13 
visual area. In hawks and other birds of prey — including insec¬ 
tivorous birds—which require binocular vision in each eye— the two 
foveas are more widely separated and the temporal macular region 
again becomes better defined and resembles the nasal area. 
A double macula, in one eye^ furnishing stereoscopic vision and all 
the advantages of binocular sight, would help to explain the won¬ 
derful range and accuracy of the monocular eyesight of birds, espe¬ 
cially in the eagles, hawks and vultures. 
Fig. 4.—Vertical section of the American Screech Owl (Strix or Megascops 
asio), showing very plainly the accommodative apparatus, scleral plates, Cramp- 
ton’s muscle, etc., as well as the peculiarly-shaped eyeball of the owl family. 
Only a small portion of the pecten is depicted. 
Color 'perception in birds. The color sense is supposed to be 
chiefly resident in the retinal cones; and this theory, Slonaker finds, 
is borne out in examining birds’ retinae. In the nocturnal birds 
he discovered few r cones, while the proportion of rods to cones in 
mammals he found reversed in day birds, where the cones far sur- 
