Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
CAN YOU SEE UV?
How Birds Employ Their Sensory Abilities
In those birds whose eyes are placed “laterally,” that is, on the side of the head, the two eyes are used for different tasks. Day-old chicks of the domestic fowl, for example, tend to use their right eye for close-up activities like feeding and the left eye for more distant activities such as scanning for predators.
—TIM BIRKHEAD, Bird Sense: What It’s Like to Be a Bird
We are constantly processing input from the environment, primarily visual and auditory signals, but we tend not to be acutely aware of our surroundings unless the stimuli are fast, colorful, or loud—in situations like crossing a busy street or playing tennis. Birds, on the other hand, appear to be exquisitely sensitive to their environs at all times. Have you ever approached friends who accused you of sneaking up on them? It has been a rare experience for me to successfully approach a wild bird as they seem to be much more aware of their surroundings. We humans miss or misinterpret signals with few repercussions. For birds it could mean life or death.
A bird’s sensory organs work basically the same as ours, but with different qualities and sensitivities to help them do things like find amphipods under the sand, see UV, or navigate over the ocean without instruments. The more we learn about birds, the more we uncover their extraordinary sensory abilities.
SENSE AND INTELLIGENCE
“Bird brain,” that negative appellation comparing someone’s lack of intellectual abilities to that of a bird, totally misses the mark. When I entered the field of ornithology as a grad student, the thinking among us zoologists was that mammals were smarter than birds because they have a relatively bigger cerebral cortex (controlling perception and behavior) while the cerebellum (regulating fine motor movements) is bigger in birds. The logical conclusion was that mammals are good learners but clumsy and birds are instinctive but agile. We were mostly wrong. It is safe to say that the cognitive abilities of birds, their social behavior, and their exceptional skill at discriminating among visual images make them a lot more capable than we thought possible, especially groups like the crows and parrots.
Birds receive input from their senses and then “decide” what actions to take. Ornithologists, behaviorists, and neuroscientists are still trying to tease out how nature and nurture interact, but it’s clear that both learned behavior (such as choosing the right food) and innate actions (like escape), often modified by learning, are based on information coming into the brain of a bird via its various senses. Young birds have to learn what to fear. Intuitively snatching every insect culminates in a learning experience when a toxic bug shocks the taste buds; blowing leaves frighten fledgling birds, which instinctively fly off as if the leaves were predators.
Tool-using birds are represented in 33 families. Some crows, warblers, finches, and woodpeckers fashion twigs to pry insect larvae out of tree bark; Egyptian vultures use rocks to break open Ostrich eggs; and crows drop walnuts in traffic for cars to crack. In the early 1900s, Blue Tits in England took to drinking cream off the top of home-delivered milk. When the milk producers covered the bottles with aluminum foil, the birds learned to poke holes through it. This behavior has an innate component, but through experience the birds learned to refine their techniques. Birds have survived for more than 150 million years by incorporating new abilities into their genes as the environment changed.
We need to be cautious in assuming that we know how birds see the world. The actions of our own friends can sometimes be puzzling—and we know what senses they use to receive input from the outside world—so you can appreciate how much more difficult it is to understand the world from a bird’s perspective. In Manaus, Brazil, where the Rio Negro flows into the Amazon at the “meeting of the waters,” thousands of Purple Martins roost at night during the Northern Hemisphere winter in a large oil refinery, just across the river from the rainforest. Steam, lights, and noise from the pipes mix with the cacophony of bird calls. Do the lights and noise bother them? Do they stay there because they are safe from predators? To figure out why birds do what they do, we first have to discover how they perceive their environment.
The Purple Martin may benefit from roosting among the hot pipes of a refinery because the heat destroys the insect midges which carry parasitic protozoans that infect birds.
VISION: SEEING IS SURVIVING
“Eagle-eye” has come to define someone with particularly good vision. Overall, birds have excellent eyesight, and no wonder—taking off, capturing prey, avoiding objects and predators, landing, and just hopping around require sharp vision. Vision is essential to birds not only because they are exceptionally active and mobile, but also because much of their communication is based on visible signals from other birds: distinguishing individuals of their own or other species, responding to courtship displays, recognizing territorial boundaries, and interpreting warning signals such as raised crests and alarm calls. Many animals do well in spite of their moderate visual capacity, making up for it with other abilities, but birds are successful largely because of their particularly acute vision and have exploited visually demanding habitats more effectively than any other animal group.
Eye Size, Shape, and Color
With eyesight being the predominant survival sense in birds, it is no wonder that evolution has selected for large eyes, especially in larger birds. In hawks, owls, and other non-songbirds, the size of the eye appears to be constrained only by the size of the brain with which it shares space in the skull. For owls, the eyes might constitute 1 to 5 percent of the bird’s total weight. Some observers have suggested that the face shape of owls is actually a result of their large eyes, which nearly touch in the midline. Ostriches have the largest eyes of all land animals, measuring 2 inches across and weighing nearly as much as the bird’s brain. The smaller size of songbird eyes is related to the bird’s body size rather than brain size, but their eyes are still large compared to ours. The European Starling’s eyes comprise 15 percent of the weight of the head, whereas the weight of our eyes is only 1 percent of our skull and contents. The larger the eye, the more light is gathered, so nocturnal foragers, like some shorebirds, owls, and nighthawks, have larger eyes. Fast-flying birds like falcons, swifts, and swallows—which need acute eyesight to avoid obstacles—also have larger eyes.
Birds that don’t need particularly sharp vision like sparrows, quail, and pigeons have “flat” eyes, which means the distance across the front of the eye is greater than the distance from front to back. This design compromise both captures the maximum amount of light and provides for wide-angle vision, but the small image that falls on the retina results in low visual acuity. Hawks, most eagles, falcons, and some songbirds have rather round eyes that narrow the field of vision, but allow more light to fall on the retina, increasing visual acuity. Owls and some eagles have tubular eyes. This shape allows the birds to focus intently straightforward, and because the birds have little or no peripheral vision, they tend to fly in a level flight path. Partly because of this habit, vehicles too often hit owls as they fly across a road. Some studies indicate that roadkill is the greatest source of mortality of Barn Owls in the United States.
The Barn Owl’s eyes are twice as light sensitive as a human’s.
