neck, or cervical, vertebrae, which vary in number with species, permit this movement. In most hawks and owls, the number is 14, and the movement 270 degrees; humans have only 7, and we move our heads at best about 180 degrees.
The bird’s skull is lighter than ours or that of any mammal, and lacks some of the mammal’s heavy structural architecture, such as a toothed jaw. The bird’s orb is supported in the eye socket by a ring of light-weight bony plates. In most raptors, a prominent eye ridge, the supraorbital ridge, extends above and in front of the eye, helping to support it. This “eyebrow” gives raptors that distinctive fierce look that goes with the label “Eagle-eyed.” This ridge also shields the eye from glare and protects it from wind, dust, and debris, important in fast, high flight or while soaring on winds. The Osprey lacks it, although the arrangement of the feathers above its eyes serves a similar function, shielding the eye from water-surface glare when the bird is hunting for fish.
Birds seldom blink, and their outer eyelids are not used in lubrication. Instead, the eye is moistened by the nictitating membrane: this is the third, concealed, eyelid that sweeps horizontally across the eye in most birds, though in owls, the membrane drops down the eye. The nictitating membrane also protects the eyes of diving falcons and raptors that hunt in forests or brush.
Raptor Nictitating Membrane: Barred Owl, Golden Eagle, Great Horned Owl
To summarize: So far, we know that “Eagle-eyed” indicates that the avian eye differs from both the mammal’s and the reptile’s eye in significant ways: its relative size, its support structures, its eyelids, the shape of the eye and control of the lens, and the make-up of the retina.
The Internal Structures of the Raptor’s Eye
The main internal structures of the bird’s eye are similar to those of other vertebrates. The outer layer is the transparent cornea, at the front, and two layers of sclera – a tough white collagen fiber that surrounds the rest of the eye and supports and protects it. The lens divides the eye internally into two chambers. The forward chamber is filled with a watery fluid called the aqueous humor, and the posterior contains the vitreous humor, a clear jelly-like substance.
The main structures of the internal eye are the lens, retina and its fovea or focus point, optic nerve connection, and pecten.
Optic Nerve & Pecten
The optic nerve is actually a bundle of nerve fibers whose function is to carry messages from the eye to the relevant parts of the brain. Like mammals, birds have a small blind spot without photoreceptors at the optic disc, where the nerve and blood vessels join the eye. At this spot in the avian eye, over the optic disc, lies the pecten, a small organ of varying shape that protrudes into the body of the eye.
The pecten is still poorly understood: Only birds have it, and it consists of a structure of folded tissue that projects from the retina (see the illustration above) and is well laced with blood vessels. It appears to keep the retina supplied with nutrients, but it may also help to oxygenate the retina, shade it from dazzling light, aid in detecting moving objects, or act to warm the vitreous humor (by blood close to the surface) when the bird is flying high. But so far, we have no way of really being sure what it does.
The lens is a transparent convex body that focuses light on the retina. Its shape can be altered by the ciliary muscles attached to the eye. Some birds also have a second set, Crampton’s muscles, that can change the shape of the cornea itself, which gives these birds a greater range of focus. The iris is, like ours, a colored diaphragm in front of the lens that controls the amount of light coming into the eye. At the center of the iris is the pupil, which controls the entry of light into the eye.
The falcon skull showing outer outline of the eye (A) as it functions as a window for light; and inner outline (B) as it functions in vision
The retina is a smooth, curved structure. Its various layers contain the photosensitive rod and cone cells, or light receptors, and their associated neurons. Rods are light sensitive but do not relay color information. Cones are not as sensitive to low levels of light, but convey color. Visual acuity depends in part on the density of these photoreceptors. Diurnal raptors depend on cones, and have few rods. Humans, too, have few rods, hence our night vision is not very good. In discussing raptor vision, we have to divide the diurnal hunters from the nocturnal, because there are many differences. We’ll get to owls and their vision later.
Humans have about 200,000 receptors per square millimeter; the House Sparrow has 400,000; the Red-tailed Hawk 1,000,000. The ratio of nerve ganglia to receptors is also very high for birds, which is another mark of visual acuity.
Summary: So another aspect of “Eagle-eyed” is the presence of many receptors per square millimeter, and a high ratio of nerve cells to receptors.
Receptors: C(one) Is for Color
In the 1970s, scientists couldn’t accurately examine cone structure, so early texts were mostly speculation about diurnal birds’ color vision, indeed the color vision of many other species. We used to believe that dogs couldn’t see color at all, only shades of gray. Today we know that dogs have two kinds of color cones and can indeed see color, though in comparison with us (we have three types of cones), their color vision is limited. (Bulls, incidentally, don’t see red at all, according to the latest investigations. They see motion quite keenly, though, and if irascible, tend to rush at moving objects.)
In the last few decades, we’ve been able to examine and even count cones, thanks to improved micro-spectrophotometry. We know that diurnal birds have more color receptors, or cones, in quantity and in kind, than mammals do, and more nerve connections between photoreceptors and brain.
Until very recently, we knew of only four types of color receptors in the bird’s eye, but new studies have found in chickens and some other species a fifth, whose function has yet to be discovered. Most mammals have only two color receptors; primates, humans included, have three. This means that some birds can see colors we don’t see, including, in some, the ultraviolet range.
