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.