Conventional wisdom has maintained for years that birds have remarkable special senses, especially vision and hearing. Olfaction, on the other hand, was relegated to the back seat.
The thinking was based on the sizes of the lobes of a bird’s brain. The lobe that handles visual signals is large, while the lobe that processes olfactory information is relatively small.
The argument goes back to Aristotle and Pliny, who declared that vultures detected dead animals by smell. (They didn’t realize that vision is most important among Old World vultures.) In 1826, Audubon presented a paper “to explode the opinion generally entertained” of the extraordinary sense of smell possessed by the Turkey Vulture. The bird would visit a taxidermy-mounted deer on its back, Audubon stated, but it failed to find a putrid dead hog covered with brush.
Many naturalists disagreed, but the matter wasn’t settled until 1964, when Kenneth Stager, of the Los Angeles County Museum, showed that Turkey Vultures were indeed attracted to the smell of dead animals, but that the carcasses had to be a bit fresher than Audubon’s hog. Stager also demonstrated that a taxidermy-mounted deer would not attract vultures. He learned from engineers that Turkey Vultures were expert at finding ruptured natural-gas lines. The birds don’t detect the gas, which is colorless and odorless; they smell ethyl mercaptan, one of the foul-smelling gases produced by rotting flesh. Utility companies routinely add the compound to make homeowners aware of leaks.
Read Kenneth Stager’s paper:
Kenneth E. Stager, 1964, The Role of Olfaction in Food Location by the Turkey Vulture (Cathartes aura), Contributions in Science No. 81, Los Angeles County Museum, June 30, 1964 (PDF).
Among birds having the largest olfactory lobes, the kiwis are the olfaction champions. Kiwis are nocturnal, and they eat earthworms, which they locate by probing the soil. The nostrils of kiwis, unlike other birds, are located near the tip of their long bills.
The tube-nosed seabirds have large olfactory lobes and good senses of smell. They forage by locating the odors of fish and squid or dead animals. Some birds, including Wilson’s and Black-bellied Storm-Petrels, can detect dimethyl sulfide, a gas given off by phytoplankton, especially when fed upon by zooplankton, including krill. The birds fly in a zigzag pattern generally at right angles to the wind. When they pick up a scent, they follow it into the wind toward the source.
Some seabirds, such as Leach’s Storm-Petrel, nest in burrows and rely on olfaction to find their nests at night. The birds land downwind and walk until they pick up the scent of their burrow. Then they go toward it. In experiments involving a maze, storm-petrels presented with the contents of different nests routinely selected their own.
Other birds that demonstrate olfaction are homing pigeons, which can locate their lofts by smelling nearby air; African honeyguides, which typically eat beeswax and are attracted to burning candles; and European Starling and Blue Tit, which select nest material from aromatic plants, such as lavender and mint. The greens reduce nest parasites.
During the 2000s, Danielle Whittaker and other biologists at Indiana University discovered that chemicals in the preen-gland secretions of Dark-eyed Juncos were characteristic of males or females. Birds with a higher proportion of the correct sex-specific volatile compounds had more offspring. Furthermore, the concentration predicted male success at raising offspring to fledging. Males with higher amounts of female-like compounds had more youngsters in their nests fathered by other males.
See photos of Dark-eyed Junco.
Similarly, biologist Julie Hagelin made a fascinating discovery while studying Crested Auklets on Alaska’s St. Lawrence Island. She noted that, during the breeding season, auklets gave off a tangerine scent that is apparently used in communication. Birds in pairs or groups bury their faces in other birds’ napes, where the scent is strongest. The source is unclear, as the oils from the preen gland lack the scent. In tests, the auklets select the tangerine scent over others. And taxidermy mounts in the breeding colony treated with synthetic tangerine scent attracted both males and females more than unscented controls. While it’s tempting to assume a role in mate selection, the function of the tangerine scent has yet to be determined.
We now know that birds definitely have a sense of smell and that it varies widely among groups. Recent work has identified olfactory-regulating genes that code for detecting particular scents. In general, the larger the optic lobes, the larger the number of genes. Birds with the largest number, more than 600, are New Zealand’s kiwis and the Kakapo, a nocturnal parrot. Birds with the smallest numbers, about 200, are songbirds. (Humans, not considered to be strong smelling mammals, have about 400.)
The discovery of avian responses to scent-related chemical signals will add to the complexity of the amazing behaviors of birds.
This article from Eldon Greij’s column “Amazing Birds” appeared in the November/December 2015 issue of BirdWatching.
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