Until the latter part of the 20th century it was generally accepted that but for a few exceptions birds had no sense of smell or it was so poor as to be virtually non-existent. It was assumed that birds relied on their acute vision and hearing to find food or sniff out predators or potential mates.
One of the reasons that this idea pervaded for so long is that scientists found it difficult to study avian olfaction. Reports were anecdotal and many experiments produced contradictory results.
There were some early pioneers. In An Introduction to the Natural History and Physical Geography of Spain,published in 1775, William Bowles, the Irish naturalist, described how he observed a woodcock probing for worms in the soil. The bird never missed its aim and Bowles surmised that it was using its nostrils rather than its bill to locate the worms.
In the 1830s Sir Richard Owen, the English biologist and contemporary and outspoken critic of Charles Darwin, dissected a kiwi and upon observing its tiny eyes and large olfactory region suggested that it relied more on its sense of smell than its sight.
But in the previous decade, the illustrious American ornithologist, John James Audubon, had conducted a series of experiments to prove whether turkey vultures had a good sense of smell, and came to a conclusion that put the brakes on almost any scientific development for over a century.
Since he was a boy, Audubon had agreed with the deeply held notion of theorists at the time that turkey vultures used their acute sense of smell to find putrid, rotting carrion. However, his own observations on how birds can find prey from a great height had led him to question whether the theory was correct.
In the first experiment he stuffed the skin of a deceased deer with dried grass, added a pair of clay eyes, and placed it in a large open field on its back with its legs in the air. He watched as a turkey vulture alighted besides the stuffed deerskin before attacking the eyes and tearing apart the stitches only to discover there was no meat inside. He later saw the vulture locate and kill a small garter snake and concluded that the vulture relied on its eyes rather than its sense of smell to seek out prey.
In a second experiment Audubon placed a dead hog in a ravine and covered it with briars. In the July heat the hog soon started to decay and the smell became unbearable. Over the next two days he watched as many vultures in search of food flew over the ravine, yet none discovered the hog’s carcass. Yet, during the same period he saw several dogs approach it to feed.
Audubon concluded that “the power of smelling in these birds has been grossly exaggerated, and that, if they can smell objects at any distance, they can see the same objects much farther.”
So Audubon may have been surprised to learn that over a century later Kenneth Stager, an ornithologist from the Los Angeles County Natural History Museum, proved that turkey vultures do indeed use their sense of smell to find dead carcasses.
In the 1960s Stager discovered from engineers working on the California gas pipeline that wherever there was a leak in the pipe turkey vultures would congregate. On speaking to them further he learnt that since the 1930s a sulphurous compound, ethylmercaptan, had been added to the gas to help humans detect gas leaks, as natural gas has no smell.
Ethyl mercaptan, it turns out, is also naturally emitted when an animal dies, and Stager hypothesised whether turkey vultures were attracted to its smell. To test the theory Stager blew wafts of ethyl mercaptan across California and sure enough the vultures flocked towards it. What Stager also showed is that birds that feed on carrion prefer fresh carcasses and Audubon’s experiment had failed because he had used rotting carcasses well past their sell-by-date.
We now know that turkey vultures have an extraordinary sense of smell and are able to smell carrion from over a mile away. In 2012 Gary Graves, a researcher at the Smithsonian Institute, led a study to examine the brains of hundreds of vultures that had been killed as part of a legal culling operation at Nashville’s airport.
The researchers found that on average the turkey vulture has an olfactory bulb that is 4 times larger than its cousin, the black vulture, and was significantly larger relative to brain volume than 143 other species.
They also found that turkey vultures have twice as many mitral cells, the cells used to help transmit information about smell to the brain, as black vultures, despite having a brain that is a fifth smaller.
It is the evolution of this highly developed sense of smell that has helped the turkey vulture to become the most widespread of all species of vulture in the world with a global population estimated at 18 million. Other vultures will find it more difficult to find food in densely populated forests and jungles where they are unable to see properly and so their survival rates are lower.
Another scientist who helped change our perception about the smelling abilities of birds was the delightfully named American biologist Betsy Bang who in the 1960s worked alongside her husband drawing specimens for his anatomical work. From detailed dissections she discovered olfactory bulbs in a number of birds including kiwis, snow petrels, and turkey vultures, that were too big not to be functioning. She also noted that the nasal conchae were much more developed than in other birds.
And in 1965 Bernice Wenzel, a physiology professor at University of California, Los Angeles (UCLA) presented a paper at the International Symposium on Olfaction and Taste, held in Japan that year, that showed when pigeons were exposed to scented air their heart rates increased. She suggested that pigeons use smell as well as sight and sound to guide themselves, a hypothesis that was not widely accepted at the time.
From then on, she devoted her career to studying avian olfaction and repeated her electrodes experiments on ravens, mallards, turkey vultures, and canaries. Every bird that she tested showed the same reaction. She also studied seabirds and found that fulmars and shearwaters were attracted to smells such as fish oil and bacon fat.
Inspired by Wenzel, Gabrielle Nevitt a sensory ecologist, and professor at the University of California-Davis has spent most of her career studying olfactory-mediated behaviour in marine animals. In 1991 she joined a research cruise in the Antarctic and conducted an experiment to find out whether tube-nosed seabirds – petrels, albatrosses, and shearwaters – use their sense of smell to find prey.
Nevitt soaked tampons in fishy-flavoured compounds and tied them to kites which she flew off the rear deck of the boat. Soon dozens of birds swooped in so close that Nevitt had to remove the kites to prevent the birds getting tangled in the strings. For her next experiment she combined the fish compounds with vegetable oils and poured the mixture into the sea. Again, the seabirds came flocking, but still Nevitt had no idea how they were using smell to locate the fishy smells.
