324 
sounds of short duration—transient 
rustles and squeaks which do not last 
long enough for the bird to make 
searching movements with its head. 
In order to locate such sounds un- 
ambiguously with two ears only it ap- 
pears, from theoretical considerations, 
that the following minimum conditions 
must be satisfied: 
(1) The sound must be complex and 
the ears competent to resolve it into at 
least three bands of frequency in such 
a way that independent comparison of 
the signals arriving at the two ears is 
possible in each band. 
(2) The two ears must have a direc- 
tion of maximum sensitivity which is 
different for each band and is different 
for the left and right ear for at least 
two of the bands. 
The exceptional length of the owl’s 
cochlea and the prevalent asymmetry 
of their external ears suggests that these 
conditions are in fact met, though the 
second condition is fulfilled in different 
ways in different genera. 
Birds are prevented by aerodynamic 
considerations from using external 
baffles like the pinnze of mammalian 
ears. Whatever is done must be under 
the cover of the feathers which insure 
laminar flow across the orifice of the 
auditory meatus and subdue the whis- 
tling of the wind. We find in Strix 
that the left and right ear cavities are 
of markedly different sizes and are 
each nearly closed by opercular flaps, 
leaving a slitlike orifice which is half as 
long again on the left as on the right. 
For wave lengths comparable with the 
length of the slit the polar diagrams of 
left and right ears will certainly be 
different. 
In Asio the external auditory meatus 
is enormously enlarged on both sides 
to occupy the whole of the lateral post- 
orbital sides of the head. Here the 
opercular slits are of similar dimensions 
on the left and right sides. But be- 
neath the opercula the meatus on each 
side is divided into two compartments 
by a horizontal shelf, one compart- 
ment communicating with the middle 
ear while the other is blind. On the 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
right it is the upper compartment 
which is blind and on the left the lower 
(pl. 3). The effect of the arrangement 
is to make the relative lengths of the 
sound paths from the upper and lower 
extremities of the slit to the tympanic 
membrane different on the left and 
right side; so that for wave lengths 
comparable with the slit lengths the 
direction of maximum sensitivity will 
be directed above the horizontal plane 
for the right ear and below it on the 
left. 
Of the two British species of Asto, 
one, A. otus, the long-eared owl, is very 
nocturnal while A. /tammeus, the short- 
eared owl, is largely diurnal, and it has 
been argued that, because of the sim- 
ilarity of the ears in species with differ- 
ent habits, it is illegitimate to correlate 
asymmetry of the ears with night hunt- 
ing. This is a misconception. The 
asymmetry of the ears indicates that 
owls are descended from nocturnal 
ancestors which were forced to rely on 
their ears for hunting. Even the most 
diurnal owls have eyes of nocturnal 
form which must be deficient in day- 
light compared with those of, say, a 
hawk. The late Miss Turner’s de- 
scription of the hunting of the short- 
eared owl does not suggest that the 
eyes are used. In fact, since the prin- 
cipal prey is the field vole, which at 
least by day lives in grass tunnels, it is 
difficult to see how the eyes could be 
used. It seems certain that the owl 
locates the prey quite accurately by 
ear and then rakes through the grass 
for it with its talons. 
Another fully nocturnal species, 
Tengmalm’s owl, shows an asymmetry 
of the ears which involves extensive 
distortion of the bony structure of the 
skull in which the right ear is set above 
and the left below the midline of the 
orbits. It is likely that the objective 
of the asymmetry is the same as in 
Asio and Strix, but the descriptions of 
the soft anatomy which are available 
are inadequate for certainty. 
It will have been noted that there is 
a sufficient body of knowledge of the 
avian eye to permit of intelligent spec- 
