The Avian Sense of Smell
photo of Turkey Vulture by Rohan Kamath	Most birds are primarily "sight animals" as their superb eyes, colorful plumage, and nonacoustic signals attest. But their sense of hearing is obviously also very acute -- as in the case of night-hunting owls, which use sound to locate their prey. Most birds seemingly would have little use for smell; in the airy treetops odors disperse quickly and would be of minimal help in locating obstacles, prey, enemies, or mates. Yet the apparatus for detecting odors is present in the nasal passages of all birds. Based on the relative size of the brain center used to process information on odors, physiologists expect the sense of smell to be well developed in rails, cranes, grebes, and nightjars and less developed in passerines, woodpeckers, pelicans, and parrots. By recording
the electrical impulses transmitted through the bird's olfactory nerves, physiologists have documented some of the substances that birds as diverse as sparrows, chickens, pigeons, ducks, shearwaters, albatrosses, and vultures are able to smell. The sense of smell seems better developed in some avian groups than others. Kiwis, the flightless birds that are the national symbol of New Zealand, appear to sniff out their earthworm prey. Sooty Shearwaters and Northern Fulmars are attracted from downwind to the smell of fish oils, squid, and krill, and when tested, investigate the area around a wick releasing such odorants. Other tubenoses such as the Ashy Storm-Petrel and Pink-footed Shearwater are also attracted to the same stimuli. When they return at night from foraging in the Bay of Fundy, Leach's Storm-Petrels appear to use odor to locate their burrows on forested Kent Island, New Brunswick. They first hover above the thick spruce-fir canopy before plummeting to the forest floor in the vicinity of their burrows. Then they walk upwind to them, often colliding with obstacles on the way. In one experiment the storm-petrels moved toward a stream of air passing over materials from their own burrow, rather than one passing over similar materials from the forest floor. In another experiment, individuals whose nostrils were plugged or whose olfactory nerves had been severed were unable to find their way back to their burrows. These results suggest that the storm-petrels locate their burrows by smell where there is heavy forest cover; they do not seem to use smell to find their burrows on unforested Pacific Islands. Interestingly, there is also some evidence that the smells in air currents near their lofts help pigeons navigate. There has been a long controversy over the degree to which vultures use odor to help them find food. Mostly the argument has been over whether sight or smell is more important, but it has also been suggested, by those with a flair for the absurd, that vultures listen for the noise of the chewing of carrion-feeding rodents or insects or even use an as yet undiscovered sense. Nonetheless, the sight-odor argument remains unsettled. While Turkey Vultures, for example, seem to have a good sense of smell, quite likely it is not good enough to detect the stench of decomposing food from their foraging altitudes. Experiments have shown that their threshold for detecting the odors of at least three different products of decay is too high to permit sniff location from high altitude. Whether or not the birds are more sensitive to the smells of other components of decomposition remains to be determined. More work will need to be done before we know whether vultures use sight or smell or both to locate the dead animals they feed on.
SEE: Hawk-Eyed ; How Owls Hunt in the Dark. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.