Diurnal birds, like reptiles, have double cones, in addition to the more common single cones, and those of most diurnal birds have colored oil droplets at their base. We believe these droplets enhance color discrimination, but we’re not sure how.
In some birds, 50 percent of their cones are the double variety, consisting of a principal cone (similar to a normal single cone – illustrations are easily available on line) and an accessory cone that curves around the principal. All receptors, cones and rods alike, have an extension that reaches into the nerve layer of the retina.
In tests, the electrical signals generated by the double cones in response to different wavelengths of light indicate that each member of the pair has strong interactions with the other. This means that the bird’s visual system may perform color processing steps within the photoreceptors themselves. (Sillman, in Avian Biology, Vol. 3)
Summary: “Eagle-eyed” indicates complex receptor construction and a highly “automated” processing mechanism, both of which add speed to the bird’s reaction to visual events.
Some birds whose sexes appear similar to our eyes have ultraviolet reflective patches on their feathers, which seems to have an effect in breeding. Some male songbirds, for instance, with the brightest and most UV-shifted colors in their plumage, hold the most extensive territories and feed their offspring more frequently than other males do.
A UV (short wave-length) receptor may also give an animal an advantage in foraging for food. The waxy surfaces of many fruits and berries reflect UV light that might advertise their presence.
Most raptors are thought not to have ultraviolet reflective feathers. An exception is the Common Kestrel, a small European falcon, which we know from tests does see the UV range. It hovers over fields in search of voles and mice, and because these small rodents lay scent trails of urine that reflect UV light, the Kestrel can find them. Few other raptor species have been tested in this area, and some other raptors regularly do hover over fields – the American Kestrel (which may also possess a UV component in some of its feathers), the White-tailed Kite, and the Northern Harrier, for example – so we might test for UV vision in these birds. In Europe in 2010, a research team studied Montagu’s Harrier, a cousin of our own Nothern Harrier, and discovered that in breeding season, the cere, the soft waxy material at the top of raptors’ beaks, in the male developed a UV component (the female’s did not), and those with the most dramatically bright ceres were more successful – that is, their nests produced more chicks. This indicates that sexual selection is at play – the females are attracted to the brightest UV-blessed partners. (Harriers, incidentally, are interesting in that unlike most other raptors, the males are not monogamous, but often breed with up to five females, who nest close together. Harriers typically nest on or near the ground and hence lose more eggs and chicks to predators than raptors who nest high, so a larger number of nests/eggs/chicks can be an important element in keeping population levels stable.)
Recently, the Rough-legged Hawk has been discovered to have UV vision.
Summary: “Eagle-eyed” also means the bird sees colors humans cannot see, and this trait can be an element in both feeding and mating.
Hawk & Eagle Field of Vision, Showing Focal Points (Detail & Movement) and the Scanning Strip of Sensitized Cells
Toward the center of the retina in most birds is the fovea, or focal point, which has a greater density of receptors and is the area of greatest visual acuity – i.e., the sharpest, clearest detection of objects.
In diurnal raptors, however, there is a second fovea for enhanced sideways viewing. In most diurnal birds, their single fovea is deeply indented. In those with a dual structure, the side fovea is deep; the other shallow. The mammal’s single fovea is quite shallow, and the owl’s single fovea is shallower than that of the diurnal hunters.
There is also, in diurnal raptors, a strip of retina between the two foveae that also has more receptors. We think this strip is used for scanning for motion.The anatomy of the hawk’s foveae suggests that the deep fovea has the higher number of receptors. Does this create distortion, making part of an image unnaturally large in relation to other parts? And if so, how does the bird obtain an overall clear view?
Several species of raptors in a recent study described by V.A. Tucker (Dec. 2000 issue of the Journal of Experimental Biology) were recorded repeatedly moving their heads among three positions while looking at an object: straight, with the head axis pointing toward the object; or sideways to either right or left, with the head axis pointing approximately 40 degrees to the side of the object.
Since raptors can’t rotate their eyes very far in the sockets, these movements presumably made the image of the object fall on the shallow fovea and then the deep one.The movements occurred approximately every 2 seconds in hawks and falcons, and approximately every 5 seconds in Bald Eagles.
The proportion of time the raptors spent looking straight or sideways at an object depended on how far away the object was. At a distances closer than 8 meters, they spent more time looking at the object straight, but as the distance increased to 21 meters, they spent more time looking at it sideways. At distances of 40 meters or more, raptors looked sideways at the object 80 percent or more of the time.
This suggests that raptors use their more acute sideways vision to look at distant objects monocularly and shift to stereoscopic binocular vision to look at closer objects. This hints that their stereo vision may be less acute.
Summary: “Eagle-eyed” includes the presence of dual focal points and other areas of increased receptors. At these places on the retina where receptors are numerous, visual acuity is greater.
How Seeing Takes Place
Interaction Between Receptors & Retina
The exact relationship between receptors – rods and cones – and sight in any animal is complex and well beyond the scope of this article. We do need to know, however, that the bird retina is multilayered and thicker than that of other animals, having a network of many cell types serving different functions.
Most of the diurnal raptor’s receptor cells are connected to a single ganglion cell leading to the optic nerve. This adds acuity of vision. In humans, the receptor cells are connected to ganglia in bundles.
