Like many reptiles and dinosaurs, birds have no external ears: only a narrow opening leading to the inner ear. Yet avian sense of hearing is typically keen.
Bird skulls act to conduct and transform sound in ways that afford highly accurate hearing from any direction. It is an intricate evolutionary adaptation.
Leaving off ears allows better sight. Eye placement on the side of the head gives birds a wide field of vision.
Some birds of prey, such as owls, have a different arrangement of eyes and ears which afford specialized capabilities. Diving birds have strong protective feathers to shield their ears from the water. During deep dives, they close the outer ear by folding the enlarged rim of their external ear.
Humans perceive sounds via relative pitch. This lets us hear a tune in one octave and recognize it played in a different octave.
Birds cannot do this. Birds hear in absolute pitch. Birds do recognize timbre: a fundamental note combined with harmonies.
Birds can hear much shorter notes than humans. A human can hear a sound 1/20th of a second long. Birds can discriminate a sound up to 1/200th of a second. What we hear as a single note may be 10 separate notes to a bird.
Many birds have a hearing range comparable to humans. The greatest sensitivity is typically between 2,000–4,000 hertz. Some birds have much more perceptive hearing.
Owls are sensitive to whisper-soft sounds, thanks to a profusion of hair cells in their exceptionally large cochlea (inner ear cavity).
Further, the flattened and pie-shaped faces of owls serve as a sound-gathering disk, passing percussion to the ears via stiff, specialized features on the disk’s circumference.
Owls also have asymmetrical ears. One ear is lower on the skull than the other, which means that sounds from a lone source reach the ears at slightly different times. This gives an owl the equivalent of binocular hearing, allowing them to pinpoint the source of a sound with tremendous positional accuracy.
So much of the lateral real estate of an owl’s skull is taken up by its ears, with frontal sculpting to optimize sound reception, that the only place left to position eyes is the middle of its face. Despite that, owls have surprising peripheral vision.