The lenses of bird eyes are more variable in shape and accommodation ability (the change of lens shape to allow for better focus) than those of any other vertebrate group. A series of thin, overlapping, bony plates called the sclerotic (from the Greek skleros, hard) ring surrounds the eyes of all birds. When the strong ciliary muscles change the shape of the lens while focusing, the sclerotic ring maintains the shape of the eye. The sclerotic rings of owls are almost like cylinders encasing the eye, maintaining a tubular shape and precluding eye movement.
Iris colors are also extremely varied, much more so than those of humans, probably because eye colors are often clues to a bird’s species, age, or sex. For example, female Brewer’s Blackbirds have black eyes while the males have yellow eyes. Because the female’s plumage is much duller than the male’s they can easily distinguish sexes so the yellow eye of the male may simply be an attractive feature to the female. The eye color of the Eastern Towhee is white and the Spotted Towhee is red, perhaps an important distinguishing sign where their ranges overlap. Sharp-shinned and Cooper’s Hawks nestlings have gray eyes, young adults yellow, mature adults orange, and older adults red; eye color stages may be involved in maintaining a hierarchical structure.
Field of View: A Bird’s Observable World
Humans, looking ahead, are always moving forward into their world (often looking down into some handheld electronic gizmo), but birds, with a substantial field of view, are surrounded by their world and with frequent head movements, take it all in.
Human eyes have about a 160-degree field of view, the extent of the observable world at any moment. With an eye on each side of the head, songbirds have a nearly complete field of vision (except directly behind). Directly in front, they have 20–30 degrees of binocular vision, providing depth perception, but most of their sideward vision is monocular, which precludes depth perception but is sufficient to detect the approach of a predator. Ducks like the Mallard can see nearly 360 degrees, with a narrow binocular view in front and a blind spot in the back of their head. The American Woodcock can see 360 degrees, including narrow slits of binocular vision in the front and back of its head since the woodcock spends a good deal of time facing down and probing the ground.
Because birds have limited eye movement, they scan their field of view with neck movements. That’s why you see birds bobbing their heads up and down, looking quickly side to side, and craning their necks. We can turn our heads another 160 degrees to scan our surroundings, but any more than that and we would cut off blood flow to the brain as muscles and vertebrae would constrict blood vessels. Owls don’t have this problem. The field of view of an owl facing straight ahead is about 70 degrees with binocular vision and about 30 degrees additional on each side with monocular vision. But they have large holes in their neck vertebrae that provide extra space for the arteries traversing through them, allowing the birds to turn their head a total of 270 degrees to scan their surroundings.
Flexible necks make up for fairly rigid eye positions, but birds still have some eye movement. Pigeons, like most birds (at least that’s the assumption because there haven’t been enough experiments), move their eyes simultaneously in the same direction. That is, if one eye moves toward the front, the other does as well. Makes sense, yes? But experiments with Zebra Finches show the contrary. When one eye moves forward, toward the beak, the other moves backward the same distance, and vice versa. So how does the brain cope with two very different images? Turns out that whatever stimulus (food, another finch) appears on one side or the other, that eye moves toward the stimulus and the other eye responds by moving in the opposite direction. Presumably, this allows the Zebra Finch to attend to the object of attraction while still getting input about predators, for example, from the other eye.
The fields of view of a pigeon and an owl.
One would think that a wide binocular field of view would be essential for flight—and the fact that the flightless kiwi has only a 10-degree binocular field would seem to support that—but in the majority of birds only 15–30 percent of their field of view is binocular. However, woodcocks that fly quickly through dense wood and filter-feeding ducks that zip through wetlands and nest in heavy vegetation have a comparably small field of binocular vision to that of kiwis. The birds make up for their narrow binocular vision and avoid obstacles by being able to compare the images from each eye. If one eye sees a tree branch move faster than the other eye sees it, for example, the bird senses it is moving toward the branch and adjusts its flight in the other direction.
Try this: put your hand in front of your face with your arm extended. You can see your hand clearly. Now move your hand slowly to the side but keep looking straight ahead. Eventually your view of it becomes less clear and ultimately unrecognizable. In the center of your retina is the fovea (Latin for pit), which contains a high concentration of sensory cells and is responsible for acute eyesight. As your hand moves to the side, less light from it hits the fovea so the image of your hand blurs. The fovea is highly developed in birds, and about 50 percent of species, especially those that depend on sharp vision, have two foveas: one fovea looks straight ahead, like ours, and another looks to the side. So a bird can look to the front and side at the same time, important while pursuing prey. Most shorebirds have one ribbonlike fovea that focuses in a strip, appropriate for the flat horizon they see. Nocturnal and crepuscular birds, like owls and frogmouths, have only one fovea.
When birds are flying, objects whiz by their fields of view. Recall the old silent films during the Charlie Chaplin era in which everyone seemed to walk in a jerky fashion? That’s because those films were shot at 16 frames per second and our vision perceives a bit of a gap between each individual frame. Today films are shot at 24 frames per second, and TV cameras project at about 30 times per second. At those speeds we do not notice the change from one frame to the next, making the action appear seamless. This is called the flicker fusion rate. When driving we don’t see objects quite in focus because our brain didn’t evolve to discern images so quickly. But it did for birds. The studies done on birds—mostly pigeons and chickens—indicate that their flicker fusion rates are more than double that of humans. That means if birds were to watch movies, they would have to be shown at perhaps 100–120 frames per second or the birds would see the flickering of individual frames. Another example: humans generally do not notice the flickering of fluorescent lights, but researchers working with chickens indoors found that the birds can distinguish individual flickers of fluorescent lights. Although humans might develop symptoms such as headaches, eyestrain, nausea when exposed to flickering lights, the chickens are apparently not bothered.
When we see a flock of birds turning rapidly in midair, we view it as a coordinated, almost balletic swarm. But if we were to slow down a high-speed film of an airborne flock it would look much more chaotic, like a frantic mob rather than a flock. Because avian eyesight is optimized for speed, birds can more readily distinguish fast-moving objects and individual birds. Thus they are able to keep track of their adjacent flock mates and usually avoid collisions.
