Hearing aids
MOST birders will agree that birding by ear is key to a successful day out in the field. Locating and even identifying a bird may depend on hearing it first. But the sounds that birds make are not meant for birders, they are aimed at other birds, usually of the same species. For any bird’s vocalisations, or other sounds, to be meaningful, they have to be heard and so a bird’s sense of hearing is extremely important.
It is easy to forget that birds have ears as they are invisible in most species, hidden by the head feathers. The exceptions are vultures, ostriches and other species with unfeathered heads, enabling the ear opening to be clearly seen behind and below the eye. The feathers which cover the ears are called the auriculars and having the ear opening screened by feathers stops unnecessary noise entering the ear when a bird is flying. The feathers can also sometimes help to channel the sound into the ear opening.
The outer ear is made up of the ear opening, or meatus, and a short passage which ends at the tympanic membrane, or eardrum. The middle ear is air filled and contains the columella, a small rod-like bone which is equivalent to the three small bones of the human ear. The columella attaches to the inner ear which is filled with fluid and contains the cochlea, sacculus and lagena.
Sensory hairs in the cochlea convert sounds into electrical impulses which pass along the auditory nerve to the brain and birds are able to regrow damaged cochlear hairs, while humans can’t. A human cochlea is coiled, but a bird’s is straight or a little curved, and the length indicates its sensitivity to sounds, meaning that larger birds hear lower sounds, while smaller birds can hear higher ones.
The hearing ability of most birds ranges from 500 Hz to about 6,000 Hz; humans can hear from 20 Hz to 20,000 Hz, so a birder with good hearing will have no problem in detecting a Eurasian Bittern’s boom (200 Hz) and a Goldcrest’s song (9,000 Hz). Most songbirds are most sensitive to sounds at 2,000-3,000 Hz, but are much more sensitive than we are to pitch, tone and rhythm.
Birds can hear a sound 10 times shorter than a human can, so what we hear as a single sound might be 10 separate sounds to a bird. This means that birds can distinguish individual noises easily among other very similar sounds which is essential for, say, a seabird which is trying to pick out a chick at a large and noisy colony.
Owls have particularly acute hearing, with their ears not only spaced far apart, but also positioned asymmetrically on the sides of the head. A facial ruff of feathers helps to focus the noise, as do their feathered preaural flaps. Owls have two pathways which interpret the sounds heard by comparing differences for each ear. One measures the time that the sound arrives while the other measures the sound intensity and with both bits of information the owl knows exactly where the sound is. Chris Harbard