Read more: “Instant Expert 34: Bird senses“
With their hard beaks and expressionless faces, birds do not appear to respond to a delicious flavour or a bad odour as we do. This may partly explain why ornithologists have been slow to attribute taste and smell to them. We still know so little that there is huge potential for discovery here. But for now, it is another little-known sense – the mysterious ability to detect the Earth’s magnetic field – that excites most interest among researchers.
A matter of taste
Dogs often swallow their food so rapidly it seems they barely have time to taste anything, but as every dog owner knows, they are certainly not indifferent to taste. The same is true of birds.
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Early evidence of a sophisticated gustatory sense in birds was found by John Weir, a bird-keeping colleague of Charles Darwin and Alfred Wallace. He noticed that, when given caterpillars of the ermine moth, his cage birds spat them out and shook their heads in disgust. At the time, though, Weir’s discovery of a sense of taste in birds was rather eclipsed by Wallace’s realisation that distasteful caterpillars often sport warning colours, and that the two traits evolved together as a signal to would-be predators.
Later investigators assumed that, like us, birds must have taste buds on the tongue. When they looked for these they found a puzzle, because they seemed to have too few to discriminate between palatable and distasteful foods. Then, in 1974, Herman Berkhoudt at the University of Leiden in the Netherlands discovered what looked like a taste bud in the tip of a duck’s beak. His painstaking microscopic examinations eventually revealed that a mallard’s taste buds are located in five clusters, four in the upper jaw and one in the lower, with none on the tongue. Although birds have far fewer taste buds than mammals – humans have some 10,000 while mallards have about 400 – they can nevertheless taste salt, sour, bitter and sweet, as we do. Whether they also respond to umami has not been tested.
A sixth sense
The navigation abilities of many birds seem almost miraculous. Release a Manx shearwater, a seabird that rarely crosses land, from a church tower in Venice, and it will unerringly find its way back to its breeding burrow on the island of Skokholm off the west coast of Britain. How? And what about barn swallows and the many other species that commute, summer and winter, between the northern and southern hemisphere – how do they navigate? Such questions have preoccupied researchers for well over a century.
Today we know that birds have a variety of navigational mechanisms, including sun and star compasses and olfactory cues. The most recently discovered, and most mysterious, is the ability to sense the Earth’s magnetic field. It was once considered impossible, but in the 1980s suspicion began to grow that birds do indeed possess this sixth sense. We still do not know exactly how it works, but what we do know is mind-bogglingly bizarre.
“Birds have a variety of navigational mechanisms, including star compasses and olfactory cues”
First, birds seem to detect the direction of the magnetic field using microscopic crystals of magnetite – a magnetic form of iron oxide – located around their eyes and in the nasal cavity of the upper beak. More recently, it has emerged that they may also detect the strength of the field via a chemical reaction – physicists have known since the 1970s that certain chemical reactions can be modified by magnetic fields. Stranger yet, studies of the European robin indicate that the reaction involved is induced by light entering the bird’s right eye only (Journal of Comparative Physiology A, vol 191, p 675). Researchers are currently striving to find out where in the body the reaction takes place. Meanwhile, some speculate that it might allow birds to “see” the contours of the Earth’s magnetic field – something that is difficult to envisage as a mere human.
Elusive smell
Ornithologists long believed that birds lack a sense of smell. This prejudice seemed to be confirmed in the early 1900s by poor research designed to “prove” its absence. One such study, published in Nature no less, involved offering a single turkey two plates of food, one placed on top of some smelly substance such as lavender oil, the other free from additional smells. The turkey promptly ate the lot, then downed a third helping contaminated with malodorous prussic acid, and died. Conclusion: birds have no sense of smell.
So the myth persisted, despite contradictory evidence from earlier anatomical studies. In 1837, for example, British biologist Richard Owen’s dissection of a turkey vulture had led him to conclude that it had a “well-developed organ of smell”. Similarly, he found evidence for large olfactory bulbs in the brain of the recently discovered kiwi. Later observations of wild kiwis snuffling around in the undergrowth at night in search of earthworms left little doubt they use their sense of smell to forage. But kiwis are so unlike other birds that it was easy to dismiss them as an exception.
Then, in the 1960s, work by Betsy Bang at Johns Hopkins University in Baltimore, Maryland, transformed ideas about avian olfaction. She found the nasal conchae – structures within the nose, which in humans warm incoming air and detect odours – to be large and elaborate in some birds. Convinced that birds must be able to smell, she turned her attention to the olfactory bulb. She found it differed in relative size between species by a factor of 12, reflecting the extent to which their lifestyle depended on olfaction (see table).
Today, attitudes have changed so dramatically that one recent study described albatrosses and petrels as living in an “olfactory seascape”, using their refined sense of smell to find food, breeding colonies and even their nesting burrow and partner (PNAS, vol 105, p 4576).
This article appeared in print under the headline “Mysterious and more mysterious”