Flying requires good vision but it also subjects the eyes to fast-moving air. To protect their eyes, birds have a nictitating membrane or nictitans (from the Latin nictare, to wink or blink), in addition to eyelids. This “third eyelid” performs the function of rapidly cleansing and moistening the eye. In many birds and reptiles the membrane is lined with feather epithelia, club-like extensions of skin cells similar to feathers that help clean and refresh the corneal surface. The eyes of flying birds, especially those of fast-flying falcons, are subjected to drying air, so surface lubrication via the nictitans is important. Woodpeckers, adept at hammering away at tree trunks, face special physical problems, a couple of which are solved by the nictitans. First, the membrane acts as a safety belt, helping to prevent the eyes from bulging too far out of their sockets as the bird exerts 1000 Gs of force on a tree. Second, pounding on a tree produces woodchips and dust, but the membranes protect the eyes from any damage. The nictitating membrane of diving birds like loons and grebes protects their eyes from the drying effects of saltwater.
Seeing Color and Ultraviolet
To say birds see color seems pretty obvious because so many birds are so delightfully tinted with subtle shades of blue or red, highlighted with contrasting colors like black and yellow, decorated with a spectacular mosaic of rainbow colors, or bejeweled with iridescence. The common names of hummingbirds—Audubon called them “glittering garments of the rainbow”—include such accurate descriptions as azure, sapphire, emerald, brilliant, and sparkling. At the Currumbin Wildlife Sanctuary in Queensland, hundreds of green-backed, orange-chested, blue-headed, yellow-accented Rainbow Lorikeets greet visitors twice a day like the shattering of a colorful mosaic. And while sitting in a leaky boat with a rotten transom in the Caroni Swamp in Trinidad, West Indies, I caught sight of a tree full of Scarlet Ibises, the bright birds declaring their personal space on various branches, like ornaments on a Christmas tree. These hummingbirds, lorikeets, and ibises were certainly not selected by evolution for my entertainment, so why such colors?
Surprising as it first seems, color appears to be the driving force in early feather evolution. As discussed in the next chapter, bird feathers arose not as a modification of scales as was long presumed, but as novel structures on the dinosaur precursors of birds. Further investigation and fossil discoveries determined that even the most primitive forms of feathers were colored and had to be there for decoration. Today the purpose of colorful plumage in species, age, and sex recognition, courtship, and establishing and maintaining territories is evident.
Birds can see colors because they have rods and cones, sensory cells in the retina that transmit information to the optic nerve and brain. Rods detect black and white and are most numerous in nocturnal birds, whereas cones perceive color. Humans have three cones, detecting red, green, and blue. Birds have these cones, too, but they see more shades of color because their range is more evenly distributed across the spectrum. Birds also possess oil droplets in their cone cells that enhance the perception of color and sharpen vision by reducing glare, like polarized sunglasses. So our usual assessment of plumage color—red cardinals and pink flamingoes—is fine for describing birds, but may not reflect what birds see.
Along with red, green, and blue, birds have a fourth color receptor capable of perceiving ultraviolet (UV, “above violet”)—the spectrum of light radiation that has a shorter wavelength but higher frequency and energy than visible light. One study on monomorphic birds (those whose sex we cannot distinguish) used instruments that measured UV reflectance and indicated that more than 90 percent of birds whose sexes appear identical to us are actually sexually dichromatic to the birds themselves. Another interesting study investigated the response of territory-holding Yellow-breasted Chats to an artificial model. Even though the sexes of the chat look identical to us, the territory-holding birds responded with sex-appropriate behaviors to the models, depending on whether they were made of female or male feathers. The only clue they could have responded to was some aspect of plumage colors such as UV reflectance.
The ability to perceive ultraviolet means that some birds can not only identify individual birds, but can find food items based upon varying amounts of reflected ultraviolet light. A variety of fruits contain anthocyanin, a pigment and antioxidant. As the fruits ripen, anthocyanin levels, the caloric value of the fruits, and UV reflectance all increase. The plants evolved to be bird-dispersed, so ripe berries signal the birds with UV. Some predatory birds find their prey the same way. European voles are small rodents that mark their trails with urine as they move through the grass. Fresh urine has a higher UV reflectance than older urine markings so European Kestrels and Rough-legged Buzzards find their vole prey by following the fresher scent trails. A study of 98 songbird species in Europe found that the eggs of hole-nesting birds had more UV reflectance than those of open nests, presumably to help the parents see the eggs.
The range of sensitivity for each type of color receptor in birds—UV, blue, green, and red.
UV reflectance can also help European Rollers with large broods determine which of their young is most in need of nourishment. A relative of the kingfishers, European Rollers begin incubation soon after the first egg is laid, resulting in asynchronous hatching and thus different-sized hatchlings in the nest. The larger young tend to compete more successfully for food brought by the parents so the smaller young may receive less food. In a small brood, one to three young, there is little difference in the UV reflectance of the young, as the parents have no problem in provisioning for the whole family. But in a larger brood, thinner nestlings will reflect more UV light than the skin of larger, healthier nestlings, allowing the parents to fine-tune their feeding strategies to raise the most young.
Seeing Near and Far
Birds that forage in watery environments face interesting challenges. Because air and water have different refractive values (they bend light differently), animals that have normal vision in air become myopic (near-sighted) in water and animals that live in water are far-sighted on land. Gannets are one of the few birds that dive from considerable heights and then wing-flap underwater in pursuit of fish. Gannets are usually far-sighted when flying over the ocean, but within 0.1 seconds of entering the water, they become near-sighted by changing the shape of their lens to facilitate chasing their piscine prey. Kingfishers have two foveas in each eye. They use the fovea near the bill to focus on its aquatic prey item while diving toward it from a perch; upon entering the water, the more centrally located fovea comes into play so that the bird now has binocular vision. The Green Heron fishes in shallow water by dropping bait such as a twig, insect, or flower onto the water’s surface and waits for its prey to rise and take a bite. Due to refraction, the fish appears to be somewhere that it really isn’t, so the bird learns by trial and error to account for the redirected light from the prey. The oil in the color-perceiving cone cells also minimizes glare. The Tricolored Heron makes its life easier by shading the water and eliminating the sun’s reflection as it dances through the water with wings spread as it follows and stirs up fishy prey.
