This article was originally published in the Journal of the Wildlife Sound Recording Society Vol4 No7 Spring 1984
When observing the well-grown young in a blackbird's (Turdus merula) nest recently, I was impressed by the way in which the seemingly dozing fledglings alerted themselves as a food-carrying parent approached through the bush foliage. The nestlings rose up and began to gape, well before I could hear any leaf rustling or, indeed, any vocalizations. In addition, if the parent blackbird gave a cluck of alarm, even at quite a distance, there was near instantaneous crouching and subsequent immobility. Again, we know that should a hawk or falcon fly towards a feeding flock of birds, for instance chaffinches (Fringilla coelebs) searching for beech mast, then a single alarm cry will result in the birds dispersing for cover.
Clearly, hearing is all-important in the lives of birds and a high degree of acuity must develop at a relatively early stage: birds chirping in the egg before hatching will stop doing so if a parent utters an alarm call. Then Vince (1969) demonstrated that clicking noises originating from the embryos of certain species of quail were concerned in the synchronization of hatching of a clutch, so sound production and hearing must have importance, at least for some game-bird species, before leaving the egg, A deaf bird would be at a great disadvantage in nature and, even if it survived to become mature for breeding, it is doubtful if successful nesting could be effected. A male could not sing a developed territorial song and a deaf female could not respond to an acoustic invitation for mating.
Presumably it is possible that some individual birds may be congenitally deaf but the life of birds in the wild is usually too brief for a senile form of deafness to arise. This is not the case in the human population, however, and, not infrequently, the time comes when a middle- aged naturalist has to admit that he can barely hear the song of the grass- hopper warbler (Locustella naevia); the high-frequency sounds disappear first, usually more readily with men than women. It is sad when flying bats no longer seem to squeak and grasshoppers stridulate, apparently, in silence. In a letter to Daines Parrington, Gilbert White (1788) wrote feelingly: "Frequent returns of deafness incommode me sadly, and half disqualify me for a naturalist.
In both mammals and birds, the essential hearing organ is the internal ear. Birds do not have an external pinna and the auditory meatus is usually feather-protected. Vibrations of the ear-drum are transmitted by movements of a bone, the columella auris, through the middle ear cavity. With the middle ear of mammals, a chain of three ossicles is involved in the same sound transmission. Sound energy, as pressure pulses, is thus converted into movements at the end of the columella and so at the oval window, which leads to the inner ear.
One part of the inner ear is the membrane labyrinth, with its utricle and three semicircular canals, each in a different plane; there are extensive nervous connections here with the cerebellum and the function of these fluid-containing structures is to maintain proper posture while walking or hopping, when swimming and, particularly, in flight when conditions are often stormy and turbulent. It now seems certain that these balancing organs play no part in the hearing process, although they are intimately connected anatomically.
There is a conspicuous difference between the hearing organ or cochlea of the mammal and the bird. That of the mammal is a thin, coiled tube while the bird's cochlea is relatively short, broad and has only a slight curve; both organs are filled with fluid. With the bird, as with the mammal, a basilar membrane traverses the cochlea; it carries sensitive hair cells with nerve fibres running to the auditory nerve and hence to the brain. The hair cells are covered by a tectorial membrane and have a far greater concentration per unit area of membrane than those of the mammalian cochlea.
Roofing over the middle cochlear canal, or scala media, is a vascular membrane or tegmentum vasculosum. Above this membrane is the scala vestibuli, with its fluid in communication with the labyrinthine system; below the middle cochlear canal is another fluid-filled canal, the scala tympani, with its exit to the middle ear cavity blocked by the round window with its occluding membrane. The tegmentum vasculosum, by its thick structure and physical properties is, theoretically, capable of a damping effect, or bringing local membrane vibrations to rest quickly.