Birds also have a very large complement of so-called association cells connected to the receptors. These cells, which birds possess in much larger numbers than mammals, interconnect not only with retinal neurons, but with other receptors. The result is that many of the functions of the visual system controlled by the higher nervous system in mammals are performed by the retina in the bird. “For example, the pigeon retina contains five types of ganglion cells, in addition to those capable of discriminating luminosities. These include cells that respond to verticality, horizontality, edges in general, moving edges, and convex edges. Thus it would appear that birds are similar to the amphibians and reptiles, which have also been found to be capable of complex responses at the retinal level.” This of course would greatly increase both visual acuity and speed of reaction. (Sillman, “Avian Vision,” in Farner and King, Avian Biology, page 356.)
Photoreceptors (yellow) and the complex of interconnecting cells in the avian retina.
Summary: Therefore “Eagle-eyed” also indicates that the raptor can react to visual stimuli more quickly than we can, in part because of the structure and function of its retinal components.
Birds can indeed resolve rapid movements better than humans can. We cannot distinguish individual ﬂashes of a fluorescent light bulb oscillating at 60 Hz, but chickens have flicker thresholds of more than 100 Hz, falcons, 70 – 80. A Cooper’s Hawk can pursue agile prey through woods and avoid branches and other objects at high speed; to us such a chase would appear as a blur.
This means that for the raptor, the world seems to move more slowly. Whereas we see more than 20 individual events per second as continuous motion (our TV picture presents 25 events per second), to falcons our TV picture would appear as a rather boring collection of still shots. This “slowing” of the vision allows the raptor a speed of response to stimuli that is vital in hunting fast prey.
Birds, including raptors, can also detect slow-moving objects. The movement of the sun and the constellations across the sky is virtually imperceptible to humans, but probably detected by birds. The ability to detect these movements is thought to help migrating birds to orient themselves properly.
To obtain steady images while flying or when perched on a swaying branch, birds hold the head as steady as possible with compensating reflexes. We find it amusing to see a bird on a moving perch, its body going back and forth, while its head is quite still. But maintaining a steady image is especially relevant for birds of prey.
Summary: “Eagle eyed” also means a finer perception of motion, both fast and slow, than we have.
The visual ability of diurnal birds of prey is legendary. Though we are still unable to say exactly how it works for all species, we know a variety of factors are involved. Raptors have large eyes for their size, 1.4 times greater than the average for other birds of the same weight, and the eye is tube-shaped (more in owls than in diurnal hunters, though according to Arnold Sillman , some eagles have tubular-shaped lenses as well). The tube shape produces a larger retinal image.
The retina has a large number of photoreceptors per square millimeter. The more receptors an animal has, the higher its ability to distinguish individual objects at a distance, especially when the receptor-to-ganglion ratio is also high.
Daytime raptors have two foveae (areas on the retina super-rich in light receptors) with far more receptors than the human single fovea (65,000 per square millimeter in the American Kestrel; 38,000 in humans), and this seems to provide these birds with spectacular long-distance vision. The fovea itself can also be lens-shaped, further increasing the effective density of receptors. Some researchers estimate that this combination of factors gives buteos and falcons distance vision at least 6 to 8 times better than that of humans.
There was once considerable argument on this, however, with some scientists insisting that the deep, V-shaped fovea would actually create distortions that could leave raptors with no greater acuity of vision, all told, than ours. However, over the years, the raptors’ excellence of distance vision and precision of detail resolution have been proven in hunting tests again and again. We may not know exactly how it works, and it may work in ways that do not seem “intuitive” to the lay person, but we do know that it works.
The forward-facing eyes of a diurnal bird of prey give binocular vision, which is also assisted by its double foveae. Owls, on the other hand, have only a single fovea, like humans and other animals that were once or are now nocturnal in habit, and that single fovea is shallower than either of the focal points of diurnal raptors. But then most owls hunt as much by hearing as by sight. And the great number of rods gives some owl species the capacity to see in near darkness.
Having their most acute vision toward the side causes a conflict in raptors like falcons that dive at prey from great heights at high speeds: At a speed of 250 or more miles per hour, turning the head sideways to view the prey straight ahead would increase aerodynamic drag and slow the bird.
Researchers have determined that raptors can resolve this conflict by diving along a logarithmic spiral path with their head straight and one eye looking sideways at the prey, rather than following the straight path to the prey with their head turned sideways. Although the spiral path is longer than the straight path, a mathematical model shows that the falcon could reach its prey more quickly along the spiral path, because the speed advantage of a straight head more than compensates for the longer path. In the PBS Nature program “Raptor Force,” videos and photographs of diving falcons illustrate the dynamics of the “stoop.”
Diurnal raptors, along with many other birds, have an additional strip of cone-rich tissue on the retina, falling between the two foveae. (On other diurnal birds, such a “ribbon” of enriched receptors surrounds their single fovea.) This may increase the acuity of the raptor’s vision even further.
Finally, many raptors have to pursue active prey using the lower part of their visual field, and therefore do not have the lower-field myopia demonstrated by many other birds.
