SENSE ORGANS OF BIRDS—PUMPHREY 
Cochlea conspicuously long (and curved): 
Bubo. 
Cochlea long: Turdus, Columba. 
Cochlea average: Vanellus, Scolopax, Cypse- 
lus, Nucifraga. 
Cochlea short: Gallus. 
Cochlea very short: Anser, Mergus, Haliaétus. 
If we exclude the owl, we can per- 
haps imagine a correlation between 
length of cochlea and musical ability 
in this series. 
A little bit more is known of another 
aspect of hearing, the ability of birds 
to determine the direction from which 
sound is coming. It might be thought 
that birds, lacking the external pinna, 
would be severely handicapped, but 
reliable work on the hen indicates 
that she can locate the cheeping of 
her chicks within 2° in the horizontal 
plane. Incidentally, the chicks are 
just about as good at locating their 
mother. This performance is rather 
better than a dog’s and not quite as 
good as a cat’s performance in similar 
circumstances. It is substantially bet- 
ter than an inexperienced man can do, 
though man improves with practice. 
Man’s directional sensitivity to sound 
is an extremely complex function. It 
depends primarily on an unconscious 
comparison of the signals arriving at 
each ear in respect of both time or 
phase differences and differences of 
intensity. For low frequencies and 
noises of short duration it is the phase 
or time difference which is the more 
important, and for high frequencies 
it is the intensity difference. If the 
sound lasts long enough and the man 
is allowed to move his head, he turns 
it until the sound paths to his ears are 
nearly equal, and in this position can 
make his most accurate estimate. 
This all seems obvious enough, but 
it is not the whole story, for a man 
with one ear plugged is quite at sea 
when asked to locate the source of a 
pure tone or a click. But if the noise 
is both complex and continuing like 
a rattle or human speech, and par- 
ticularly if the noise is familiar to him, 
he can locate it nearly as well as if both 
ears were free. The explanation is 
that the sense of direction for complex 
323 
sounds depends rather intimately on 
an analysis of the relative intensi- 
ties of the frequency components. 
Because the head casts a sound shadow 
which is more complete for high fre- 
quencies than low, the perceptual 
character of a complex sound differs 
with the angle at which it reaches the 
ear. Ifitis familiar, it is unconsciously 
compared with the memory of what it 
should sound like when coming from 
directly in front, and the head is moved 
until the sound heard corresponds with 
the memory. 
There is no evidence that birds are 
capable of monaural localization. In 
fact Englemann’s experiments with 
the hen indicate the contrary, but the 
possibility must not be ignored. In 
any case it is likely that frequency 
analysis is even more important in 
direction finding for some birds than 
it is for man. 
The hen, like dog and man, appears 
to be almost incapable of localization 
in the vertical plane. Man invariably 
refers an unfamiliar noise to the hori- 
zon, and has to learn to look up to 
see an aircraft. It is likely that this 
limitation is inherent in all animals 
whose ears are symmetrical about the 
median plane of the head, a class which 
includes very nearly all the vertebrates. 
It has long been known, however, that 
the ears of many owls display a very 
remarkable asymmetry. So far as is 
known it is only the outer ear which 
is asymmetrical. And on the whole 
it is in the most nocturnal species that 
the asymmetry is most marked. Be- 
fore discussing these structural peculi- 
arities it is illuminating to consider 
what are the requirements of a bird 
which is so nocturnal that it has to 
find its prey by ear while in flight. 
It obviously must possess a directional 
sensitivity which is effective in the 
vertical plane as well as in the hori- 
zontal, and this is quite obviously im- 
possible if the ears are mirror images 
of each other and the median plane 
of the head remains vertical. It must 
be further assumed that the bird’s 
directional clues must be derived from 