Although penguins tend to be near-sighted in both air and water, their eyes appear to function equally well in both environments. They don’t need to see very far on land and the suspension of particles in the water disturbs the clarity so that they cannot see very far while swimming. Underwater, penguins see green, blue, and violet best, but not red, probably because the low energy of red light does not allow it to penetrate deeply. The birds can probably see ultraviolet but it may be that the underwater vision of penguins depends more upon contrast than on color.
Tricolored Heron shading the water to attract aquatic prey.
CAN YOU HEAR ME NOW? HOW BIRDS DEPEND ON SOUND
Hearing is better developed in birds than in any other terrestrial vertebrates. Diurnal birds use sound in many ways—to listen for prey, predators, warning calls, mates, competitors, calls from their young, and so on, and in combination with their eyesight, they are acutely aware of their environment. Nocturnal birds, however, depend much more on sound even though they have excellent night vision.
Like that of a mammal, the ear of a bird has external, middle, and inner sections, and an eardrum to transmit acoustical vibrations. Instead of the three vibration-transmitting bones of mammals—the malleus, incus, and stapes (hammer, anvil, and stirrup)—birds have only the columella (Latin for small column, which it resembles). When sound waves hit the ear, vibrations are transmitted from the eardrum to the columella to the cochlea, the fluid-filled, slightly curved organ that contains hair cells with their nerve endings. The hair cells move in the vibrating cochlear fluid, amplifying weak sounds and converting them to electrical signals to the brain via the auditory nerve. Unlike humans, whose hearing declines with age and exposure to loud noises because of damage to and loss of hair cells, birds maintain their hearing all their lives because avian hair cells continually regenerate—important to such aural animals. Since this discovery in birds in the late 1980s, research to regrow ear hair cells in humans has restarted.
The external ear of birds lacks a pinna, the cartilaginous flesh-covered outer ear like we have. We cannot see the ear opening of birds except for those with unfeathered heads such as vultures and storks. If you were to hold a parakeet in your hand, though, and blow gently on the side of its skull, you can expose the opening that leads to the inner workings of the ear. This hole, the external auditory meatus (Latin for passage), is covered by auricular feathers that protect it from rushing air as the bird flies and helps to funnel sounds into the ears. The auricular feathers of diving birds protect their ears from water pressure. Because the Ostrich and its relatives are not flying birds and ear protection is less important, they only have a thin covering of feathers over their ear holes. The structures of owls that are sometimes referred to as ears are actually feather tufts that play no role in hearing but instead indicate the bird’s mood; the real ears are not visible.
California Condor; note ear opening behind eye.
Most birds receive sounds more or less equally because their ears aren’t far enough apart for the separation of the sounds to be significant. Barn Owls have a flattish facial disk that funnels sounds toward the ears. The majority of birds determine the source of a sound by moving their head, like we do, and some evidence indicates that the ear closest to the sound registers the sound at a louder and higher frequency, helping to localize it. It has been said that if you see an American Robin or European Blackbird walking with its head turned toward the soil they are listening for their prey—worms or insects crawling in their burrows or under the litter. But in reality the birds are simply turning one eye to the ground to look for worm castings or other visual signs of prey.
Owls have fleshy, cartilaginous ears, not unlike ours, but asymmetrical in shape and location—they don’t look exactly alike, and one is higher on the head than the other. Sometimes, when I give a talk on birds I ask for a volunteer in the audience to help explain why owls can locate sounds better than we can. I ask the volunteer to close her eyes and tell her I will snap my fingers in front of, over, or behind her head and that it is her job to determine the direction of the sound. Since I snap my fingers in the vertical plane that bisects her head from front to back, the volunteer rarely guesses correctly because the sound hits both ears with the same frequency and volume.
Volume and Frequency
Volume is the loudness or strength of a sound. Human conversation, for example, is rated at about 60 decibels, a dishwasher 80 decibels, and a motorcycle 25 feet away about 90 decibels. Whispering and the rustling of leaves only register at 20 decibels. A person with excellent hearing might be able to hear a sound at -15 decibels; Tawny and Long-eared European owls can detect sounds as soft as -95 decibels. (Seems curious, but decibels, like degrees of temperature, can be registered as negative.)
The sound-producing organ of birds is called the syrinx (Greek for pan pipes). Air moving over the syrinx produces a vibrating sound wave. The number of vibrations per unit time is the frequency and what our brains interpret as pitch. The measurement for sound frequencies is Hertz or Hz, defined as one cycle or vibration per second. Humans hear between 20 Hz (the lowest pedal on a pipe organ) to 20,000 Hz (a dog whistle), but are most sensitive to those between 1000 and 4000 Hz. Birds are most sensitive to sounds ranging from 1000 to 4000 Hz but there is considerable variability. In general, humans have a wider range of hearing than do most birds. The range of frequencies that a bird can most easily detect differs among bird species, and what birds can hear is closely related to the frequencies that species of bird can produce. The Horned Lark hears best between 350 and 7600 Hz, the Canary from 1100 to 10,000, the House Sparrow from 675 to 11,500 Hz, and the Long-eared Owl from 100 to 18,000 Hz, one of the broadest hearing ranges of all birds. Birds are more sensitive to frequencies, though: humans hear sound in bytes of about 0.05 of a second; birds hear the same sound in bytes of 0.005 of a second, so birds might hear 10 sounds in the time a human hears one. This allows birds to easily pick out sounds even in a rather noisy environment. The Hairy Woodpecker and probably other woodpeckers have the ability to hear beetle or bee larvae crawling under the bark of a tree. The Great Gray Owl can hear the rustling of a mouse below 13 inches of snow.
During my lectures on songs and calls in my ornithology class I analyzed a recorded bird song with a sonograph and showed the graphed printout of the sound frequencies the bird uttered. Then I asked students to try to imitate the bird sound and analyzed the best imitation with the sonograph. Although a student might produce a good imitation as judged by our ears, the graphs of the two songs were typically very different. Our vocal cords are not nearly as complex as the syrinx of birds and our hearing is not nearly as discriminatory. The sonograph of a Willow Warbler shows that the highest frequency the bird sings is at the beginning of the song and that it declines somewhat over a period of 4–5 seconds with a note sung about every 0.2 of a second.
