BIRDS 



149 



DlAG RAMM ATIC REPRESENTA- 

 TION OF THE STRUCTURE OF 

 THE VANE OF A FEATHER. 



Sch., Rachis or shaft ; St. ; barbs; 

 Nst., barbules. 



(nestling plumage) is generally of simpler colour than that of the old birds 

 (see partridge). The plumage of the female also is generally less brilliant 

 than that of the males (see chaffinch). 



(d) Lubrication or "Preening" of Plumage. 

 Being composed of a very light substance 

 (horn) and almost filled throughout with air, 

 feathers contribute to a marked degree to the 

 diminution of the specific weight of the avian 

 body (see Sections 6, 7). If wetted by the 

 rain, tney would not only tend to increase the 

 body-weight, but their function as a heat- 

 conserving agent would be considerably af- 

 fected. Accordingly, the feathers are periodi- 

 cally lubricated ("preened ") with an oily fluid, 

 secreted by a special oil-gland (uropygial gland) 

 found in all birds (with few exceptions) at the 

 root of the tail above the posterior caudal vertebrae. This gland is specially 

 large in birds which are much exposed to wet (swimming birds). (Com- 

 pare also in this respect the 

 length and mobility of the 

 neck. How does the beak of 

 a bird serve it as comb and 

 brush ?) 



11. The Flight of Birds. 

 (a) How does a bird maintain 

 its position in the air without 

 falling ? The body of a bird 

 is heavier than the air. To 

 prevent itself from sinking to 

 the bottom of the aerial ocean i.e., the surface of the ground the bird, 

 except when kept up by the wind (soaring), is obliged to execute suitable 

 movements, exactly like a man swimming in water. As the latter pre- 

 vents himself from sinking by powerful upward strokes of the arms, and 

 rises upwards with each repeated stroke, so in like manner the bird 

 prevents itself from falling by downward strokes of its wings, and with 

 each new stroke raises its body further upwards (see also Section c). 



(b) How are the wings adapted for this function 1 A bird deprived of 

 its flight-feathers is incapable of flight, and only recovers this power after 

 their renewed growth. It is these feathers, accordingly, which render 

 the wings capable of serving as an organ of flight ; for, by converting the 

 narrow limbs into broad surfaces, they enable these organs to execute 

 powerful and effective strokes against the resistance of the atmosphere. 



TlP OF WlNG FROM BELOW. 



Vf., Anterior vane ; Hf., posterior vane. 



