ELEVENTH ANNUAL REPORT. ] (,5 



ceeding the mobility of this region in mammals — the great ma- 

 jority of which possess but seven separate cervical vertebrae. 



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 

 tlie 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 owds 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 Ol^^ 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 difi^erent, 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, (Stirnia). 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. 



