Myth of the bad-nose birds: study of avian sense of smell recovers from Audubon's blunder

Science News, August 20, 2005 by Susan Milius

Blame Audubon. Animal behaviorist Timothy Roper says that the renowned naturalist propagated die-hard misunderstandings of birds' sense of smell. In 1826, he published on the habits of the turkey vulture "with a View to Exploding the Opinion Generally Held of Its Extraordinary Power of Smelling." Observers as far back as Aristotle had claimed that various vultures sniff their way to prey. Yet Audubon argued that they couldn't smell. He described hiding smelly meat close to caged turkey vultures. They showed no sign of noticing.

Many people today still think that birds don't have working noses, and biologists often assume that the sense of smell doesn't matter much in avian lifestyles, says Roper. Yet decades of studies have shown that birds do have a sense of smell, sometimes as fine as some mammals'. Turkey vultures, for example, can pick up faint traces of certain odors. A classic report in the 1960s noted that California-gas-company repair crews found leaks in their lines by watching for circling turkey vultures attracted to the carrionlike scent added to the gas as a safety alert.

During the past 2 decades, sophisticated tests have identified more and more avian species that use a sense of smell. Some follow their noses home. Others follow their noses to food when a landscape, such as open ocean, offers few other clues. An ongoing study is exploring whether birds communicate by scent, as mammals do.

So what did Audubon do wrong? A century and three-quarters is a bit late for second-guessing an experiment, but Roper, of the University of Sussex in England, questions the naturalist's choice of meat. Vultures have standards, and the test meat may have smelled so bad that the birds weren't any more interested in eating it than Audubon was.

OLD NOSE-HOW Since the early 19th century, other naturalists tested birds and agreed that they probably don't have the sense of smell, says Roper. It's an understandable point of view: Most birds just don't seem as interested in smells as mammals are. Birds don't go around "peeing on lampposts and sniffing each other's rears," he says. And many do seem to rely on sight for hunting food.

The subtle olfactory behavior in many bird species led to a paradox, according to Roper. Despite the scarcity of behavior suggesting that birds can detect smells, morphologists during the 20th century found that birds had the right equipment to do so.

The pair of holes typically located toward the upper end of a bird's beak is, in fact, the avian version of nostrils. In pelicans, the holes are tiny, and a flap of skin shuts out water during dives. The air holes of kiwis, however, lie at the tips of their beaks and produce little sniffling noises as the kiwis hunt along the ground for food.

The rest of a bird's smelling equipment is typical of that in any other vertebrate. Air enters the holes and moves through a series of chambers. A bird's lungs draw much of the air downward, but some passes into a side chamber in the head. Receptors in an inner chamber's lining send signals via olfactory nerves to a part of the brain called the olfactory bulb.

During the middle of the past century, several researchers compared olfactory bulb sizes of different animal species as a way of gauging how much smell matters to the individual. Turkey vultures, a group of seabirds called tubenoses, and kiwis tend to devote a relatively large proportion of their brains to olfactory processing.

These birds have attracted interest from behavioral scientists, and in the past several decades, experiments have demonstrated how certain citizens of the bird world put their olfactory equipment to use.

WAY TO GO Interest in how homing pigeons find their way kick-started the new era of behavioral research on smell, says Roper.

The first major experiments on sniff navigation came from the lab of Floriano Papi in Italy during the early 1970s. He proposed that pigeons memorize patterns of odors in the wind--a pine-forest tang here, a seashore saltiness there. In otherwise unfamiliar landscapes, he suggested, the birds use their memories of these odors much as a person uses a map.

When Papi and his colleagues sabotaged pigeon noses in various ways, they found that the birds were less likely than unimpaired pigeons to find their way home from an unfamiliar place. As human navigators do, various bird species may also orient themselves by some kind of compass, perhaps the sun, Earth's magnetic field, or the stars. Yet even a navigator with a compass needs to know which direction is the right one.

The idea that birds can inhale a map was "unexpected," admits olfactory-navigation researcher Hans Wallraff of the Max Planck Research Center for Ornithology in Seewiesen, Germany. And he acknowledges that the idea remains controversial. Yet evidence from pigeons released in Morocco, the United States, Brazil, Germany, and England support the idea, he noted in the February 2004 Animal Behaviour.

Scientists have also tested a few other types of birds that home, he says. Smell sabotage has confused swifts. And European starlings released far from home can't find their way back if they can't smell anything.