ELEVENTH ANNUAL REPORT. 165 
ceeding the mobility of this region in mammals—the great ma- 
jority of which possess but seven separate cervical vertebre. 
Necessity for wider vision in the diurnal birds of prey is shown 
by the 69 degrees divergence of the optical axes in the kestrel, and 
from 55 degrees to 65 degrees in the turkey vulture, the rolling of 
the eyes in this species extending through a horizontal arc of about 
10 degrees. 
It has often been asserted that owls are near-sighted, but this 
is untrue. Careful tests made with the ophthalmoscope show the 
eyes of recently caught birds to be perfectly normal, there being 
not a trace of either myopic or hyperotropic tendency. Although 
as we have seen, owls have become especially adapted for a noc- 
turnal life, a number have changed their habits secondarily, and 
are as diurnal as any hawk. This change is of such comparatively 
recent date, however, that no radical structural changes are 
apparent. The facial disks are incomplete or almost lacking in 
some of these diurnal forms.* 
The ear opening is of large size in most owls and a fleshy 
flap-like outer ear serves to direct the sound more accurately. An 
interesting condition of asymmetry is not unfrequently present, 
the opening of the outer ear being larger on one side of the head 
than on the other, and this unlikeness may extend even to the 
bones of the skull itself. 
ADAPTATIONS OF THE FEET. 
The structure of the feet and more particularly the toes of owls 
is very peculiar. When perching, the arrangement of the toes is 
zygodactyl, that is, two in front and two behind, but when an 
owl pounces upon its prey, the four toes point to the four quarters 
of a circle. Again when the bird alights upon the ground, the ar- 
rangement is still different, being more like that of a typically 
avian foot—three toes in front and one behind. This facile ad- 
justment to different conditions makes the foot of great efficiency 
in all its functions. The osprey or fish hawk shares the digital 
elasticity of the owls. Not only in present mobility but in past 
evolution, the owl’s foot has proved extremely plastic, and we 
find considerable differences between the feet of various species. 
*Measurements of the optic axes in the skulls and mounted specimens 
of the diurnal hawk owls, (Surnia), together with the apparently more- 
lateral position of the facial disks, indicate a wider divergence, perhaps 
of as much as 43°, but this evidence cannot be confirmed until living speci- 
mens have been examined. 
