Birds possess a well-developed sense of hearing, surpassing that of humans, despite lacking external ear structures. Instead, their ears are concealed beneath overlapping feathers called auricular or ear coverts. But how does it function?
Birds' Hidden Hearing Abilities: Exploring How They Perceive Sound Without External Ears
Structure of an Avian Ear
According to Bird Spot, birds possess ears similar to those of lizards, located just below and behind the eye, with earholes that can be as large as the eye itself. These ears are protected by ear coverts, which shield them from wind noise, dust, and water without obstructing sound entry due to their lack of barbs.
Similar to humans, birds have three ear sections: the outer ear, middle ear, and inner ear. The outer ear directs air onto the eardrum, or tympanic membrane, through a short passage called the meatus. Many birds also have a muscle around the meatus that can partially or fully close the opening.
The middle ear transmits vibrations from the eardrum to the columella bone and cochlea in the inner ear. Nerve receptors within the cochlea carry these signals to the brain, where they are interpreted as sound.
The cochlea in birds resembles that of crocodiles, with a short, slightly curved bony tube ranging from 2.5 to 4.5 mm in most species, but reaching up to 10 mm in owls.
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How Do Birds Locate Sounds?
Birds possess the remarkable ability to locate sounds despite lacking external structures, as per BirdNote. With regards to how birds do it, a 2014 German study examined the hearing capabilities of ducks, crows, and chickens to unravel this mystery. The researchers discovered that the shape of a bird's head plays a crucial role in this process.
As per an article in the Forest Preserve District Will County, researchers measured the volume of sounds reaching the birds' eardrums from different angles and observed that the frequency at which sound registered on each eardrum varied.
This allowed the bird's brain to determine the sound's direction, whether it came from above, below, or at the same level, indicating that the bird's head reflected, absorbed, or diffracted the sounds.
Owls, known for their acute hearing, demonstrate this phenomenon vividly. They possess concave-shaped facial discs that guide sound to their ears. The Owl Pages explains that the shape of the disc can be altered using special facial muscles, while the owl's downward-pointing bill increases the surface area for soundwave collection by the facial disc.
Certain owl species exhibit asymmetric ear openings, with one ear higher than the other on the sides of their heads. Nocturnal species, like the barn owl and great horned owl, rely on their razor-sharp hearing for hunting in darkness.
The Owl Pages clarifies that the asymmetric ear openings enable owls to determine whether a sound is higher or lower. For instance, in a barn owl, the left ear opening is higher than the right, causing a sound from below the owl's line of sight to be louder in the right ear. The owl's brain instantly combines the left, right, up, and down signals to create a mental image of the sound source's location.
However, owls with what appear to be ears on the top of their heads have ear tufts-skin projections covered in feathers-that serve purposes unrelated to hearing. BBC Wildlife Magazine explains that these tufts, found in approximately one-third of owl species worldwide, primarily serve for display and visual communication. They also play a role in camouflage, breaking up the bird's outline against the background.
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