Songs, Calls, and Why Birds Sing
Usually complex and relatively long, songs are those bird sounds that are considered melodious and pleasant to the human ear. Songs are most highly developed in the songbird group (order Passeriformes, also called the perching birds), which contains about 56 percent of all bird species. But not all members of the songbird group are singers; crows, jays, and jackdaws are certainly not known for pleasant warbling. Some naturalists construe bird song anthropomorphically, stating that birds sing because they are happy and enjoy singing. I always hate to disabuse them of this interpretation, but as a scientist, I know natural selection rarely allows frivolous behavior. The problem with the idea of a bird singing for pleasure is threefold. First, it’s usually only the male singing; why not females? Second, singing is usually restricted to breeding season when courtship and mating are taking place. Does that mean birds are not happy the rest of the year? Third, singing advertises a bird’s presence, which is ok if a female or competitor notices, but risky if a predator does. So, in the non-breeding season, a singer would just be announcing to predators he’s there for the taking. Exceptions include the European and American Robins, which hold winter feeding territories and sing to defend them. Female Northern Mockingbirds, Northern Cardinals, and Black-headed Grosbeaks have songs as complex as the males, but again, typically sing only in the spring.
Willow Warbler song, indicating a slight decline in pitch over its duration.
Calls are typically short, only a note or two or three, and are heard most of the year. Birds may use calls to communicate a threat, keep a flock together, indicate a food source, intimidate a predator, or announce their location. The number of distinct bird calls or songs depends on the range of the bird, the habitat, and neighboring birds, which might have a similar voice. The Common Raven has at least 25 calls. Chaffinches have a song plus 10 different calls: the flight call, social call, aggressive call, injury call, three courtship calls, and three alarm calls. Chaffinches mob a predator with low-pitched sounds described as “chink” calls. But when hidden in cover, the birds give a high-pitched “seeet” call, a thin note that causes other Chaffinches to seek cover. The chink note is easy to locate because of its low frequencies that differ in phase (sound vibrations of the same frequency occurring at slightly different times), whereas the seeet call is impossible to locate because it is composed of high frequencies with little phase difference.
Many birds have species-specific warning calls. Crows give a warning call that will frighten away only other crows. Nestling Dunnocks and European Robins only respond to the alarm calls of their parents by ceasing begging and hunkering down in the nest; reacting to the alarm calls of other birds would interfere with their feeding. But birds may also respond to the warning call of other species, whether or not the call resembles their own. A brochure from the Minnesota Department of Natural Resources states in bold print that “gull distress calls are specific to species and region.” Nope. Great Black-backed and Laughing Gulls will respond to the alarm calls of Herring Gulls. Fledglings just out of the nest suffer from a high risk of predation and immediately face the task of determining which alarm calls to respond to. Young Australian White-browed Scrubwrens, immediately after fledging, respond only to the alarm calls of adult scrubwrens. However, two weeks later, they also respond to the similar-sounding alarm calls of Superb Fairy Wrens and the very different alarm calls of the New Holland Honeyeater. But scrubwren fledglings in areas without honeyeaters do not respond to recorded honeyeater alarm calls. This demonstrates that the fledglings learn appropriate responses to the calls of other species.
Savannah Sparrow singing.
Communication between parents and young begins even before hatching. Young quail, still in the egg, chirp to their mother and ask her to turn the eggs to the correct position for hatching if they are upside down. The chicks also chirp to each other; the older ones closer to hatching chirp slowly and the younger ones chirp faster so that they coordinate their hatching times. Mallard ducklings-to-be still in the egg can tell their mother to turn them if they are too warm or cool on one side. The ability of young and parents to recognize each other by calls seems to be related to the sociality of the species. The more social interactions that occur among the members of the group, the more it is necessary for communication to develop among individuals. Young birds in nesting colonies, such as guillemots, penguin chicks, and Cliff Swallow nestlings recognize the calls of their parents. And the degree of colonization (bigger, more closely packed) is related to the degree of recognition. In non-colonial birds no such recognition occurs.
Birds can often distinguish the songs of neighbors, mates, and strangers, and some bird species can recognize specific individuals. Studies of White-crowned Sparrows indicate that each individual voice is sufficiently different that birds are able to recognize one another. They need to hear only the first three notes of another’s song; a difference in pitch distinguishes the bird. Many birds also exhibit varying degrees of vocal mimicry and imitate call notes or songs of other species. Scrub, Blue, and Steller’s Jays can imitate a Red-tailed Hawk either to warn others that a hawk is present or to scare nesting birds away from their nest so that the jay itself can eat their eggs. Mockingbirds and others in the family Mimidae are well known for imitating other birds’ calls although the main reason seems to be for a male to impress a female with its repertoire, which might exceed 200 different songs.
Although birds are primarily vocal, those with minimal oral abilities have developed other sounds. Kiwis stamp their feet when annoyed; Boat-billed Herons, storks, and albatrosses rattle or clap their bills; and the Ruffed Grouse “drums” with its wings. The male Common Nighthawk, to entice females, dives through the air and while pulling up, spreads his wings so that the feathers vibrate in the breeze much like the reed of a woodwind, making a noise commonly described as booming. The bird was once called a bullbat because its flight is reminiscent of a bat and its wing sounds boom like that of a bull. Male hummingbirds behave similarly; they dive and the feathers of their spread tail flutter and produce chirping sounds, different for each species of hummingbird.
Woodpeckers have calls and chatters, but they also advertise by drumming on anything that will produce a sound—dead trees, fence posts, houses, traffic signs, utility poles. Each species of woodpecker has its own drumming tempo and rhythm; I’ve watched Northern Flickers banging away at metal transmission towers, making an impressive racket. Both males and females drum as a form of communication and territory establishment, although males do it more frequently.
