376 ZOOLOGY 



Blacks, reds, browns, and usually yellows are due to such 

 pigments. The bluebirds, however, furnish no more 

 blue on analysis than the rainbow ; the pigment present 

 is not blue at all, and the brilliant effect is due to the 

 manner in which the surface of the feathers reflects the 

 light. This can be determined by examining the 

 feathers by transmitted light under the microscope. 

 In all such cases the underlying pigment is connected 

 with the effect produced, but the manner in which the 

 light is reflected is the more important factor. 

 Moulting 4. A caterpillar, as it grows, sheds its skin from time 



to time ; a snake does the same. The scales of fishes 

 and reptiles, and the feathers of birds, are renewed 

 when lost. In birds, however, we find a periodical loss 

 of feathers, the moult. Feathers, like clothes, wear out, 

 and were they not renewed the bird would become "a 

 thing of rags and patches." Moulting renews the 

 plumage, replacing the old clothes by new and clean 

 ones. Usually the moult is annual, after the rearing of 

 the young; but it may occur more frequently. The 

 feathers do not all come out at once, or the bird would 

 be disabled. Some water birds, as ducks, do indeed 

 shed their primary or large wing feathers at once, and 

 for a while are unable to fly. If the external physical 

 conditions, such as the amount of moisture, are greatly 

 altered, the color of the feathers after a moult may be 

 modified. In the ptarmigan, however, the plumage 

 ' regularly changes to white in the winter, to harmonize 

 with the snow, on which the bird is almost invisible. 

 This is not due to a change in the feathers themselves, 

 but to an alternation of white and brown colors in 

 successive plumages. Once a feather is formed, it is a 

 dead structure, like a hair, and cannot be modified, 

 except through wear or dirt affecting its appearance. 