Eagle-eyed: The Latest Assessments
Diurnal raptors, except for falcons, are now thought to lack complex colored oil drops in their cones, and are said to have similar color perception to humans. They are now also thought to lack the ability to detect polarized light (though raptors do migrate, some for long distance). The generally muted plumage of this group and the absence of color displays in courtship suggest to some students that color is not as important to these birds or important in the same ways it is to many other species. There is, however, a great deal of work to be done in this area, and raptors are often subtle creatures. Speculation can be misleading.
Hawk-eyed: Testing Vision
Despite questions and speculations, the diurnal raptor’s adaptations for optimum visual resolution gives us “Hawk-eyed” and “Eagle-eyed” as metaphors for keen sight. Lately, an America Kestrel actually demonstrated that it could see a 2-millimeter insect from the top of an 18-meter tree.
None of these daytime hunters, however, sees well at night. That’s when the “owl-eyed” creatures come out to prowl. And while their eyes may seem simpler, they perform perfectly in the dim light of the moon.
January 6, 2014
Raptor Vision Part II:
The Acute Senses of the Winged Night-time Hunter
The Dance of Sight & Sound
We humans like to make classifications. Nocturnal – night. Diurnal – day. Nocturnal – owl. Diurnal – hawk. But living creatures can’t be categorized quite so simply. Everyone has seen owls in the daytime, and most of us have seen a hawk or falcon hunter well after sunset. True, hawks and falcons have acute vision and powerful flight that give them advantages in daylight. And owls have nightvision and the soft, silent feathers that give them an edge after dark. But owls also have another advantage in the dark – acute hearing. Which gives their particular hunting style a nice twist no matter when they are on the wing.
Owls actually can see well by day, though they do not see colors, probably not even as much color as a cat or dog. But they can hunt in any light. Some are mostly geared to hunting in dim light – the Great Horned, for instance. Others hunt during in broad daylight, like the Burrowing Owl; some hunt at both times, like the Snowy and the Great Gray. Finally there are those, such as the Strix owls and those in the Tyto family (the Barn Owl, in this country) who are able to negotiate in nearly total darkness. Still and all, though, owls developed a body plan that is most useful dusk to dawn, and those who changed their habits have kept the night-oriented adaptations.
The Barn Owl, flying at night, has earned the nickname Ghost-bird. They can capture mice in total darkness, under inches of leaf-litter. A single Barn Owl will eat up to 2000 mice a year, and a pair, with their whopping-big broods of up to 15 young, provide free, and safe, rodenticide services for farmers and grape-growers.
All owls long ago adapted themselves for dim-light conditions that require vision like that of cats and other nocturnal animals, with receptors that are light sensitive – rods – and very few color receptors – cones – which are so important to diurnal birds of prey. Owls are also built for slow, silent flight. Since we humans are by nature classifiers, we’d like to get more precise than that – to decide a specific category that every owl will fall into. But life is messier than our ordering proclivities would have it, and so that’s probably not possible.
For a long time, we were taught that owls with bright yellow eyes, such as the Great Horned, were dawn and dusk hunters; and those like the Barred and the Barn owls, with dark eyes, were active only in the depths of night. Today we know that doesn’t always hold true. Many yellow-eyed owls, the GHO among them, do hunt mostly in the crepuscular hours. But some are also active late at night. Some occupy both time slots. We often hear Great Horned Owls hooting in our woods well past midnight. The Long-eared Owl is most active in conditions of dark, but has been observed by day. And while the Barn Owl is an excellent stealth hunter in the darkness, she is also out occasionally in the daylight. So the golden-eye rule doesn’t hold.
Another idea that enjoyed popularity for awhile had us look at the facial disc – that ring of specialized feathers around the eyes and ears of owls that seems geared to increasing the hearing ability. Around the edges of the disc is the ruff, a band of stiff small feathers, which the owl can raise and lower, and in the interior are soft short feathers that let sound pass through easily. Over the ear openings themselves we find yet another kind of feather, the auriculars. The idea was that owls with really large facial discs, like the Barn Owl, were the true night hunters, because the larger the disc, the greater the increase in hearing acuity. Those with small discs, like Burrowing Owls, were daytime hunters, not depending on their hearing as much. Great Horned Owls hunt at various times of day and have a medium-sized disc.
Great Horned Owl face with disc. Barn Owl and Burrowing Owl profiles, showing discs.
To some extent, this is true. But as with eye color, it doesn’t always hold up. The Great Gray Owl has a huge disc and yet it hunts both day and night, often in snow-bright conditions. Evolution seldom occurs by rational steps. It is a relatively crude tool that usually works over the long haul, its sometimes elegant precision a matter of refining a general pattern to fit specific habitats and habits over many generations. Actually altering physical body parts moves much more slowly than the quicker adaptation-to-local-conditions type of adjustments that allow some populations of some species to change eating, breeding, or defense behavioral habits fast enough to survive catastrophic events.
So, alas, we can make few firm rules about owls and their hours of activity. “Nocturnal” indicates a generality. Owls evolved from an nocturnal avian line that produced Nighthawks and the like, and share many nocturnal characteristics with them. The refinements are more complicated and less subject to “rules.”
