284 BIOLOGY FOR BEGINNERS 



tions to secure greater strength and less weight. At the base is 

 an opening through which nourishment was supplied during its 

 growth. The vane of the wing feathers is wider on one side of the 

 rachis than the other. When the wing strikes against the air it 

 tends to turn up, but rests against its neighbor and is held flat, 

 while on the return stroke it is free to turn. The air passes through 

 the wing as each feather partly turns on its axis (" feathering ") 

 and the wing meets less air resistance. 



Uses of Feathers. The feathers provide the means of flight, 

 and aid in easy locomotion, by giving the angular body a smooth 

 outline. Moreover feathers, being one of the best heat-retaining 

 substances, serve to keep the bird warm, even in the coldest 

 weather, no matter how high or swift its flight. Their great 

 activity necessitates their high body temperature and the feather 

 covering retains this heat and makes possible their life in the upper 

 air. The feathers of most birds are oiled by a secretion taken 

 from a gland near the tail and spread on them by the beak. This 

 makes them waterproof and is best shown in swimming and diving 

 birds, which can spend hours afloat and suffer no discomfort. 



Feathers have a further use in providing a colored covering 

 which helps birds in escape from discovery by enemies because of 

 its resemblance to their surroundings. This coloring may also be 

 used to attract mates. 



Moulting. Birds shed their feathers at least once a year, so that 

 new ones may replace any that are lost or damaged. This is 

 especially important in the case of wing feathers. Some species 

 moult twice annually and may have differently colored plumage 

 at different seasons. This change of color is sometimes used for 

 protection and sometimes to attract mates. Wing feathers are 

 shed in pairs and gradually, so as not to impair flight. 



SUMMARY OF USES OF FEATHERS 



1. Flight. 



2. Giving regular body outline. 



3. Protection from cold and water. 



4. Protective coloration. 



