Hearing

The structure of a bird's ear is simpler than our own but their hearing seems to be much the same - the range of frequencies (pitch) is similar, though the sensitivity to loudness is reduced. While people with healthy hearing can hear sounds in the frequency range 50 Hz to 18 000 Hz (cycles/seconds), birds are generally limited to about 12 000 Hz.

There are some species, such as owls, that have exceptionally good hearing. For example, Barn Owls can locate prey by sound alone and this remarkable sensitivity and accuracy is achieved by having:

• A facial disc (like a satellite dish), which funnel lots of sound to the eardrums.
  Try cupping your hands and placing them behind your own ears and notice the difference it makes.
  
• Ear openings that are asymmetrical, i.e. one opening higher than the other, which means sounds appear louder in one ear than the other and helps to determine the distance and direction of the source of the sound.
  Try rotating your head slowly from side to side while listening to a distant sound to experience this effect.
  
• Flight feathers that have soft edges to them, which allows the owl to fly silently, but also means they do not hear their own wing beats.
  Try flapping your hand near your own ear to hear what wing beats sound like and how they mask other sounds.

Structure of the Avian Ear

Birds have a ruff of feathers, called ear coverts or auriculars, around the ear opening instead of external ears as in mammals. The ear opening is just below and behind the eyes. The auriculars funnel sounds toward the eardrum, which is a thin membrane stretching across the ear canal. A small bone, called the columella, is attached to the rear of the eardrum and transmits the vibrations through the middle ear to another membrane across an opening, called the oval window, in the cochlea. The cochlea is the hearing organ and is a fluid-filled chamber with hair-like cells, or cilia. Movement in the fluid caused by the vibrations causes the cilia to move, like trees swaying in a breeze, and these in turn create nerve impulses that are then transmitted to the brain along the auditory nerve. The round window, another membrane covered opening, protects the cochlea by allowing the pressure waves in the fluid to dissipate through it.

The inner ear also comprises the semicircular canals, which regulate balance and orientation, as they do in humans. The canals are fluid-filled and have specialised sensory cells that convert movements of the head, for example, to nerve impulses that the brain can interpret as a position.