So let’s look at the owl “theme,” with a bow to the variations on that theme from species to species. All owls have relatively large heads almost completely taken up by huge forward-facing eyes – eyes more than twice as large as the average for birds of the same weight – and huge ear openings. In a very real sense, owls are flying eyes and ears, for their lives depend upon those senses.
Other aspects of the owl theme, which will become clearer when we study species, are: adaptations for night life – soft, thick feathers, slow flight, camouflage coloring, resonant voices; hunting and digestive adaptations – a toe that can swivel from front-facing to rear-facing for accurate grabbing, the lack of a crop; life-style habits – cavity nesting and a disinclination to build an actual nest.
In structure, owls’ eyes are unique. They are so large that they cannot move at all in their sockets, but are literally held in the head by the sclerotic ring – a circle of cartilaginous plates, in small owls; hollow bone plates in the large ones. If our eyes were as large for our heads as an owl’s, they would be about the size of oranges.
To see to the side, above, or below, the bird must turn its entire head. And not only to focus its eyes, but to focus its hearing as well. When at attention, owls will often move the head so fast it looks as if it is spinning. Indeed, children often ask if the owl is turning its head all the way around. Of course it can’t do that. But the owl is turning the head 270 degrees, like the hawk. And like the hawks, owls’ cervical vertebrae number 14 – to our seven – which makes possible that motion, to say nothing of feather preening and nasty bites to unexpected parts of a handler’s person.
In all owls, the eyes are placed at the front of the head and have a field overlap of 50–70%, giving better binocular vision than diurnal birds of prey (overlap 30–50%), whose eyes are slightly to the side of the head. Owl eyes are positioned more like our own. And that round head-shape is familiar to us: One of the reasons many of us are so strongly attracted to owls is that the large round head with large round eyes reminds us of human infants, and we respond, on sight, with feelings of affection.
Owl’s eyes look round, but in actuality they are tubular in shape. And the cornea, also tubular, is quite large, which creates a large image on the retina. The lens beneath the cornea is also large, making it possible for more light to enter the eye as needed. The pupil, which admits the light, can be closed in bright conditions by the muscles in the iris to a small pinprick, or opened, in dim light, so wide that almost no iris is visible at all. Owls can adjust their pupils independently – just as they can their eyelids and nictitating membranes.
In the photo above, a Western Screech Owl hangs upside down in a flight cage after being rescued from a horse trough. The picture was taken in near-dark and shows pupils fully dilated. When rescued, he was a sopping wet skeleton with an attitude. He’s fluffy and dry now, but still has the attitude!
The convex shape of cornea and lens allows for good light-gathering, and produces that large image on the retina. This increases the accuracy of the owl’s vision. But it is exaggerated in larger owls, with the result that those birds are rather far-sighted and can’t focus well on near objects.
The smaller owls, dependent on insects, have a slightly less extreme convexity, and therefore better near-field vision.
Most owls have bristles around the beak – we know they have a function, but just what is it? We assume they at least partially compensate for accommodation problems as the short-sighted bird is feeding its babies or closing in on prey. But as yet we have no hard evidence. We know that cats use their whiskers to help them negotiate in the dark, particularly in small passages. But we cannot make that assumption about owls, who fly in relatively open spaces rather than creeping through the brush. There are now some studies suggesting that in nocturnal mammals, the facial bristles collect other sensory information, even perhaps an enhanced response to certain sounds.
Northern Pygmy Owl, a tiny guy who hunts in daylight and dusk. Note the small facial disc but numerous rictal (mouth) bristles. The small disc gives evidence to the daylight hunting, using eyes more than ears. The bristles indicate that this bird may not see well close up and uses the bristles to explore objects in front of his beak.
Barn Owl showing extreme facial disc. The brown ring of short stiff feathers can move slightly forward and back at the bird’s will, sometimes surrounding the face with a cone to direct sound. The rictal bristles are relatively even in length and soft – hardly bristles at all.
Short-eared Owl – hunts by day and in dusky hours. Large facial disc – note the tweed-colored ring of stiff short feathers. The bird’s ear is under the dark patch in the ring, slightly under the eye. This owl’s bristles are more prominent than those of the Barn Owl, but not nearly as large and forceful as those of the little Pygmy.
Owls’ ears are large openings into the skull, behind and a bit beneath the level of the eyes. The Great Horned Owl’s and the Barn Owl’s ear openings are so large, you can easily see them when you are examining one of these birds, by simply blowing the feathers gently away from the side of the head.
Owl’s ears and eyes – note large size of the ear opening, and how much of the skull area is taken up by the eyes and ears. These drawings do not show the asymmetric placement of the ears on the head, however.
On some species (Saw-whet and Boreal, for example), one ear is higher than the other, and often the opening of the high one is turned upward, while that of the lower points downward. This facilitates pinpoint location of a sound. The owls possessing this asymmetry are usually those most successful in their hunts during the darker hours.
Some species have external ear flaps, called “opercula” – and some have them in front of as well as behind the ear opening. These seem to be variously useful in guiding sound into the ear openings. They surely also offer some protection to the delicate inner ear.
There is some evidence from a number of research studies that owls locate sounds by shifting their heads as they listen until the sound is equalized in both ears.
Western Screech Owl – the ear tufts are like those of the Great Horned Owl; for show or communication, nothing to do with hearing.