Now Helmholtz's resonance theory is based on the supposition that, in the hearing process, particular portions of the basilar membrane come to vibrate, according to the sound or pressure frequency to which they are subjected. On this basis, therefore, sound frequencies are analysed at the cochlea's basilar membrane and the information is relayed to the brain by means of the auditory nerve. Nevertheless, it does seem that the solution of the problem is not as straightforward as was once thought and it is likely that a significant proportion of the sound nerve impulses travel straight to the brain for analysis. It is not possible, as far as I know, to be certain of what method of sound frequency analysis predominates for either bird or mammal; however, at present most authorities appear to favour the theory that a large proportion of sound frequency analysis takes place directly in the brain where birds are concerned.
Birds appear to be able to respond to sounds, detecting minute noise differences, far more rapidly than can human beings. Birds produce rapid syllable alterations in their songs and calls, often so quickly that man's hearing ability is incapable of comprehension of details of the pattern. For this reason, the re-playing of one's bird-sound tapes at reduced speeds aids sound localisation. Now it is suspected that birds may be able to detect vibrations, if not sounds, by means of the corpuscles of Herbst. These touch-type nerve endings, rather like the mammalian Pacinian corpuscles, are to be found in the follicles of feathers, in the deep tissues of the leg and in the beak. Birds are said to respond to distant low-frequency sound, such as gunfire, and which is inaudible to humans. It may well be that the corpuscles of Herbst, rather than the ears, are the receptor organs in such instances. Then, in birds, another structure is located at the top of the cochlea; this is the lagena which contains hair cells with a rich innervation, The function of the lagena is uncertain, but it is considered that it is more likely to be concerned with balance rather than hearing.
As already mentioned, birds appear to be capable of sound analysis at a speed not possible in humans, To illustrate this further, the work of Tschanz (1968) showed that young guillemots (Uria aalge) reacted significantly to the calls of their parents and ignored those of other adults. The young guillemots responded to a loud-speaker emitting a parent's cry but not to a speaker which played the call of another bird. Such a capability of recognition is obviously of importance in a large sea-bird colony where the noise, both from the sea and from the colonially nesting birds, is often of remarkable amplitude.
As Thorpe (1961) has pointed out, the fact that one bird species will often respond to the alarm calls of another, does not necessarily mean that a learning process has occurred. Thorpe gives the example of the call given by a chaffinch as a hawk flies over: under the same circumstances, the sound uttered is very similar to that produced by a blackbird. Calls of this variety, of course, differ so much in their acoustic characteristics from those given by the same birds when they are mobbing a perched owl or other predator. The high, whistling calls with gradual commencements and endings, which are made by small birds when a hawk is spotted in the sky, are difficult to localise and it is generally assumed that a bird predator experiences the same difficulty as do humans.
Yet Shalter (1978) has demonstrated, experimentally, that some birds-of- prey will quickly turn towards the source of the call. Whether this is a new phenomenon or whether the conclusions can be applied to conditions in the field, remains to be seen. But there can be no doubt about the ease of localisation of the alarm calls of birds which are occupied in mobbing. The calls are loud, definite in form, start abruptly and the frequencies are usually well modulated; the cries are often harsh rasps or clicks. Calls of this type are distinctive for most small birds and if an ornithologist hears a mobbing chorus at a distance, he can normally identify the species involved before he sees them. It follows that auditory information about a bird-predator situation is quickly made available; considering the speed of the time-scale which is used, the information is doubtless made available more rapidly for birds than would be the case for humans under similar circumstances.
The localisation of sound depends on several factors. There must be a difference in the time of arrival, and of the intensity, in the sounds heard in the two ears and, moreover, there will be a phase difference between them, although the last two factors depend on sound wavelength. Obviously, the size of the head and hence the distance between the two ears is important: the time taken for sound to travel depends on the distance. Low-frequency sounds, say less than 1kHz, with long wavelengths, enable the better detection of phase differences and modulated, complex sounds are more readily localised than pure tones. Of course, small passerines necessarily have narrow heads and, to detect intensity differentials between the two ears, it is preferable to deal with sounds of high frequency, which have small wavelengths.
The sense of
hearing, like that of sight, is acute and well developed in birds;
compared with man, birds appear to vocalize and to respond to
vocalizations which are more complicated than they at first appear and,
overall, with a greater avian efficiency. It seems that birds, with
their powers of rapid sound analysis, communicate acoustically at a
speed with which the human cannot compete.