Hearing is undoubtedly a very important sense for birds, second only to sight, as they rely on constant vocal communication with others of their species.
Calls and songs help them avoid trespassing on another’s territory, keep in touch with their flock, receive warnings of predators, relate to a mate and tend to their young. Additionally, many bird species use their hearing to detect prey, whether it’s a rodent scuttling through the grass or a worm burrowing underground.
Do birds have ears?
It’s strange, therefore, that there are none of the external signs of hearing apparatus you find in almost every mammal. Visible ears, known as auricles or pinnae, funnel sound waves down the ear canal to the eardrum. When they hit the drum, the vibrations of air are converted to electrical signals the brain can interpret as sound. Having one pinna on each side of the head establishes from which direction the sound is coming.
- Can butterflies hear? And why do moths have ears and butterflies not?
- Why do rabbits have such long ears?
Birds have ear canals that are located slightly behind and below their eyes. But they don’t have pinnae, for good reason. Almost all of a bird’s life is conducted in flight, where wind noise is a constant. Having external ears would be like trying to hear a conversation with your head sticking out of a car window.
Instead, birds have funnel-shaped canals covered in very specific types of feathers called auriculars. These are soft and sweep backwards from the eye to cover the ear holes. Thisprotects the internal canals from dust and cuts down on the effects of rushing wind. In certain owl species there is also a valve over the opening, called an operculum.
In this way distracting background noise is dealt with, but what about pinpointing the sourceof a sound in their three-dimensional air space? This is an aspect that is most important to a bird in both evading predators and finding prey.
To find out how a bird copes with the lack of an external direction-detecting device, researchers from the Technical University of Munich, Germany, examined birds’ heads. They discovered that instead of employing external ears, the animals use their whole heads.
They have slightly oval skulls and so sound waves passing over them are heard by each ear at a different volume and frequency. A noise coming from one side is perceived by the ear on that side to be louder and pitched at a particular frequency. By the time the sound wave has passed over the head to the other eardrum, the volume and frequency have been changed by distance and the head’s contours. The bird’s brain interprets these different frequencies to give the incoming sound a direction and height.
In addition, many birds such as owls have one ear hole positioned slightly higher than the other. Their disc-shaped faces are equipped with muscles that allow them to alter the faces’ angles, like a radar dish swivelling to catch the soundwaves. The resulting very precise changes in volume allows the birds’ brains to create a highly accurate soundscape even in total darkness – a particularly valuable adjunct to keen eyesight.