Much of the initial information we have on owls’ hearing comes from Roger Payne, in the 1960s – he is the researcher who first described the sub-sonic sounds of elephants and fin and blue whales. Follow ups and refinements to Payne’s ground-breaking studies are still going on.
The Barn Owl’s hearing has been described by many researchers, who have come up with often contradictory conclusions. One fact that all seem to agree on, though, is that the measured area of their greatest sensitivity falls between about 40 Hz to 9 kHz. In that range, their acuity has been shown to be remarkable. The full human range is between about 20 Hz and 20 kHz, from just above the lowest pedal note on the organ to a mosquito buzz. The active audible range for most of us is around 40 – 50 Hz and 12 kHz, because over time (starting when we are about eight years old!), we lose acuity at both extremes. So while the owl probably cannot hear sounds at our low threshold, nor sounds as high as our upper threshold, its hearing, in action, is “better,” in that the owl undoubtedly hears quite acutely in its range, and processes the auditory information with exquisite precision to its needs. Humans, on the other hand, use their eyes with exquisite accuracy and we tend to often ignore the signals we are getting through our hearing.
As researchers look into the senses of other species, they often uncover information about us. We humans seem to have sense capacities that go beyond our present need for them or our ability to interpret them to our advantage. (Our actual capacity may well explain how some blind people are able, over time, to increase the accuracy and dependability of their hearing.)
The European Tawny Owl’s vision has been studied more than any other, and many writers make assumptions about all owls using what science has discovered about that one species. It’s good to keep in mind, though, that even among related species, birds differ in many of their systems. We really can only speculate in the absence of direct studies. Many of the technical details in the discussions that follow are based on the research on the Tawny. It’s hard to deny ourselves the pleasure of extrapolating them to others!
The Retina & Its Elements
Rods & Cones
Rod cells in the retina respond to photons of light – that is, they are sensitive to the very presence of light. The rod’s nucleus can respond to a single photon, and in nocturnal animals, the high number of rod cells adds up to acute photosensitivity. This is true of owls, certainly. Owls have mostly rods in their retinas, and those rods are connected in groups to the bipolar cells, which in turn form a link to the optic nerve. This grouping increases the sensitivity to light, but decreases acuity. (In diurnal raptors, as we’ve seen, the cones, which react to types of light waves, or color, connect to the bipolar and other auxiliary cells one on one. This increases acuity.)
We know that in hawks, the auxiliary cells connecting the cones to the optic nerve often connect to one another, as well, indicating that some response to visual stimuli takes place on the retinal level, by-passing the optic nerve/brain connection. I haven’t found any research on this in owls and their rod connections.
In addition to the rods, all owls also have some cones, and contrary to popular belief, they can see in daylight. Even those who hunt in the darkest hours of the night often begin their forays shortly before or shortly after sunset. And if they fail in their night hunts, they will continue their efforts rather than go to bed hungry when dawn breaks. Of course they do not see in daylight as well as diurnal birds, nor indeed, as well as humans, though they do outperform both in various degrees of darkness.
The Tawny Owl looks a good bit like our Spotted and Barred owls and is a close cousin to those birds. It is strongly nocturnal, as are the other two. Its retina has about 36 million light-sensitive rods per square inch. It was once thought to see in the infrared part of the spectrum, but more recent studies have thrown doubt on that.
You may recall from the section on hawk vision that there are colored oil drops at the base of the cones in most birds’ eyes. In owls, there seem to be few colored oil drops, which might reduce the light intensity for owls, but in some species, perhaps as a compensation, the bird may have a light-reflective layer, the tapetum lucidum, behind the retina that reflects available light back into the retina. This gives the animal a second chance to see objects it is focusing on.
Note I say that owls “may have” this. Nocturnal mammals do have this tapetum, and their eyes shine with various colors – often a shade of “neon” blue or green. For a long time it was assumed that all owls possessed the tapetum as well. Now, however, there is some disagreement about whether that is true.
On one side, Paul Johnsgard, in Owls of North America, an excellent resource, says that various owl species do possess the tapetum. He cites studies in which the Tawny Owl and the domestic cat have performed similarly in tests of nocturnal visual sensitivity. In these tests, both animals seemed to perceive light at close to the limit of light perception.
The cat of course has a well-developed tapetum, and its eyes shine green in the dark with reflected light. Johnsgard discusses reports from the 1920s to the 1970s that list bright orange-red eye-shine in Barred Owls, and intense red to golden reflections in photographs he himself had made of Spotted, Barred, Boreal, and Great Gray Owls. He writes: “Walker (1974) listed eight relatively nocturnal North American owls in which he had observed weak to strong eye-shine (strongest in Spotted, Barred, and Long-eared), compared to the more diurnal Burrowing Owl that lacks it.” (Note for the curious: Spotted and Barred Owls have dark eyes; the Long-eared, bright yellow.)
On the other side of the question, Lynch, in Owls of The United States and Canada, says point blank that no owls have the tapetum, that the red eye-shine we see when we photograph them with a flash is simply “red-eye,” caused, in his opinion, by blood vessels near the surface of the retina and not by the presence of a reflective layer. In his discussion of owl vision, he cites many of the same sources as Johnsgard, though in the case of tapetum versus no-tapetum, Lynch gives no citation.
