The hidden life, and surprising depth, of the avian mind.
Oct 21, 2013, Vol. 19, No. 07 • By DAVID GUASPARI
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.