404 ZOOLOGY 



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 temporary 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 appar- 

 ent 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. 



433. 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 dififerences 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. 



434. Endoskeleton. — The chief points of importance to the 

 elementary student are as follows : 



1. 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. 



2. The cranial bones fuse closely, and the bones of the face 

 are prolonged into the core for the beak (Fig. 219). 



3. The sternum is normally well developed and provided 

 with a keel to which the muscles of flight are attached. Pinger- 

 like processes also increase its surface for the attachment of 

 muscles and the support of the viscera. 



4. The ribs are double-headed, and each has a process on 

 the posterior margin, joining it to the rib behind. 



5. The pectoral girdle has its clavicles fused ventrally in the 

 flying birds, forming the "wish bone." 