To further confuse the matter, most texts agree that birds in general have few blood vessels close to the surface of the retina, depending instead on the blood-rich pecten to “feed” the retina with oxygenated blood.
Great Horned Owl showing “eye shine” or “red-eye?” This photo was taken with a flash.
So – tapetum in owls, or no tapetum? My inclination is to lean toward Johnsgard, that some owl species possess some kind of tapetum, and the red to gold color of the eye-shine is indeed a reflection from that layer. Even Lynch acknowledges that some species of nightjars have been found to possess a tapetum, and according to DNA studies, owls developed from the family of nightjars and related nocturnal hunting birds (Caprimulgiformes). Of course that doesn’t guarantee that owls “inherited” the genes for the tapetum layer, but it at least suggests the possibility.
So I will leave the question at that.
Again, unlike hawks, which depend on their dual foveae, those dimples in the retina rich in cones that increase acuity of vision, owls have only one fovea, and that one is shallow and poorly developed except in mostly diurnal hunters, such as the Burrowing Owl. The fovea is most useful in increasing acuity, rather than light sensitivity, so this probably should not be surprising. (We too have a single fovea. Cats seem not to have one at all.)
To recap our discussion above: Owls, even those who hunt in the daytime, use their hearing. Those who hunt in darker hours are of course more dependent on auditory cues than are the ones flying at dawn and dusk or even into broad daylight. All owls use the two senses in tandem to provide the accuracy in hunting that the diurnal raptors achieve with their combination of super-acute vision and precise and powerful flight.
The internal parts of owls’ ears have been studied closely. They have ear-drums, of course, and stapes of different sizes and shapes, according to species, and even more inclusive classifications. The shape of the Barn Owl’s stapes is much different from those of the many species in the Strigidae group. I won’t go into the physiology of the inner ear, however. What is more critical in understanding how hearing works is something more difficult to get hold of: the connections between the sound and the auditory nerve and the brain. It would be wonderful to discover that the owl’s hearing is as complex in interpretation as the hawk’s sight, some aspects of which, you will recall, take place on the retinal, and even receptor, level rather than in the brain. There is so much space in the owl’s skull devoted to the ear and the eye, and so little to the brain, that it seems not unreasonable to speculate about where and how the interpretation of sound actually occurs. But we don’t yet know for sure. One interesting thing that has come to light on avian ears in recent research has to do with the cilia, or “hair cells” that serve as sound-wave receptors. In humans, loud sounds can destroy these cells, and the effects – decreasing our acuity at various frequency levels. In birds, it seems that these cells can regenerate, so that they may not become less sensitive to sounds over time.
Hunting by Sound
Owls either sit and wait or fly relatively silently and slowly, sometimes turning their heads until they pinpoint a sound (or sight and sound) with precision. Their silent flight makes them effective hunters, right down to the limits of auditory perception, where noises vanish into ambient sound. The softer feathers on owls’ bodies and wings make them harder to detect by prey animals, and let the owl itself listen undistracted by the whirring of its own wings.
Owls are capable of fine triangulation of sound. Several studies show Barn Owls catching mice in leaf litter under research conditions of total darkness. Other studies show the nocturnal/diurnal hunter, the Great Gray Owl, catching mice in the day time by diving feet first into more than 20 inches of snow. Surely they have found the mouse by hearing alone.
All owls, once they have focussed their attention on a spot, do not deviate from it. That’s probably one reason so many are injured by cars: they zero in on the prey animal and then see and hear little else. As they reach the animal, their heads lift slightly, their legs swing forward, their outside front toe swivels to the back. They brake with their wings for the strike. Using both senses, sight and hearing, they are amazingly accurate.
Some Random Nocturnal Thoughts
Lately, I’ve read articles that put forth the idea, backed with some tests, that owls’ actual sound-pinpointing capacity in the horizontal plane is comparable to our own. And that in the vertical plane, our perceptions may be slightly better. The same sort of studies suggest that the acuity of our vision is nearly equal to that of hawks, in spite of the fact that we possess only three cone types and birds possess four to five, giving them color perception we cannot really comprehend.
Remember, though, that testing animals in these areas is not like testing people, whom you can ask to report when they see or hear something. In light of the lack of inter-species communication, there are many ways we can mistake an animal’s responses. We depend on behavioral cues to tell us what the animal is experiencing. What if the sound we’re using in our tests is not of great interest to the animal? What if that sound has a certain meaning we are completely unaware of, but one that influences the animal’s response in ways we don’t know? Finally, what are we doing to get the animal’s full attention?
There is much more going on in actual diurnal and nocturnal hunting flights than measurements of simple sense apparatus and capacity can tell us. Think of yourself running a race with a friendly rival. Now think of yourself running to escape a maddened bull. There will be critical differences in your behavior!
If these new studies are correct about what human and raptor eyes see and ears hear, we still need to figure out why the birds are, in basic life behaviors, so much better than we are in performance. For hawks, as we’ve seen, some of their skill lies in the speed of their reactions. In owls, it could be the same, or a matter of a more refined interpretation of what the senses are relaying, or – as has been recently suggested – by a superior spatial memory. It is worth questioning, however, what cues led to that last speculation! An owl in a strange place is more likely to bump into obstacles in the dark? How well has that actually been tested?
