Vision that extends into the ultraviolet. Hearing that extends to frequencies well above what the keenest teenager can perceive. Most birds have senses of touch, taste, and smell that may be quite similar to our own, or poorer, but they can also perceive the earth’s magnetism and polarized light. How well can they really see and hear? Those are just some of the questions people ask about how birds experience the world.
Q How good are “eagle eyes”?
A Birds in general have extraordinarily good eyesight. Imagine how acute a hawk’s vision must be if it can soar hundreds of feet in the air, scanning the ground for the slightest movement to home in on a mouse.
Bird eyes are bell-shaped, with a large retina. A 3-pound Great Horned Owl’s retina is larger than an adult human’s. Bird eyes move very little, and so the only parts that are not covered by skin and feathers are the iris and pupil. But beneath the surface, their eyes are huge. Eagle eyes are as large as or larger than human eyes, and in some species the combined weight of the eyes is heavier than the brain!
Rather than being shaped like mammalian “eyeballs,” avian eyes are concave in back, giving them a relatively larger retina than humans have. Bird eyes also have a distinctive feature: a large projection from the rear surface of the eye near the optic nerve, called the pecten oculi. Scientists don’t yet understand why birds have this, but it probably nourishes the retina.
We know that birds’ visual acuity is far better than ours, allowing them to perceive objects much smaller and farther away than we can. The cones — the cells in the retina responsible for visual acuity — are packed about five times more densely in areas of a raptor’s retina than in ours. There are about a million cone cells per square millimeter!
In birds, each eye sends information to just one half of the brain, allowing birds to process information from separate visual fields independently, but an area of the brain called the “Wulst” processes the information from both eyes to provide stereoscopic vision, the means by which birds have depth perception, which is critical when an eagle or heron is homing in on a fish or when a flycatcher is snapping up a moth.
Q Do birds see colors?
A Yes, they do, and they can even see some wavelengths in the light spectrum that are invisible to humans. Birds use their excellent color vision to find food, such as ripe fruits and flowers. Their colorful plumage is also important in courtship. Many studies have shown that when given a choice, female birds often prefer males with the most colorful feathers.
Birds can detect polarized light that humans can’t see. Experiments show that pigeons and migratory songbirds use polarized light as a cue to help them navigate in the right direction.
Birds can also see ultraviolet light, which is invisible to us. Feathers reflect in the ultraviolet spectrum, and birds can see and use this information to help them discern sex and age differences in one another and even recognize individuals. In experiments in which female flycatchers could choose between a male with normal feathers and one treated with sunscreen to block the UV-reflection of his feathers, females showed a strong preference for normal, ultraviolet-reflecting feathers.
Many birds, including seabirds such as terns, gulls, and albatrosses, have red or yellow oil droplets, containing high concentrations of carotenoids (the same pigments that make carrots orange) within the cone cells in their eyes. Light travels through the oil droplets before it reaches the visual pigments. The oil droplets filter out some wavelengths of light, narrowing the color range that each cone perceives, which means the cones perceive colors more accurately than we can; birds with these oil droplets apparently also see better in hazy or watery conditions.
The ability to see ultraviolet light also comes in handy for finding food. Kestrels can actually see where their prey — small rodents called voles — have been walking. Fresh trails reflect UV light from urine that the voles have left behind, so kestrels can detect which trails are most likely to lead to a meal. How do we know this? Finnish researchers put out fresh trails of rodent urine in an experimental field and found that Eurasian Kestrels hunted along these trails. In the laboratory, kestrels spent more time hovering above and inspecting urine-treated paths illuminated by UV light than those illuminated by artificial white light.
Q I read that Peregrine Falcons can dive toward a duck at speeds of over 100 miles per hour. How do they protect their eyes from dust and insects, and from simply drying out, at such high speeds?
