Read more: “Instant Expert 34: Bird senses“
On the face of it, studying the hearing or tactile sensitivity of birds looks unlikely to yield benefits for humans. Yet discoveries about bird hearing could hold clues to treating deafness and neurodegenerative diseases. And as we learn more about a bird’s sense of touch, who knows what practical applications it may inspire.
Sounds familiar
Given the extent to which birds rely on vocalisation and how much effort has gone into the study of their calls and songs, it is remarkable how little we still know about their hearing. I wonder whether this lack of interest is partly a consequence of birds having no external ear or pinna, which is part of their reptilian heritage.
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We know that birds can hear an almost identical range of sounds to us. They achieve this with a single middle ear bone, rather than the three we have – again a product of their reptilian ancestry. Intriguingly, the hearing ability of birds living in temperate climes fluctuates through the year. The auditory regions of their brains grow during the breeding season, then shrink when song becomes less important. Understanding this process could provide clues to treating Alzheimer’s and other neurodegenerative diseases.
Another important difference between bird and human hearing occurs in the inner ear, and especially in the cochlea – the structure containing the vibration-sensitive “hearing” hairs. It is snail-shaped in humans, hence its name, whereas in birds it is banana-shaped. In both, the hair cells detect changes in pressure and transform these into electrical signals, which are interpreted as sound in the brain. Crucially, we cannot replace damaged hair cells, making deafness a scourge in older people. Birds, have no such problem: they can grow new hair cells. If we can discover the genetic basis underpinning this difference, it could give us the potential to solve a common cause of age-related hearing loss.
Extraordinary ears
The ability of owls to function in the dark is legendary, but exaggerated. Although they have good vision in low light, in total darkness they cannot see at all. What owls do excel at is hearing, which is of course useful at any time.
The ears of several owl species – not to be confused with their feathery “ear” tufts – are positioned asymmetrically on and in the skull (see diagram). In the great grey owl, for example, the left ear is located at 7 o’clock, the right at 2 o’clock. This asymmetry results either in a sound reaching the two ears at slightly different times, or in minute differences in volume, allowing the owl to pinpoint its source.
Great greys – among the largest of all owls – are daytime hunters. Using their acute hearing, they can locate rodents beneath the snow, then power through the surface with pinpoint precision to grasp their prey.
Dabbling with touch
There are few bird behaviours that seem more banal than a duck dabbling in muddy water. Familiarity breeds contempt. In fact, the sensory processes going on inside a duck’s bill almost defy belief.
At the tip of the beak is a semicircular arrangement of tiny pits, visible to the eye, each housing a receptor whose end protrudes just above the surface. Within each one, and stretching back inside the bill, lies a tubular structure a few millimetres long containing a blood vessel and two types of nerve endings called Grandry and Herbst corpuscles. These are touch receptors. A nerve fibre from each corpuscle joins the major nerves running along the upper and lower jaw, eventually connecting to the brain.
It is these two types of corpuscle – in combination with taste receptors inside the mouth (see “Bird senses: Taste, smell and magnetism“) – that allow ducks to distinguish between what is edible and what is not. This is a sophisticated process, and the sensitivity of a duck’s bill tip may rival that of our fingertips.
Besides ducks, many birds have touch receptors in their bill tips. They are especially abundant and well developed in birds such as kiwis and waders, which probe for hidden food. Birds probing in wet sand, for example, can set up a pressure wave whose shape enables them to detect edible bivalves (Proceedings of the Royal Society B, vol 265, p 1377).
As well as being crucial in foraging, touch plays an important role in birds’ social relationships. Many spend hours preening each other – equivalent to the social grooming of primates. Such contact between individuals must be felt via touch receptors in the skin, or possibly through the modified sensory feathers called filoplumes. Presumably, as with primate grooming, preening triggers the release of feel-good chemicals that promote bonding.
This article appeared in print under the headline “The mechanical senses”