A rare ability in birds, echolocation—the emission of high-pitched sounds whose reflection indicates the direction and distance of objects—has been discovered in 16 species, notably the Edible-nest and Black-nest Swiftlets and Oilbirds. (The cave-nesting swiftlets of Asia are known for their contribution to “bird’s nest soup,” an expensive delicacy in China at $2,500 a kilogram. The nests, held together by saliva, form a gelatin when dissolved in water, producing the soup.) The swiftlets feed on flying insects by sight but nest in dark caves and have evolved a form of echolocation to find their way around. Unlike bats, however, the clicks they emit are audible to humans as well as to other birds in the cave. Oilbirds belong to an unusual group of birds found all over the world (order Caprimulgiformes) variously called nightjars, goatsuckers, potoos, and nighthawks. Oilbirds of northern South America got their name from the native Venezuelan practice of collecting and killing the young birds and using their oil for cooking. Like the swiftlets, oilbirds are colonial cave nesters. They leave the cave at night to feed on fruit (the only nocturnal birds to do so) and use echolocation, audible to humans, to find their way; they also emit screeches that appear to be communication between individuals as a response to disturbance. There seem to be no special adaptations to the ears of these birds to perceive echolocation.
The Red-bellied Woodpecker employs a simple drum roll at about 19 beats per second. The bird was named for the reddish wash on the belly, though it is often not visible.
SMELL: FIND FOOD, HOME, AND MATES, AND IDENTIF2 PREDATORS
For many years birds were considered to be poor smellers but recent evidence shows clearly that some birds use smell to find food, communicate, and navigate. The dinosaurian ancestors of birds apparently had a pretty decent sense of smell, as judged by the size of their olfactory bulb (the part of the brain that processes smell), which was about 30 percent the size of the cerebrum (cognitive part of brain). The olfactory:cerebrum ratio of modern birds averages 20 percent although a few, like the kiwi, come in at 30 percent. The olfactory bulbs of small forest-dwelling songbirds comprise only 3 percent of the brain whereas those of some seabirds take up to 37 percent. Having a good nose for food is an essential survival skill for ocean-living birds. I have been on a few pelagic (from the Greek pelagios, of the sea) birding trips off the coasts of California, Connecticut, and New Zealand. These daylong boating expeditions are great for spotting seabirds that we can’t see otherwise. On one journey, armed with saltine crackers, Dramamine, and image-stabilizing binoculars, I saw nothing unusual for the first hour or two as we motored out of sight of land. Then the bird guides began heaving buckets of buttered popcorn over the transom and birds started arriving almost immediately. Albatrosses can apparently smell food from as far as 12 miles away. I even heard a report of a Black-footed Albatross attracted by bacon drippings from as far as 20 miles away.
Smelling is also important for seabirds in orientation, navigation, and recognition. The Leach’s Storm Petrel, a small seabird, spends most of the year at sea, returning to land only to breed. It digs burrows in the soil with its bill and feet and lays one egg inside; about 42 days later it hatches. After hatching, the parents spend the day at sea feeding, returning to the burrow at night to avoid predators and then regurgitate the victuals into the young’s gaping mouth. To find the correct burrow among hundreds, the parent bird approaches the colony downwind and walks upwind, following the scent that will take it to its nestling. The importance of olfaction in finding young was demonstrated by experimentally plugging the nostrils of adult nesting prions (a type of small petrel) and Wilson’s Storm Petrels. As a result, the birds were disoriented and had difficulty finding their burrows. Larger seabirds, less at risk for predation, return to their burrows during the day without olfactory clues; experimental blocking of their nostrils had no effect on navigation to their nest site.
Next time you visit the ocean shore, watch the birds sitting on the water, most likely gulls. Every once in a while, one will move its head as if it were sneezing. What the bird is doing is ejecting salt out of its external nares, openings in the bill like the nostrils of our nose. To smell, birds inhale air through their external nares; some birds also possess nasal glands in a depression in or on top of their eye sockets to remove salt from ingested seawater. Gulls, terns, shorebirds, some ducks, and all ocean-dwelling birds have these salt glands. The tubenoses, such as albatrosses, petrels, and shearwaters have a tubular structure on their bill used for olfaction and filtering saltwater. In a study of several dozen hawks and eagles, researchers found that these birds also excreted salt solutions from their nares. Although they do not excrete as much salt as equivalent-sized seabirds, ridding themselves of salt ingested with their prey is important for water conservation.
Southern Royal Albatross showing its “tube nose.”
A number of land birds are able to distinguish scents. When I was director of a field station in the mountains of northeastern California for several years, one of my non-academic (and less-than-pleasant) duties was cleaning the grease trap outside of the kitchen. The job entailed uncovering the cement-lined pit full of waste kitchen water, skimming the solidified fat off the surface and tossing it on the ground nearby knowing that porcupines, coyotes, and other creatures would devour it overnight. Within minutes, chickadees, juncos, nuthatches, and jays were picking away at the unappealing puddle of grayish sludge. Surely they were attracted by the odor.
In 1826, J. J. Audubon, artist and naturalist, conducted a couple of crude experiments to test whether vultures could smell. He put a stuffed deer complete with artificial eyes out in the field; the vultures shortly found the deer and shredded it. Then he placed some decaying carcasses of hogs outdoors, some uncovered and others swaddled with burlap, and noticed that the birds only found the uncovered carcasses. Audubon concluded that vultures cannot smell and that they find their carrion by sight alone. This misconception persisted until the 1960s when an ornithologist demonstrated that Turkey Vultures can find carcasses by scent, later identified as ethyl mercaptan, exuded by decaying bodies. Smelling skunkish, ethyl mercaptan is put in gas pipelines to detect cracks in the pipeline because it is harmless but detectable to humans at low concentrations. Although pipeline workers use more sophisticated ways to detect gas leaks, they also keep an eye out for vultures, as the birds will circle over any pipe cracks from which gas is seeping out.
The kiwi is among the few terrestrial birds whose lifestyle is largely dependent on olfaction. Given their nocturnal habits and flightlessness, one would think that large eyes would be advantageous. But in a sort of regressive evolution, the kiwi developed an enhanced sense of smell while its eyes and visual center of the brain diminished. They are the only bird with nostrils located at the tip of the bill, which helps them find food by smell and touch while probing in the soil for earthworms. Kiwis also react to the feces of other kiwis, which are often placed in conspicuous places such as the top of a log or root, by sniffing rapidly, sticking their beak into the air, and moving their head back and forth, similar to a mammal’s response to an odor. This seems to indicate that kiwis use feces for social signaling, perhaps to declare a territory.
The rare Little Spotted (Grey) Kiwi exists in a few small populations in New Zealand.
