A long-standing myth says that birds have no (or very little) sense of smell (olfaction). Myths usually have long lives, and this one became dogma despite many casual observations suggesting that birds can smell. Only gradually, over many decades, and facing lots of resistance, has the myth lost strength—and finally we can say with certainty that the myth is effectively dead. Birds do have a very functional sense of smell and they use it in many ways (just as mammals do).
Scientists have determined the genetic basis of the olfactory sense and birds have the requisite genes for appropriate receptors. Studies of birds’ brain activation during exposure to odors show definite responses, usually very specific to particular odors. The olfactory bulb of brains varies in size, but even the very small ones (representing a lower number of olfactory receptors) have exhibited clear functional responses. Anatomy, electrophysiology, and genetics back up interesting observations and experiments that clearly demonstrate the variety of uses for the avian sense of smell. Some examples follow:
The myth of no-sense-of-smell began to be perforated by the late 1900s; although at first, the few documented examples were just ‘exceptions’ to the dogma. The examples accumulated, and a number of birds were generally acknowledged to use olfactory cues for foraging. For instance, kiwis of New Zealand have a long bill with nostrils at the tip, and they use that long sniffer to grub for earthworms in the soil. Woodcocks in the northern hemisphere do that also. Carrion-eating turkey vultures can detect the aroma of freshly decaying meat (not rotten) far downwind of a carcass (sometimes misled by the same odor coming from leaking natural gas pipelines). Seabirds such as albatrosses and petrels can follow the odor plume of an aggregation of krill for miles, until they reach a concentration of that favorite planktonic crustacean. In Europe, storks can trace the smell of newly mown grass to find meadows for hunting bugs and rodents. European great tits and blue tits track down the volatile chemical aromas emitted by pine trees that are assaulted by insects, and there they find lots of prey.
More myth-perforating information appeared from studies of navigation. For example: Homing pigeons exhibit an ability to locate their home lofts by smell, especially in conjunction with using visual, landmark cues. European starlings can return to their nest sites after being experimentally displaced for long distances, if their sense of smell has not been blocked. Shearwaters can navigate over the open ocean, back to their nesting colonies, using their noses.
Furthermore, several kinds of song birds, including European starling and blue tit, can detect the presence of predators such as weasels; parent birds spent less time visiting their chicks if the nest cavity was decorated with the scent of weasels. Hummingbirds often forage on flowers, but they are deterred by ants—not just the presence of ants but even the aroma of the ants’ formic acid (which they can spray to deter their own attackers). Several kinds of birds (e.g., starlings and blue tits in Europe, russet sparrows in China) add aromatic herbage, such as yarrow, milfoil, and wormwood, to their nests. The effects of the greenery seem to differ among the species, often leading to better chick growth–in some cases reducing parasites (ticks, bacteria), or reducing mosquito bites at night, or somehow improving the parents’ incubation patterns, and even affecting mate choice.
Such observations and experiments completely shredded the myth and demolished the dogma. But there was still skepticism about the use of smell in social relationships among birds— Canbirds identify kin, sex, and the identity of individuals? Can theyuse olfaction in avoiding conflict or in courtship and mate choice? Oh yes! Many studies now have shown that the avian sense of smell is sufficiently finely tuned to be used in these ways too. Here are some examples:
Some birds can self-identify: kiwis can discriminate between the odor of their own feces and that of other individuals, and are said to show territorial aggression when another individual has been detected nearby. Some petrels and prions can identify their own nest burrows by scent alone, avoiding conflict that would occur if they mistakenly entered someone else’s nest burrow. Blue petrels can identify their own eggs and avoid those of other conspecifics.
House finch males can assess the quality and social rank of other males. Spotless starlings can tell the sex of other individuals by their odor, and male mallards get really revved up by the smell of females in the courtship season. Kin recognition by smell can be accomplished by some species (e.g., storm petrels, house sparrows, zebra finches), in some cases avoiding mating with kin, in other cases preferring to associate with kin. Antarctic prions can recognize their mates by smell. Young zebra finches can recognize the odor of their siblings and the natal nest.
How do the birds make such particular identifications? It’s likely that genes involved with the immune system are involved (as they are in mammals, including humans). These genes vary a lot among individuals and are known to affect odor (somehow).They may be dispersed over the birds’ bodies when oils in the preen gland at the base of the tail are spread over feathers as the birds preen to keep feathers in good condition. These genes have been associated with mate choice in house sparrows and petrels. House sparrows make mate choices in part on the immune system, avoiding individuals with too few immune-system genes, preferring those with a good diversity of those genes.Blue petrels prefer mates with immune systems differing from their own, although that was not the case for Magellanic penguins.
An intriguing example comes from crested auklets, a colonial nester on sea cliffs around the Bering Sea. In the courtship season, they engage in the endearing behavior of ‘ruff-sniffing’—nuzzling each other’s feathers at the nape of the neck. The feathers there are specialized, emitting a citrus-y aroma, which comes from certain volatile lipids called aldehydes. These compounds can deter ectoparasites such as lice, and one of them seems to indicate that the owner has good metabolic stress responses (perhaps indicating status and making it a good potential partner). Auklets are attracted to the scent, and sometimes the ruff-sniffing involves several individuals. Birds emitting lots of this aroma can transfer more parasite deterrent and they are more attractive; they are likely to be favored as mates. If the possible status-indicator is transferred, the recipient might then falsely advertise its (unearned) status. These parts of the story needs more study.
We can expect to see more and more research revealing that birds can use their olfactory sense in many different social ways.
For more on auklets’ ruff-sniffing, see this article.