In addition to owls, nightjars, nighthawks, bat hawks, and frogmouths also have good night vision, and most display even more interesting auditory capacities. Some nest deep in caves that are too dark for vision, and find their way around with a form of echolocation. This means, incidentally, that each must be able not only to avoid smacking into stalactites and such, but to distinguish its own nest from those of the other birds in the colony. The Oilbird and several swiftlets, none of which live in the US, have this capacity, and one swiftlet even uses echolocation outside its caves, hunting like a bat. (Oilbirds are also said to be among the few avian species with a keen sense of smell. Are they smelling their nests?)
Research into owl vision/hearing would surely be one of the most rewarding fields of study.
New Research in Bird Vision (2010)
Researchers at Washington University School of Medicine in St. Louis have recently studied the eye of the chicken and mapped five types of light receptors. They discovered that the receptors were laid out in interwoven mosaics that maximized the chicken’s ability to see many colors in any given part of the retina. . . . (Study published in Plos One, quoted in Science News, date not available).
“Based on this analysis, [these] birds have clearly one-upped us in several ways in terms of color vision,” says Joseph C. Corbo, M.D., Ph.D., senior author and assistant professor of pathology and immunology and of genetics. “Color receptor organization in the chicken retina greatly exceeds that seen in most other retinas and certainly that in most mammalian retinas.”
Corbo plans follow-up studies of how this organization is established. He says such insights could eventually help scientists seeking to use stem cells and other new techniques to treat the nearly 200 genetic disorders that can cause various forms of blindness [in humans].
Earlier studies in Britain found that [Common] Kestrel vision was complex and included color receptors not found in mammals. Ongoing studies continue to map the vision capabilities of many bird species.
Diurnal birds probably owe their superior color vision to not having spent a period of evolutionary history in the dark, according to some researchers. Birds, reptiles and mammals are all descended from a common ancestor, but during the age of the dinosaurs, most mammals became nocturnal for millions of years.
Most birds, now widely believed to be descendants of dinosaurs, never spent a similar period living mostly in darkness. As a result, diurnal birds have more types of cones than mammals.
“The human retina has cones sensitive to red, blue, and green wavelengths,” Corbo explains. “Avian retinas also have a cone that can detect violet wavelengths, including some ultraviolet, and a specialized receptor called a double cone that we believe helps them detect motion.”
In addition, most avian cones have a specialized structure that can be compared to “cellular sunglasses”: a lens-like drop of oil within the cone that is pigmented to filter out all but a particular range of light. Researchers used these drops to map the location of the different types of cones on the chicken retina. They found that the different types of cones were evenly distributed throughout the retina, but two cones of the same type were never located next to each other.
“This is the ideal way to uniformly sample the color space of your field of vision,” Corbo says. “It appears to be a global pattern created from a simple localized rule: you can be next to other cones, but not next to the same kind of cone.”
Corbo speculates that extra sensitivity to color may help birds in finding mates, which often involves colorful plumage, or when feeding on berries or other colorful fruit. It has further implications in birds of prey: the Kestrel, for example, can see the ultraviolet component in mouse and vole urine trails in the grass, which helps them locate their prey.
“Many of the inherited conditions that cause blindness in humans affect cones and rods, and it will be interesting to see if what we learn of the organization of the chicken’s retina will help us better understand and repair such problems in the human eye,” Corbo says.
Much further work is needed to understand the vision of different species of birds. We’d like to know, for instance, if hawks have the same color reception as falcons, two types of predatory birds that we recently learned developed along somewhat different evolutionary paths.
Update on Research Methods:
“To gain insights into the evolution and ecology of visually acute animals such as birds, biologists often need to understand how these animals perceive colors. This poses a problem, since the human eye is of a different design than that of most other animals. The standard solution is to examine the spectral sensitivity properties of animal retinas through microspectrophotometry—a procedure that is rather complicated and therefore only has allowed examinations of a limited number of species to date. We have developed a faster and simpler molecular method, which can be used to estimate the color sensitivities of a bird by sequencing a part of the gene coding for the ultraviolet or violet absorbing opsin in the avian retina. With our method, there is no need to sacrifice the animal, and it thereby facilitates large screenings, including rare and endangered species beyond the reach of microspectrophotometry. Color vision in birds may be categorized into two classes: one with a short-wavelength sensitivity biased toward violet (VS) and the other biased toward ultraviolet (UVS). Using our method on 45 species from 35 families, we demonstrate that the distribution of avian color vision is more complex than has previously been shown. Our data support VS as the ancestral state in birds and show that UVS has evolved independently at least four times. We found species with the UVS type of color vision in the orders Psittaciformes and Passeriformes, in agreement with previous findings. However, species within the families Corvidae and Tyrannidae did not share this character with other passeriforms. We also found UVS type species within the Laridae and Struthionidae families. Raptors (Accipitridae and Falconidae) are of the violet type, giving them a vision system different from their passeriform prey. Intriguing effects on the evolution of color signals can be expected from interactions between predators and prey. Such interactions may explain the presence of UVS in Laridae and Passeriformes.” (Odeen & Halstad, “Complex Distribution of Avian Color Vision,” Molecular Biology & Evolution, Journal, Vol 20, 2003)
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