AVES. 399 



distinct value in sexual attraction has been believed by many 

 naturalists. The feathers, together with the scales of the 

 shank, the claws, and the beak, are epidermal growths and 

 represent the remnants of the exoskeleton so well developed in 

 some of the lower forms. Feathers are not usually produced 

 uniformly over the body, but are grouped in regions \vhich 

 differ in different species. They also vary a great deal in form, 

 from the down feathers of the young to the stiff quill-feathers 

 of the wings and tail. Most birds shed their feathers either 

 a few at a time the year round or within a short period. In 

 the former case the change may be scarcely noticeable. When 

 the moulting takes place rapidly, it may be accompanied by 

 profound disturbance in the health and habits of the animal. 

 The new feathers may differ in color from the old, and thus 

 a periodic change is apparent in the dress of some of our birds. 

 This is not infrequently of such character as to accord in color 

 with the changes in nature outside, giving a real protective 

 value. 



424. Supplementary Topics. In what various ways are the feathers 

 of birds protective? Explain how the protection is realized in each case. 

 What varieties of feathers may be found in birds, and what are the chief 

 differences in structure? How are the color patterns obtained? Are they 

 made up of feathers of one color so put together as to form the pattern, 

 or is a single feather of more than one color? Does a single feather 

 ever show an independent complex color pattern? Where is the boundary 

 between feathers and scales on the legs of various breeds of chickens? 

 Do you find any evidences that feathers are highly modified scales? Are 

 any of the feathers like the hair of mammals? 



Secure further data from nature and from reference books concerning 

 the moulting habits of birds. 



425. Endoskeleton. The chief points of importance to the 

 elementary student are as follows : 



i. There is a fusion of several vertebrae in the sacral region 

 (including some of the thoracic, all of the lumbar, the sacrals 

 and the caudals) with the dorsal bones of the pelvic girdle, to 

 form a strong dome-shaped structure above the viscera. The 

 cervical vertebrae vary much in number (eight to twenty- four) 

 with the length of the neck. 



