Hearing and the Bird Ear

For information on
bird calls and songs
see :-Bird Songs
One of the endearing and endlessly fascinating things about birds is their song. Nearly all birds make some noises. Sound, as a prelude to, and part of, courtship, as a means of simply staying in touch with the flock. As a way of identifying either your young or your parents and as an efficient method of sending out a quick warning of approaching danger is an integral part of both a birds and a bird watchers life. Naturally enough if birds are using sound as a means to communicate then they need to be able to hear as well as create sounds. For this, they have, like you and me, ears.

Birds have good ears but they tend to hear things differently to us. Within sounds birds recognise and remember something akin to absolute pitch whereas humans perceive sounds via relative pitch. Very few humans can hear and remember absolute pitch. Relative pitch however allows us to hear a tune in one octave and still recognise the tune in a different octave. Birds cannot do this. Birds do however recognise 'timbre' (a fundamental note combined with harmonies). Recognising timbre and harmonic variations gives birds great versatility in the sounds that they can respond to, and in some cases reproduce. Birds also hear shorter notes than we can. Humans process sounds in bytes about 1/20 of a second long whereas birds discriminate up to 1/200 of a second. This means where we hear one sound only, a bird may hear as many as ten separate notes. Some birds such as Pigeons can hear much lower sounds than us. Birds (Pigeons) can be music buffs and can distinguish between human composers such as Bach and Stravinsky.

 

The range of hearing in many species of birds is comparable with that of mankind. Having a greatest sensitivity between 2000 and 4000 hertz (cycles per second). This is partly why bird song is so useful in bird identification - it is easy for us to hear - and partly why we find bird song so pleasant. In birds as a whole, the known hearing ranges vary from a lower limit of below 100 hertz to over 29000, though not all birds have this range. The common Mallard (Anus platyrhynchus) for instance has a range from 300 hz to 8000 hz.

Some birds have hearing much more sensitive than ours. Owls not only are more sensitive to small sounds but have asymmetrical ears (one ear being lower on the skull than the other) this means sounds from a single source reach the ears at slightly different times. This gives the owl the equivalent of binocular hearing, allowing them to pinpoint the source of a sound extremely accurately. Barn Owls, Tyto alba, can locate and catch small mammals in complete darkness using only their hearing. Finally, a number of species of owls have tufts of feathers which look like ears and give rise to names like 'Long Eared Owl' and 'Short Eared Owl'. These 'ears' are not ears at all, however, and have nothing to do with hearing.

Birds lack the externally visible part of the ear that we think of as an animal's ear and which is strictly speaking called the pinna. The ear of a bird has three chambers much like ours. The outer ear is simply a tube leading to the tympanum or ear drum. Behind this is the middle ear which has a single bone stretched across it called the columella. This is where, in mammals, you have an arrangement of three bones (Hammer, Anvil and Stirrup/Stypes). The inner ear is bathed in fluid, the outer and middle ears being air filled. It consists of five parts, of which two, the semicircular canals (see above) and the utricle are concerned with balance. The other three are the cochlea, the lagena and the sacculus. The lagena is used to help detect low frequency sounds, the sacculus to help detect high frequency sounds and the cochlea contains special sensory hears which change the physical vibrations caused by the sound waves into electrical impulses to be passed along to the brain.

The ear evolved in fish as an organ of balance and it still performs this function today in both birds and mammals. The part of the ear that is the organ of balance consists of three semicircular canals situated in a part of the inner ear called the utriculus. These three canals lie one each in the three different planes of the material, i.e. one horizontal, one vertical and one sagittal. They contain a fluid and sensitive hairs. Movement of the head causes the fluid in the canals to move which energises or triggers the sensory hairs. The degree of movement of liquid in each canal combines to tell the bird exactly where its head is at any given moment in time. This is very similar to how the human sense of balance works. Naturally, for creatures who move in a fundamentally more fully three dimensional universe than we do a good sense of balance is very important.

 

Birds start using calls early in their lives, in some species even before they are hatched. Quail chicks use calls to communicate with each other and their mother from inside their eggs. They are able this way to synchronise their hatching so that they all emerge from the eggs within the space of a couple of hours. Pelican chicks tell their mum if they are too hot or cold from inside the eggs. Chicks also listen to their parents while inside the eggs. This way they come to recognise their parents even before emerging form the eggs. Some birds such as Mallards have special maternal calls that they give while incubating the eggs so that after hatching the mother only has to give this call to have the chicks rush to her for protection.

Sound is often more important than sight in parent-offspring recognition. A deaf female turkey is unable to recognise her own chicks and chickens cannot recognise silenced chicks (with a belljar over them). Experiments have also shown that, in colony nesting birds at least young birds can recognise their own parents by their calls alone, though they all sound the same to us.

Not all young birds learn to recognise their parents or vice versa immediately. In Herring Gulls, about 5 days pass before this recognition takes place, while Kittiwakes take up to 5 weeks for recognition to register.

Birds also distinguish their mates by call. Gannets are colony nesting birds and a nesting site can have thousands of birds coming and going in a noisy melee that would befuddle a human listener, yet gannets can distinguish the calls of their particular mate from all those around them on the basis of only 1/10 second of the total call.

Birds use sounds other than those created by their vocal chords. Sounds can be created by stamping as in Coots, or by clacking the mandibles together as in Frigate birds, Albatrosses and Storks. Birds also use their wings to create sounds, simply by clapping them together as the wood pigeon or by having modified feathers which vibrate at a set frequency when exposed. Snipe use this during courting. Two feathers on either side of the tail vibrate as the bird falls out of the sky. Other birds which make sounds with their wings include Mute Swans, Broad-tailed Hummingbirds, Bellbirds and the New Zealand Tui. Perhaps the best known of these percussive sounds is the drumming of woodpeckers. Each woodpecker, in those species tested, has its own drumming pattern so male and female birds can easily recognise each other while they are out foraging.

Perhaps the most unusual however is the Palm Cockatoo which makes drumsticks from twigs and beats them against a hollow log in time with a pirouette during courtship.

Several species of cave dwelling birds use echolocation similar to bats to detect objects around them in the dark. Swiftlets from S.E. Asia, also known because some of them produce the nests used in making bird-nest soup, use sounds with a frequency between 4.5 and 7.5 kHz to navigate in the caves they nest in. Oilbirds in South America also nest in caves and use sounds in a range between 1.0 and 15 kHz emitted in staccato bursts to navigate inside the cave. Unlike the Swiftlets, Oilbirds are nocturnal but they do not use their echolocation outside of the cave. The sounds both these species use are audible to the human ear and sounds caused by a flock disturbed by a human intruder into their nesting caves have lead to many tales of devils and demons. The echolocation of both these species is considerably less efficient than that of bats because the sounds are lower and therefore have longer wavelengths. This means that they cannot distinguish smaller objects. Oilbirds cannot, apparently, detect anything smaller than about 15cm diameter while Swiftlets have a lower size limit of about 6 cms diameter.

 






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