A You’re right that keeping eyes lubricated and dust free are important issues for birds. Birds have a translucent, semitransparent inner eyelid called the nictitating membrane that sweeps across the surface of the eye, allowing them to blink without blocking their view. Some birds, especially diving species such as loons and some ducks, have a clear “window” in the middle of the nictitating membrane that they probably use as we do goggles to improve underwater vision.
In addition, birds have two different tear glands. The lacrimal gland has many tear ducts along the lower eyelid. There is a second tear gland at the base of the nictitating membrane to maximize lubrication as that membrane blinks.
Q I spend a lot of time watching birds. On TV I see hawks and owls holding their heads upside down when looking at something, but I never see them do that in nature. Why?
A Virtually all birds and lizards, and also primates, have an area of the retina called the fovea, where vision is especially keen. The fovea of hawks and owls is above the midline, so from high perches or in flight they can see objects far below especially keenly. This is important because their food is normally found below, and they seldom need to see objects above their heads as clearly. Those TV hawks and owls holding their heads upside down are almost always captive birds, perched much lower than they would be in the wild in the presence of a photographer. When something looms above a bird’s normal field of view and the bird can’t change its position, it holds its head upside down to get the object in the fovea.
Q When robins cock their heads toward the ground, are they listening for worms?
A Nope, they’re looking for them. A scientist named Frank Heppner designed an elegant series of experiments to establish that vibrations, odors, and sounds do not help robins find worms. Instead, robins see worms inside their subterranean burrows by peeking into the tiny holes at the surface, or they watch for them wiggling on the ground. Robins cock their heads to focus close up with one eye. The pupil of the eye looking up is adjusted for daylight while the one focused down is open wider for seeing inside the dark little holes.
Male robins sing most intensely while it’s still too dark to see worms, and females lay their eggs at midmorning rather than at dawn when most songbirds do. This enables them to focus their attention entirely on feeding during the time of day when worms are most visible, at first light before the sun sends them underground for the day.
The other bird most specialized for feeding on earthworms is the American Woodcock, a plump shorebird with a very long bill. Like robins, woodcocks pick up any earthworms they might see wriggling about on the surface. Woodcocks probably detect underground worms at least partly by touch, probing beneath the soil with the sensitive tip of their long bill.
Q Why are owls the only birds with visible ears?
A All birds have ears, usually hidden behind feathers on the sides of their face. The “ear tufts” on owls aren’t ears at all, but feathers sticking up on their heads. Ornithologists speculate that when the feather tufts are raised, typically when an owl is alarmed, they make the bird look somewhat like a broken branch, which may help the owl avoid detection.
The feather tufts may also help owls recognize and visually communicate with one another. A few people think the feathers may enhance their sense of touch around their face, since every feather is attached to a nerve. These ear tufts also enhance many yellow-eyed owls’ catlike appearance, and so may give them a second or two of extra time to escape when approached by a predator, because even large predators are at least a little intimidated by the claws, teeth, and fierce fighting ability of cats.
Q How well do birds hear?
A Birds have more acute hearing than we do. Although audiology tests indicate that we can hear higher frequencies than the birds tested in some experiments, the range at which we hear best is lower than that of some songbirds, and they can resolve notes in songs that are too rapid for us to easily distinguish. Young humans can usually hear from roughly 20 to 20,000 hertz (Hz). (One hertz equals one vibration per second. To give you a sense of what that means, the lowest key on an 88-note piano keyboard produces a sound at 27.5 hertz, Middle C is at 261.6 hertz, and the highest note is at 4,186 hertz.) As another point of comparison, dogs can hear sounds in the 67 to 45,000 Hz range, while cats have the keenest ears, capable of distinguishing sounds from 45 to 64,000 Hz.
