THE IRIS OF BIRDS. 113 



kind and the same age always have the same colored eyes. 

 Often this makes an easy way to tell one species from another. 

 The red-eyed vireo and 

 the white - eyed vireo 

 could easily be distin- 

 guished by the color of 

 the eye alone. So, too, 

 the common towhee of 

 the North with his red eye is readily distin- 

 guished from the white-eyed towhee of the 

 South, which otherwise is almost precisely 

 similar. But here we find a curious fact. 

 The white-eyed towhee is not a distinct spe- 

 cies, but is a race of the red-eyed, and we find 

 that part way between the northern and the 

 southern limits of the two forms, in Geor- 

 gia, for example, the towhees are neither FIG. 37. HEAD OF 

 white-eyed nor red-eyed, but have brownish GOOSANDER (MALE). 

 eyes. 



A curious in- 

 stance of seasonal 

 change has been 

 observed in the 

 Louisiana egret. 



In the spring, during nesting time, 

 both the male and female have a red 

 eye, surrounded by bare blue skin. 

 In the female the iris gradually 

 changes to yellow after the breeding 

 season is over, while the blue skin 



becomes yellow also. The male keeps 



FIG. 38. HEAD OF Goos- 



his red eye. ANDER (FEMALE). 



i 