Experiments with seabirds indicate that some birds recognize each other by smell well enough to determine genetic relationships and avoid inbreeding. Hector Douglas, a doctoral student at the University of Alaska, studied Crested Auklets in the Bering Sea and found that both sexes produce a tangerine-scented chemical, the concentration being highest during breeding season. The chemical acts as a tick and lice repellent; females prefer males with the strongest and perhaps the healthiest smell.
In another experiment, D. J. Whittaker of Michigan State University and several colleagues captured Dark-eyed Juncos from two different populations in California. One population inhabited an urban setting and the other a nearby montane region. The birds were housed in cages in identical environments and fed the same food for 10 months. Gas chromatography determined that the chemical odors emitted by the preen glands of each group were different. Differences between the sexes were found as well. This indicates a genetic basis for odor, suggesting that mate selection or species isolation (keeping different species from interbreeding) might be partially based on smell.
European Starlings incorporate strong-smelling herbaceous plants into their nests. The birds spend a considerable amount of time searching for the plants and it appears that the starlings choose the plants based on scent rather than appearance. The bacterial count in nests with green herbs is less than that in nests without herbs and the nestlings in herb-containing nests are healthier (as measured by blood cell counts). Not surprisingly, starling females prefer to mate with males with green herbs in their nests.
Scent as a Defense
The ability to detect the smell of a predator helps birds enhance their survivability. Experimenters in Spain placed the scent of a weasel on an active Blue Tit nest box when the young birds were eight days old. Video recordings revealed that when the parent birds arrived at the nest with food for the young they hesitated quite a while before entering. The parents did not make any fewer visits to the nest box because of the odor, but they spent less time there, reducing their chances of encountering a predator. Sleeping Great Tits exposed to the scent of a predator did not wake up or change their metabolism. They used anti-predator strategies such as roosting high in a tree or in a cavity before going to sleep rather than relying on olfaction.
Birds can also use scent as a defense against predators. The Northern Fulmar, a gull-like bird related to petrels that is commonly seen following fishing vessels, has a defense mechanism known for its “ick factor.” If a predatory bird approaches, the fulmar vomits a stream of orange-colored liquid up to 10 feet, covering the intruder. Smelling like decaying fish, the liquid coats the potential predator’s feathers, reducing their waterproofing properties, and making it difficult or impossible to fly. (Fittingly, “fulmar” comes from the Old Norse, meaning foul gull.) The Southern Giant Petrel has a similar defense mechanism, ejecting oil from its gut when threatened. Eurasian Roller nestlings regurgitate and cover themselves with a foul-smelling orange liquid when frightened, not only becoming less palatable but also warning the parents returning to the nest that a predator was near. Northern Shoveler, Mallard, and Common Eider ducks eject feces onto their eggs when frightened by an approaching predator (or researcher). Some have speculated that the feces odor deters predators but it may simply be that the sitting birds eject feces as they take off.
A common misconception is that parent birds will abandon their nest if a person touches the nest, young, or eggs, leaving a scent. I’ve heard and read this hundreds of times and even a nature center website stated that parent birds will kill their young if humans touch them. I did years of research on blackbirds, grebes, grackles, cormorants, gulls, and ducks, counting nests and eggs. I have climbed Osprey nests and weighed and banded the young while the parents flew overhead and complained. But I was never aware of any nest being abandoned because of my actions. Parent birds, once they have invested the energy to lay and incubate eggs and especially after hatching them, are unlikely to give everything up and fly away because of a minor human disturbance. Like any animal (or person) in a desperate situation, birds will persist to the last minute. Survival depends upon it.
FLAVOR, BITTERNESS, SUGAR, AND SPICE: WHAT DO BIRD2 TASTE?
From carrion-seekers to berry-eaters, the sense of taste varies greatly in the avian world with some birds rejecting distasteful fruits and others devouring hot peppers. A good deal of information is still yet to be uncovered and elucidated about the sense of taste in birds. Experimenting with birds and various substances in the lab tells us something, but we need more information from ecological and behavioral observations of birds in the wild to complete the picture. Compared to other vertebrates, birds have few taste buds. Catfish have around 100,000, rabbits 17,000, humans 10,000, and cats and lizards 500. Among birds, European Starlings have 200, the Japanese Quail 62, Bullfinches 46, and the barnyard chicken only 24. The taste buds of chickens are located so far back on the tongue that by the time the chicken tastes what it has eaten, it is swallowed.
Numerous examples have shown that birds can taste at least some flavors. Rice farmers lose millions of dollars each year in lost crops and expenses incurred trying to control pest birds. Dozens of techniques are employed, including flags, noise cannons, electric wires and fences, and model and real airplanes, to name a few. One of my former students used to ride shotgun (literally) in a cloth-winged biplane, shooting at blackbirds out a window. He moved on to another occupation after he blew a hole in one wing of the plane. Chemical deterrents to make the rice taste bad have been tried but they have proven expensive, ineffective, harmful to the birds, or not safe for food crops. One chemical that has seen moderate success is methyl anthranilate, which has been used to protect rice, fruit, and corn crops against some avian pests, and golf courses against the ravages of Canada Geese.
Lincoln and Jane Brower, along with their colleagues and students, studied various examples of plant chemical defenses. Their classic 1971 study of birds being stymied by bad taste is that of Blue Jays eating a Monarch butterfly and vomiting shortly afterward. Monarch caterpillars feed on milkweed and ingest cardiac glycosides. A Blue Jay chomping down on a caterpillar gets a nasty taste in its mouth almost immediately and the encounter is so intense that the bird learns not to eat Monarchs or the look-alike Viceroy. Recent studies indicate that Viceroy caterpillars accumulate salicylic acid from eating willow and poplar leaves and that they may be unpalatable on their own so perhaps the Monarch and Viceroy are co-mimics, both protected from bird predation. Clearly birds can taste and learn from the experience.