Two species of grouse from Europe and Asia, called caper-cailles, can produce sounds below 20 Hz in their breeding displays, and North America’s Ruffed Grouse drums at 40 Hz. They produce these sounds to attract mates and to defend territories from rivals. Behavioral studies show that pigeons can detect sounds as low as 0.05 Hz. This may provide one of the cues that pigeons use to home, because such sounds as wind blowing over different kinds of terrain and ocean waves can carry over hundreds or even thousands of miles. Great Horned Owls in some lab experiments appear to hear better than we do at the lowest frequencies, but they may be perceiving these sounds through their sense of touch as we perceive the lowest bass sounds by feeling the vibrations. Grouse may have evolved to produce sounds at the limits of owl hearing abilities so that they can safely display at dawn, dusk, and night, when owls are most actively hunting, preferably for something as large and tasty as a grouse.
Most songbirds seem to hear best in the range between 1,000 and 5,000 Hz, about the range of the top two octaves on a piano and a bit higher. Most people can easily hear songbirds singing at the mid-range, but as we grow older, we start losing the highest and lowest frequencies. Some bird songs are simply too high for most older people to detect without hearing aids: the sibilant trills of Cedar Waxwings in the 6,000 to 9,000 Hz range, the sweet notes of the Cape May Warbler at 10,000 Hz, and the lovely but sky-high Blackburnian Warbler songs that ascend at the end to sometimes top 11,000 Hz.
Q Do birds use sonar?
A Sonar, or echolocation, in the animal world is a method for detecting and locating objects by sending out sound waves that are reflected back by the objects; it’s usually found in nocturnal creatures or those that hunt in the blackness of the ocean deep or in dark caves. Sperm whales use sonar to find and catch squid in the ocean depths, and bats use it to catch insects and to avoid colliding with objects. Bats emit extremely high frequency sounds; these ultrasound wavelengths are tiny enough to bounce off very small insects.
Birds can distinguish the individual notes of complex, rapid songs much more easily than we can — to say nothing of being able to produce those songs in the first place. A Winter Wren in western North America sings an average of 36 notes per second! We can only resolve those individual notes by playing back a recording of the song at slow speed.
Among birds, the Oilbird — a nocturnal species of Trinidad and northern South America — and cave swiftlets of Southeast Asia are known to use echolocation. These birds produce audible clicks that bounce off the cave walls and other impediments, such as stalactites and stalagmites, allowing the birds to safely negotiate their treacherous homes. But neither the swiftlets nor the Oilbird can produce ultrasound notes, so their sonar is at best only one-tenth as functional as that of bats — Oilbirds can detect items only larger than 20 millimeters in diameter and will collide with anything smaller.
Q Can birds smell?
A People once believed that, with very few exceptions, birds couldn’t smell at all. They don’t have a specialized nose but simply nostrils, necessary for breathing and usually located near the base of their upper beak. But some species, including several ground birds and also some North American vultures and marine species called “tubenoses,” do have fairly large olfaction (smell) centers in their brains.
Many “tubenose” seabirds, especially petrels, shearwaters, and fulmars, locate their oceanic food by smell. Petrels are attracted specifically to the smell of dimethyl sulfide, an aromatic substance released by microscopic algae when tiny drifting invertebrate animals called zooplankton are feeding on them. The petrels don’t eat the algae; they’re after the zooplankton. This zooplankton isn’t easily seen, and petrels feed by night as well as by day, so their sense of smell is very useful, and very keen.
Tiny Wilson’s Storm-Petrels, smaller than American Robins, can detect slicks of dimethyl sulfide from great distances. Their albatross relatives also have a good sense of smell but are not attracted to dimethyl sulfide, probably because these larger birds feed on fish and squids rather than plankton.
Recent studies have shown that even some songbirds, with relatively tiny olfaction centers in their brains, can smell. For example, Cedar Waxwings, which eat berries that can ferment and make them sick, have a better sense of smell than Tree Swallows, which probably can’t take in the odors of flying insects as they snap and swallow them in flight.
Kiwis (flightless birds from New Zealand) are the only birds with nostrils close to the tip of their bills. Their olfactory centers are about 10 times the size of those of other birds relative to their size, and they apparently find the earthworms they feed on by smell.