The next year Nevitt was back at sea but unfortunately an accident in which she tore a kidney forced her to stop her work prematurely. In a twist of fate, she met Tim Bates an atmospheric chemist who came on board with a new scientific crew when the ship was docked. He was studying dimethyl sulphide (DMS), a gas emitted by plankton, and how it might combat climate change.
As the pair got chatting Nevitt picked up a familiar smell. The DMS was emitting an odour that smelt like oysters. Although Nevitt had heard of DMS she hadn’t until that point known it had a smell.
Nevitt realised that it was DMS that the tube-nosed seabirds were able to detect. They picked up the smell and followed it to schools of fish. A few months later Nevitt was able to prove her theory by sailing out to the Antarctic again. She sprayed aerosols of DMS across the water and watched as the birds followed the boat, lured by the smell.
Since the beginning of this century scientists have increased their efforts to find how important the sense of smell is to birds in particular the impact it may have on navigation.
In 2010 researchers from the Max Planck Institute for Ornithology in Radolfzell, Germany, conducted a field study that found that during migration birds may rely on smell even more than the magnetic field of the earth or their orientation in relation to the sun.
Led by Richard Holland and Martin Wikelski, the team captured 24 adult and 24 juvenile catbirds in New Jersey. They applied a saline solution to the nasal mucous membranes of 8 of the birds in each group so the birds were unable to smell properly. They also used strong impulses to distort the magnetoreception capacity in another 8 birds from each group.
At the same time a further 19 catbirds were captured in Illinois which were transported overnight to New Jersey. These birds were added to the three groups of New Jersey catbirds and they were all fitted with radio transmitters, before being released.
During autumn migration catbirds from New Jersey usually fly in a south-west direction, whereas birds from Illinois fly directly south.
However, the birds from New Jersey that were unable to smell could not find their bearings and flew directly south, compared to the control group who flew in the right direction. The birds from Illinois with an impaired sense of smell also flew directly south, while the control group compensated for the change in location by flying south-west or west.
There was no effect observed in either the adult or juvenile catbirds who had been exposed to magnetic impulses and the researchers questioned just how important the magnetic field is for bird migration over long distances.
Another idea that has recently been debunked is that smell plays no part in birds’ reproduction. A few studies have started to look at the role of scents in mating rituals, and in 2003, in a scientific first, a small seabird was found to be sending fragrant signals to potential mates.
Crested auklets live in Alaska in breeding colonies of up to 100,000 birds. When birds meet their press their bills against each other’s neck feathers which give off a strong smell of tangerines. Both sexes emit the citrus-flavoured fragrance and it only appears during breeding season. Just like humans, crested auklets seem to be using perfume to make themselves attractive.
Scientists are unsure what the scent signifies but have theorised that it could convey the bird’s quality as a mate, if for example, well-fed birds were able to produce more scent.
Another study in 2013 by a team led by Danielle Whittaker, from Michigan State University, demonstrated that birds do communicate via scents, and odour can reliably predict their reproductive success.
Prior to the study, which looked at dark-eyed juncos, scientists thought the act of preening, where birds extract oil from glands near their tails and rub it on their feathers and legs, was simply to strengthen and protect their feathers.
The Michigan study showed, however, that there was a strong correlation between the way birds smell at the beginning of the breeding season when they are choosing their mates and how successful they are at reproducing. The team found that females that smell more ‘female-like’ and males that smell more ‘male-like’ were better indicators than size or attractive plumage.
And what about birds closer to home? Do the birds in our garden have a sense of smell?
Compared to some of the birds we have looked at above a lot of British songbirds do have relatively small olfactory bulbs. However, scientists have discovered that some birds practise ‘aromatherapy’ during breeding season.
In one study it was found that Corsica blue tits line their nests with aromatic plants such as mint and lavender to act as a disinfectant and kill bacteria, creating a sterile environment which will give chicks a better chance of survival.
They choose Mediterranean plants with medicinal properties and at first Adele Mennerat, a biologist from France’s National Centre of Scientific Research, who was involved in the study, thought the plants were being used to kill the blow fly larvae that often live in blue tit’s nests and feed on the chicks’ blood.
However, after finding that the plants had no impact on blow fly infestation Mennerat and her team turned their attention to the plants’ anti-bacterial properties. They found that aromatic plants reduced the number of bacteria in the nest and the chicks had a higher proportion of red blood cells, a strong indicator of a chick’s health and chances of survival after fledging. Although she can’t be sure Mennerat suspects that the chicks can devote less energy to their immune systems and more to physical growth.
The researchers also found that different female blue tits preferred different plants, regardless of their local availability. For example, in a territory with big bushes of lavender blue tits at this site would still travel far away from their nests to collect mint.
A previous study investigating the presence of aromatic herbs in starlings’ nests had come to a similar conclusion. It found that the presence of plants such as mistletoe, goutweed, yarrow, and cow parsley triggered reactions against infection, changing the composition of the chicks’ blood and boosting the production of red blood cells. Helga Gwinner and colleagues from the Max Planck Institute for Ornithology suggested that the volatile compounds in the herbs stimulate nestlings’ immune systems which they called the drug hypothesis.
In another study a team from North Carolina State University found that the presence of fragrant plants in the nests of European starlings improved parenting behaviours and birds were more likely to attend the nest, provide better incubation, and become more active earlier in the day.
We still have a lot to discover about avian olfaction. Although it is now accepted that birds are not anosmic, it is still generally believed that their sense of smell is less developed than in other vertebrates. But we do know that smell is important for navigation, food location, and reproduction success, and therefore the survival of birds.