114 STRUCTURE AXD COMPARISON. 



A notable difference in colors in the two sexes is that of the 

 sheldrake, or goosander, which we have already studied. The 

 male's eye is red, the female's yellow. The red-breasted mer- 

 ganser, however, has a red eye in both sexes, and the hooded 

 merganser has a yellow eye in both sexes. "What is odd is that 

 though the males of the first two species are remarkably 

 different, the females are so nearly alike that they can hardly 

 be told apart excei)t by the color of their eyes and by their 

 nesting habits. 



There is sometimes a change of color with age in birds as in 

 cats. A young crow, or raven, is as blue-eyed as a kitten, but as 

 he grows older his eye becomes as black as his reputation. I 

 have seen a young goshawk, taken from the nest, which was 

 blue-€yed, though at a later period the goshawk's eyes are 

 yellow, and at maturity they are red. From blue to yellow, 

 yellow to red, what a change for one bird ! 



We have seen that the color may vary with age, sex, and 

 season, or may even form a racial mark, as in the towhee, yet 

 that it is usually constant in the same individual and species 

 through life. 



There is another interesting thing about birds' eyes. Any 

 one who has watched an owl will remember the third eyelid, 

 or " winking membrane," which the owl draws sleepily over 

 his eyes. You may observe something similar, though not 

 nearly so complete, in your cat when she is lying on the rug 

 half asleep. Even in your own eye there is a trace of this 

 winking membrane in the little folds of membrane in the 

 inner corner of the eye; but you have no power to draw it 

 over the eye as the cat and the owl do. 



