"What is it like,” asks Tim Birkhead, “for an emperor penguin diving in the inky blackness of the Antarctic seas at depths of up to 400 m[eters]?” And what is it like “to feel a sudden urge to eat incessantly, and over a week or so become hugely obese, then fly relentlessly—pulled by some invisible force—in one direction for thousands of miles, as many tiny songbirds do twice each year”?
He acknowledges that these questions can’t really be answered—so this book can’t really make good on its subtitle. A sighted person cannot explain to one blind from birth what sight is like, and a blind person cannot explain to a sighted one what it’s like to navigate without it. And these are easy cases, beginning as they do from a mutual understanding of what purposes our abilities serve: the wants and needs and lives of humans.
What Bird Sense does provide is fascinating: a survey of current knowledge about birds’ abilities to sense and respond to their environment. Until relatively recently, Birkhead says, the subject was a backwater, one he himself avoided as a graduate student. He tells of meeting a scientist who had spent his career studying the sensory biology of birds, but, having stirred little interest in his work, burnt his papers on retirement. When Birkhead asked to discuss them, the man was both “dismayed and delighted.”
This book is, necessarily, a bit of a miscellany—anecdote, experiment, history—but two general themes emerge: that the sensory world of birds has persistently proven richer than was previously supposed, and that vast amounts remain to be discovered. Separate chapters cover their vision, hearing, touch, taste, and smell, as well as “the magnetic sense” and emotions. Each begins with a story from the field. The chapter on sight quotes a 19th-century account of falconers capturing their birds—using a pigeon as bait and a shrike as falcon detector. At the approach of a raptor far too distant for a human eye to see, the shrike would become agitated, and in its behavior an experienced falconer was said to be able to read not only that a bird of prey was approaching, but also what species, how fast, and how low.
Like most people, I take a sub-scientific interest in the gaudy and the amazing, including a fondness for animals possessing Clark Kent-like powers and abilities far beyond those of mortal men. The basic evidence comes from anatomy and behavior. The shrike’s behavior shows it has detected a great deal about the approaching bird, but doesn’t tell us how that trick was done. Comparing the bird’s eye to a human eye gives one answer, without ruling out the possibility that other mechanisms are also involved. A fovea is a structure in the retina capable of especially sharp image processing—identifiable by a high density of cones (photoreceptors responsible for both color vision and acuity) and the absence of blood vessels and non-photosensitive neurons. A human eye has one fovea; the eye of a shrike (and falcon and eagle) has two.
Anatomical explanation often invokes the plausible principle that the relative size of an organ (and of the part of the brain that processes its signals) indicates its importance. The ratio of eye size to body size in birds is typically twice that found in humans. This principle is well and astonishingly displayed in the large seasonal variations that can occur in the size of a bird’s internal organs: e.g., the part of a male songbird’s brain associated with singing grows in preparation for the mating season and shrinks thereafter.
We all can see birds respond to song, and an anatomist can find in birds analogues to the structures of a human ear (inner-ear bones, cochlea, hair cells), so we easily credit birds with hearing, sometimes in a superhuman way. Birkhead reports that in the large, densely packed colonies of guillemots he has studied, parents and chicks can identify one another’s calls even against a background cacophony of others. Well-known experiments have shown that owls can hunt in complete darkness, tracking their prey by sound—to which Birkhead adds a poignant detail: Owls are not keen to fly in complete darkness, except in surroundings that they know; and even then, an owl that has seized its prey will fly straight back to its perch, retracing a path known to avoid obstacles.
It was much harder for ornithologists to discover the structures responsible for touch, taste, and smell. Whether birds have such senses was, for a long time, in dispute, despite an abundance of anecdotal and behavioral evidence. Not until the 1970s were the first avian taste buds discovered—in the tip of a duck’s bill. And Birkhead notes a brilliant speculation about taste from Alfred Russel Wallace, co-discoverer with Charles Darwin of natural selection. Some caterpillars are brightly colored, as if to flaunt their presence; they seem to be asking for trouble. And the colors cannot be useful in mating displays, since caterpillars are sexually immature. Wallace suggested that bad-tasting caterpillars would have an adaptive benefit from looking conspicuously different from those that tasted good. Subsequent experiments found birds acting as if they found the brightly colored caterpillars distasteful.
Audubon himself, Birkhead says, performed a highly influential but flawed experiment purporting to show that turkey vultures lacked the sense of smell and had to locate food by sight alone. His error was to test his theory with putrefying carrion; turkey vultures like it fresh. My favorite of the amazing smell stories is evidence that superimposed on the ocean is a “landscape” of smells related to underwater topography, and that far-ranging petrels and albatrosses, whose brains have huge olfactory bulbs, use not only local plumes of smell to locate food, but also the olfactory landscape to find their way back to the tiny island specks on which they nest. They can’t do it if their olfactory nerves are cut.
Birds’ feats of navigation have been a subject of wonder and speculation for centuries, and ingenious tracking technology has made it clear just how spectacular they can be. Geolocators, for example, are electrical devices that make it possible to track a bird’s movements by periodically recording the level of ambient light. From these data, one can determine the length of day, which correlates with latitude, and the time of solar noon, which correlates with longitude.
Important early studies of migration were based on caged birds, which can become restless at migration time, hopping up and down. Placed in “orientation cages,” allowing them to see the night sky, they hop in the direction of their migratory destination. These experiments provided evidence that some birds could use the stars for navigation; but more is involved, since some could orient themselves in total darkness. That realization revived a possibility, first suggested in the 19th century, that birds have a compass able to detect the earth’s magnetic field. The suggestion had been dismissed because there seemed to be no physiological mechanism to account for it. But experimenters in the 1950s showed that changing the magnetic field inside the cage with externally applied magnetic coils caused birds to reorient their hopping to the direction of this new field.
Birkhead sketches the two leading explanations for how this happens. The more charming goes like this: Magnetic fields can affect the rate at which certain chemical reactions take place; thus, the rate of reaction can serve as a detector. Further, those reactions are also induced by light; so a magnetic field may alter a bird’s response to light, which suggests that the presence of a magnetic field may be, literally, visible. This possibility gets support from astonishing experiments showing that a robin’s magnetic compass works only if the bird can see clearly out of its right eye. (An obvious question not discussed: Why, then, don’t humans also see magnetic fields?)
Birkhead begins his chapter on emotions with the story of a goose whose mate had been shot, and who spent the next week doing what might be described as standing vigil beside the body. Although we can explain this, he says, without reference to emotions—as a programmed response—we don’t have to. Fair enough. Birkhead is inclined to believe that birds do experience emotions, and hopes that behavioral observations and physiological measurements (birds secreting certain hormones, as humans do, in presumptively “emotional” situations) will be illuminating. It’s hard to see, however, how such measurements can ever count as evidence against the view that birds are simply automata. The point of Thomas Nagel’s famous essay “What Is It Like to Be a Bat?” is not (merely) the difficulty of knowing what it is like to belong to some other species, but that “no presently available conception gives us a clue” how an essentially subjective experience could be accounted for by a purely physical explanation.
Bird Sense cites a claim that we are currently in the golden age of sensory research on humans and expresses the hope that a golden age in the study of sensation in birds is to come. Perhaps Tim Birkhead will be able to write its chronicle in the not-too-distant future.
David Guaspari is a writer in Ithaca, N.Y.