72 THE BIOLOGY OF BIRDS 



the sail-area can be increased by a lengtliening of the 

 feathers without necessarily involving much corresponding 

 increase in the size of the bird. The character of the flight 

 depends mainly on (a) the ratio of sail-area to size and 

 weight of body, (6) the strength of the pectoral muscles, 

 which often bulk largely in the total weight of the body, 

 and (c) the shape of the wings. 



In addition to the characters of the feathers, other 

 primary adaptations are : (i) the shape of the bird's body 

 which is well suited for reduction of resistance in passing 

 quickly through the air ; and (2) the insertion of the wings 

 high up on the thorax so that the centre of gravity is far 

 below the centre of suspension, this being assisted by the 

 ballasting of the aerial vessel, with the heavy organs below, 

 e.g. pectoral muscles, heart, liver, and stomach, and with 

 light organs above, e.g. lungs and air-sacs. 



§ 2. Movements in Flight 



A physical account of the flight of birds is beyond our 

 power and scope, but certain elementaiy considerations 

 must be stated. The outstretched and upstretched wing 

 which strikes the air is almost impermeable, partly because 

 of the structure of the individual pinions and partly because 

 the gaps between these are filled up by the wing-coverts. 

 The under-surface is concave, both as regards the wing as a 

 whole and each individual pinion. 



In raising the wing for the next stroke, it is very im- 

 portant, as in rowing, that there should be economy of 

 energy. This is effected by an automatic reduction of the 

 size of the wing when it is not pressing against the air, by 

 a movement of the individual feathers so that air passes 

 between them, and by the convex upper surface which 

 allows the air to glide off easily. Running across the quills 

 of the pinions there is an elastic band which brings the 

 feathers back into position again as the wing begins to 

 descend. It is automatically tightened on the down-stroke, 

 relaxed on the up-stroke. 



