The Audubon Societies 



305 



stand what actually happens in the case of these birds, it is necessary to know 

 something of the structure of the feather as shown under the lens. 



All feathers are composed of a mid- vein or shaft and the web. If the web 

 is examined carefully it will be seen to be composed of a series of fibers called 

 'barbs' attached on each side of the shaft. Each barb, similarly, bears two 

 rows of barbules. When the barbules are examined under the microscope, 

 they are found to bear a number of minute recurved booklets which fasten 

 into the booklets of adjacent barbules and give to the feather its firmness, 

 being best developed in the flight feathers, which require the greatest strength. 

 Some feathers, and the innermost parts of most feathers, lack this device and 

 are, therefore, always soft and fluffy, giving little resistance to the passage of 



MALE HOUSE SPARROWS IN SUMMER AND IN WINTER I'EUMAGE 



Note that in winter the black throat and breast is nearly concealed by the gray tips of the feathers. In 



the spring they wear off, revealing the black beneath 



air through them. Now in the case of the Purple Finch and Indigo Bunting, 

 the red and blue colors are located mostly in the barbs, while the barbules and 

 booklets are dusky. With the wearing away of the barbules and hooks on the 

 body feathers, the barbs become more conspicuous and the color of the bird 

 becomes apparently more intense. 



That molting is not confined to birds is well recognized and its homology 

 to the 'shedding of the skin' in reptiles and amphibians is conceded. Of course, 

 the snake or amphibian does not actually shed its skin but merely the hard 

 outer cuticle. This cuticle, as in the case of the birds' feathers when fully 

 formed, is a dead structure and it is inelastic. Consequently, as the snake or 

 frog or salamander grows, this 'shell' becomes too small for it and must be 