In 2004 the genome of the chicken was sequenced and researchers discovered that chickens lack the gene that allows the tasting of sweet substances. Another dozen or so birds tested also lack the gene so it’s probably the case for many if not most birds. Red-winged Blackbirds will even choose plain water over a weak sugar solution. On the other hand, sugar lovers include parrots, hummingbirds, and other nectar and fruit feeders, even though they don’t perceive the taste as sweet. Hummingbirds easily detect differences between the content of high and low sugar solutions and the amount of time hummers spend feeding is based upon the concentration of the solutions; higher sugar levels mean less time feeding. Douglas Levey of the University of Florida demonstrated that tropical tanagers can distinguish between solutions of 8, 10, and 12 percent sugar and that they prefer the latter, but South American manakins did not detect any differences. The explanation is that tanagers crush the fruit as they eat it and manikins, like the Araripe Manakin, swallow it whole, so the tanagers are exposed to the chemicals of the fruit and manikins are not, at least not very much. The Long-tailed Manakin eats unripe fruits when nothing else is available but only enough to maintain its weight. The message here is that being unfussy and eating every available fruit, even those low in nutritive value, is a successful survival mechanism for birds that gobble bad-tasting fruits whole. This gives them access to fruits that other birds generally ignore and will only eat in times of food shortage.
Bitterness is common in toxic plants and is often a signal for birds to avoid a plant. For example, birds avoid foods high in tannins, an important chemical component of plant defense, especially in oaks. Tannins reduce the digestibility of proteins and are sometimes toxic in high concentrations. However, Blue Jays can subsist on a largely acorn diet for the winter if they have access to supplementary protein such as acorn weevil larvae. A species of aloe plant in South Africa produces dark nectar with a bitter taste; the plant is rejected by nectar-feeding bees and sunbirds, but is fed upon by bulbuls, white-eyes, chats, and others that are apparently unaffected by the nectar’s taste. The plant has apparently evolved to allow only a narrow selection of birds to distribute its pollen. Variation in the sensitivity to bitter tastes among different species is to be expected, but recent studies of the White-throated Sparrow indicate that there may also be differences among individuals within a species as their genetic makeup encodes for 18 different bitter taste receptors.
Not much is known about the frugivorous Araripe Manakin, a rare endemic bird of eastern Brazil, only discovered in 1998.
Chile peppers contain the chemical capsaicin, which, as you know if you have eaten one, causes mammalian pain receptors to produce mild to painful burning sensations. Capsaicin is such a powerful chemical that it is used in bear repellent at a 1–2 percent concentration; mail carriers use a weaker solution (0.35 percent) to deter dogs. Capsaicin has no smell or taste; its only stimulus is pain. Mammals learn to avoid eating peppers with capsaicin in concentrations as low as 100–1000 ppm (parts per million) but birds can eat peppers with concentrations up to 200,000 ppm as they lack the chemical receptor that reacts to capsaicin. This evolutionary strategy assures that pepper plants will not be eaten by mammals—which would chew and destroy the seeds in their guts—but by birds, which, for the most part, will pass the seeds through their digestive system unscathed and disperse them.
TOUCHY, FEELY: A LESS-EXPLORED SENSE
We know more about the touch sensations of mammals than we do about how birds respond to tactile stimuli. A bird’s body is mostly covered with feathers that are insensitive to touch, although receptors are present in the skin at the base of the feathers. The scaly feet and smooth ramphotheca are alive but do not appear to be very good touch receptors. Birds have four kinds of tactile receptors: Herbst’s corpuscles, Grandry’s corpuscles, thermoreceptors, and nociceptors.
HERBST’S CORPUSCLES are the most widely distributed of the tactile receptors, found in the feathered skin, beak, tongue, legs, and feet. The receptors in the skin are associated with feather follicles and detect plumage disarray, causing the bird to commence preening or alter its flight behavior. Shorebirds, waterfowl, kiwis, parrots, ibises, spoonbills, and Whimbrels have abundant Herbst’s corpuscles in their bills. Probing in the sand produces a pressure gradient; an object such as a mussel interferes with the normal gradient by blocking the water flow, which the Herbst’s corpuscles detect, informing the bird of the mussel’s presence. Herbst’s corpuscles can also detect the movement of invertebrate prey burrowing through the substrate. The bill-tip organs of ibises function similarly to shorebirds, but the more aquatic ibises have a denser concentration of corpuscles than the other species, suggesting that these birds might be able to detect prey items in the water as well as below it. Rock Doves have a string of Herbst’s corpuscles about 40 mm long. Anatomical, electrophysiological, and behavioral studies indicate that the pigeon can detect vibrations in the environment via these nerve endings. No definitive reason has been established, but it is logical that the movement of a branch, perhaps a predator, could awaken a bird sleeping in a tree. Some have speculated that birds can sense high-frequency vibrations that precede an earthquake; possible, but it’s not likely this system evolved to escape earthquakes.
Whimbrels and other shorebirds use Herbst’s corpuscles to forage nocturnally as well as during the day. The corpuscles allow the birds more time to feed and help them avoid diurnal predators and coordinate their feeding times with the tidal schedule.
GRANDRY’S CORPUSCLES are nerve endings found in the bill tip of aquatic birds; these sense organs, along with Herbst’s corpuscles, detect bill tip movement. Kiwis have both Herbst’s and Grandry’s corpuscles. The bill-tip organs of three families (sandpipers, kiwis, and ibises) appear to have evolved independently, so it may be that other probing birds have similar organs that we have yet to discover.
TEMPERATURE RECEPTORS or THERMORECEPTORS consist of free nerve endings in the skin but are primarily located in the beak and tongue. Cold thermoreceptors are more abundant than heat receptors and various areas of the feather-covered skin are differentially sensitive to temperature. The skin on the back of the Rock Dove is more sensitive to heat than the skin on the wings and breast, possibly because the back of the bird is more often exposed to the sun than other parts of the body. Unfeathered skin areas such as the legs and feet are fairly insensitive to either warm or cold input. The brain also contains thermoreceptors, which signal the bird to make appropriate physiological or behavioral adjustments to maintain the proper body temperature.
NOCICEPTORS are free nerve endings that are receptive to any stimulus that threatens or causes pain. They are located in both the skin and the beak and are sensitive to strong mechanical forces, chemical irritants such as toxins in plants, and heat over about 113°F. Birds respond with increasing blood pressure and heart and respiratory rates.
The more advanced our technology becomes, the more we discover about the senses of birds. It has been only since the 1980s that we have begun to understand that birds can see ultraviolet, their hearing does not diminish because they replace damaged hair cells in the ear, they detect objects by differences in pressure, and they can eat hot peppers because they simply can’t taste them. What else do we have yet to learn?