"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.