472 VERTEBRATE LIFE AND ORGANIZATION 



Shaft 



Aftershaft 



Shaft 



Figure 24.3. Types of feathers. A, Contour feather; B, enlargement of two barbs 

 to show interlocking barbules; C, filoplume; D, down feather. (A-C, modified after 

 Young; D, after Thomson.) 



is commonly a gradual process occurring after the breeding season. 

 Ducks, however, molt abruptly; most of the flight feathers are lost simul- 

 taneously and ducks cannot fly until new ones develop. 



Scales and feathers are the major derivatives of the skin in birds, 

 but there is one conspicuous skin gland (the uropygial gland), located 

 on the back at the base of the tail. It produces an oily secretion that 

 some birds spread over their feathers with their beak during preening. 

 The gland is particularly well developed in water fowl and its secretions 

 are important in waterproofing the feathers. Its secretions may also have 

 other functions, including the maintenance of the horny covering of 

 the beak. 



Skeleton. Many adaptations for flight are apparent in the skeleton 

 of birds (Fig. 24.4). The bones are very light in weight, for they are 

 hollow and remarkably thin. Extensions from the air sacs enter the limb 

 bones in many species. Robert Cushman Murphy of the American Mu- 

 seum of Natural History has reported that the skeleton of a frigate bird 

 having a wingspread of seven feet weighed only four ounces, which was 

 less than the weight of its feathers! This is an extreme example, but the 

 skeleton of all birds weighs less relative to their body weight than 