Some homing pigeons use their sense of smell as one cue for navigating home. And some seabirds use their sense of smell to locate their nest.
Great Horned Owls eat skunks. Their sense of smell hasn’t been evaluated carefully, but if they are good at smelling, apparently their sensibilities about what smells good are quite different from ours!
Q Do vultures find dead animals by smell or by tracking predators or other scavengers on the ground?
A Researchers proved fairly long ago that Turkey Vultures can smell. In 1938, the Union Oil Company discovered that by injecting a strong-smelling organic chemical called mercaptan into gas lines, they could readily find leaks by monitoring vulture activity above the pipelines. Some mercaptans smell like rotting cabbage or eggs. They and related chemicals are released as carcasses decompose. To us, mercaptans smell horrible, but for vultures they are associated with fine dining.
In a 1986 study in Panama, Turkey Vultures found 71 of 74 chicken carcasses within three days. There was no time difference between finding concealed and unconcealed carcasses, and the only carcasses the vultures seemingly had trouble finding were the freshest ones. Even though the older carcasses emitted a stronger odor, the vultures showed a definite preference for eating fresher carcasses.
Greater and Lesser Yellow-headed Vultures of Central and South America, which are closely related to Turkey Vultures, seem to have comparable reliance on their sense of smell for finding food, and King Vultures may also use smell to find food. These species must all be able to find carrion in forests where the canopy visually obscures dead animals. Unlike these species, Black Vultures, which find their food primarily in open country, depend far more on vision and are believed to have a relatively poor sense of smell. Of course, one strategy that all vultures use to locate food is to watch for other circling vultures to drop down suddenly; in that sense, even Turkey Vultures find much of their food visually.
BAD-TASTING MEDICINE
Some strong tastes elicit interesting responses in birds. I once watched a fledgling Blue Jay that I was rehabbing pick up a large ant with the tip of its beak and then open wide to pull it into its mouth. Suddenly its crest went up and it spit out the insect and shook its head hard, all the time running its lance-shaped tongue against the roof of its mouth. Suddenly it picked up the ant again and started smearing the insect against its feathers.
Many birds engage in “anting.” Ant bodies are covered with a bitter chemical called formic acid, which may afford birds some protection from mites and lice. One European researcher studying a population of pipits that were suffering from a heavy infestation of feather mites learned that mites on birds that had been anting suffered much higher mortality than the mites on non-anting birds. In the case of my Blue Jay, apparently the taste of the ant elicited the behavior. People have reported birds anting with other items, including mothballs, cigarette butts, and onions.
Q How well do birds taste their food?
A Bird taste buds are similar in structure to mammalian ones, but they have significantly fewer than we have. Chickens have 24 taste buds and pigeons have fewer than 60, whereas we humans have about 10,000 and rabbits have about 17,000! Most of our taste buds are on our tongues, but birds have very few on the tongue, and none at all on the tip. Instead, most of their taste buds are on the roof of the mouth and deep in the oral cavity.
One researcher fed bread mixed with quinine to parrots, and they ate it without any apparent objections. In taste preference tests on pigeons, one researcher found that the birds rejected sour or bitter solutions, preferred low concentrations of salt and high concentrations of sucrose, but didn’t seem to respond to glucose at all. In another experiment, pigeons didn’t seem to have any response to quinine, and half responded to saccharin.
Birds do respond to some bitter tastes. If a Blue Jay bites into a monarch butterfly, the strong bitter taste makes the jay spit it out. Monarch caterpillars feed on milkweed, and foul-tasting toxins from the milkweed, called cardenolide aglycones, are taken up by the caterpillar’s tissues, remaining in the adult butterfly. The bright orange color of monarchs protects them from any bird that tasted one and either found it too bad-tasting to eat or swallowed it and got sick.
Q Do birds have a sense of touch?
