CLASS AVES. BIRDS 



435 



Contour 



Diagram of the shaft 



barbs and barbules 



of a contour feather 



Contour Contour 



(outer surface) (inner surface) 



Figure 307. Left, kinds of feathers. Right, microscopic detail of a contour feather to show 

 the curved edge, along which the hooklets slide to make the feather flexible. 



3. 



the body. These include the general body 

 and wing (flight) and tail feathers. 

 The down feathers are without a shaft, the 

 barbs arise as a fluffy tuft from the end of 

 the quill; they provide the first plumage, 

 natal down, of newly hatched birds and pro- 

 vide an insulating covering beneath the con- 

 tour feathers in such birds as the waterfowl. 

 The filoplumes are hairlike feathers with a 

 few barbs at the tip. These are the feathers 

 singed off a chicken before it is cooked. 



Only certain portions of the pigeon's body 

 bear feathers; these feather tracts are 

 termed pterylae, and the featherless areas, 

 apteria. The feather tracts vary in different 

 species of birds; those of the pigeon are 

 shown in Fig. 308. 



Birds shed their old feathers, that is, they 

 molt usually in the late summer and acquire 

 a complete new set of feathers, which are 

 formed within the follicles and from the 

 papillae of those that are cast ofT. There may 

 be a partial or complete molt in the spring 

 when the bird assumes its breeding plum- 



age. At this time the plumage often changes 

 color; this is usually due to wear or to the 

 breaking off of the tips of the feathers, thus 

 exposing new colors beneath. 



Skeleton 



The principal differences between the 

 skeleton of a pigeon and that of a reptile are 

 those made necessar}- by the methods of 

 locomotion of the former. The hindlimbs 

 and pelvic girdle of the pigeon are modified 

 for bipedal locomotion; the forelimbs and 

 pectoral girdle are modified for flight; the 

 skeleton of the trunk is rigid; the sternum 

 has a distinct median ridge, the keel; short 

 projections, called uncinate processes, ex- 

 tend backward from some of the ribs, mak- 

 ing the thoracic framework more firm; and 

 most of the bones are very light, many 

 containing air cavities. The skeleton of the 

 common fowl (Fig. 309) is larger and more 

 easily studied than that of the pigeon and 

 is similar to the latter in most respects. 