A Many songbirds have very few tactile nerve endings on their feet. Gray Jays have been known to stand on cast iron pans where bacon is sizzling without apparent discomfort! But birds have an excellent sense of touch that works in other ways. Beneath the feathers, bird skin is very sensitive, especially where the flight feathers attach and at the wing joints, allowing birds to sense and adjust to the tiniest changes as they fly. Thanks to their exquisite sense of touch, hawks and other soaring birds can feel the rising air of a thermal air current to capitalize on easy lift during migration.
Furthermore, bird beaks and tongues may not have many taste buds but they do have a great many touch receptors, as shown by the following examples.
Woodcocks, snipes, and some sandpipers have exquisitely sensitive tactile sensory receptors at the tips of their bills, providing them with the means to detect and grasp worms more than two inches below the surface by touch alone.
Woodpeckers have dense, sensitive touch receptors at the tips of their tongues, which they use to feel out insects dwelling deep in wood.
Finches have a great many touch receptors in their beaks and tongues, allowing them to hold and crack open a seed shell in the corner of the beak as they deftly manipulate and swallow the seed inside.
The bills of ducks and geese are extremely sensitive, especially at the tip and along the outer edges. Just along the edge of the palate, a Mallard has 27 touch corpuscles per square millimeter, which compares with 23 touch corpuscles per square millimeter in the most sensitive part of a person’s index finger.
The bills of Wood Storks are so sensitive that when blindfolded, these birds could close their beaks on a live fish in as little as 0.019 seconds after first touching it; this is less than half the time it takes for us to blink.
The feathers around the mouths of Whip-poor-wills, called semibristles, have a tactile function similar to that of a cat’s whiskers and may also increase the useful area of the gape for sweeping up insects.
Q Can birds really feel barometric pressure?
A We have an abundance of field data that implies that birds must be able to feel barometric pressure. Before storms, as pressure is falling, birds feed more actively than at other times. During nocturnal migration or in foggy conditions, birds can maintain a safe altitude for long distances without being able to see or visually gauge the distance to the ground beneath them. Both of these abilities seem to depend on being able to detect barometric pressure, but scientists haven’t yet been able to discover exactly how birds do this. However, scientists have proven experimentally that homing pigeons are extremely sensitive to small changes in air pressure, comparable to about a 25-foot difference in altitude.
Q Do birds have any senses that we humans lack entirely?
A Many birds can sense the earth’s magnetic pull and use this information to help them navigate. In one experiment, scientists fitted homing pigeons with tiny metal helmets. Half of the birds wore magnetic helmets of iron and the other half wore helmets made of a nonmagnetic substance of the same weight. The birds were taken away from their home loft and released.
The pigeons with magnetic helmets usually became lost on cloudy days, probably because the helmet interfered with their ability to perceive the earth’s magnetic field, which they use to help them navigate when they can’t see the sun. (When the sun came out, pigeons with magnetic helmets found their way home, too.)
Tiny crystals of magnetite have been found near the olfactory nerves of pigeons between their eyes, and magnetite has also been found in similar tissues or the upper beaks in many migratory species. In a recent experiment, scientists trained caged pigeons to hop to one end of their cage when the magnetic field was normal and to the other end when the scientists switched to an abnormal magnetic field. The birds could only learn this task if, indeed, they could detect differences in the magnetic field.
When scientists attached magnets to the pigeons’ upper beaks, the birds could no longer perform the task. And when scientists temporarily froze their olfactory cavity (where deposits of magnetite are concentrated), the birds also failed to demonstrate this behavior.
Some birds may be able to detect magnetism by way of light receptors in their eyes that may be able to convert light and magnetic fields to nerve impulses. However they do it, the world of a bird must be filled with sensations that we can only imagine!
During the breeding season, you might see three Mourning Doves flying in tight formation, one after another. This is a form of social display. Typically the bird in the lead is the male of a mated pair. The second bird is an unmated male chasing his rival from the area where he hopes to nest. The third is the female of the mated pair, which seems to go along for the ride.
