45« 



THE BIRDS 



xvi. 14-16 



maintained almost straight and does not follow a wavy path as it would 

 do if the parts of the wing vibrated together. In small birds the wing 

 works more nearly as a whole and the flight differs in several respects 

 from that of larger birds. In general the wing is a very labile system 

 and regulates itself automatically with changes in the aerodynamical 

 forces. This regulation is produced partly by feather plasticity and 

 joint mobility, with participation of reflex muscular adjustments that 

 are little understood. 



The whirring flight of some small birds, especially of humming 

 birds, enables them to remain almost in one 

 place in the air, or even to move backwards. 

 The wings beat backwards and forwards 

 (Fig. 269), often as fast as 200 times a second, 

 and the 'pectoralis minor' is almost as large as 

 the major. 



15. Soaring flight 



Many birds economize the energy needed 

 for flapping flight by making skilful use of 

 the possibilities presented by movement of the 

 air. All birds glide for short distances, some 

 small birds with wings folded, others with 

 wings outstretched. Sustained gliding and 

 soaring upwards without flapping the wings 

 is found only in large birds, probably because 

 considerable weight is necessary to provide 

 kinetic energy sufficient to ensure continuous 

 flight and efficient use of wind variations. As 

 has been suggested, there are two distinct types of soaring birds: 

 (1) land birds using thermal up-currents, (2) marine birds using 

 variations in wind above sea level. 



Fig. 269. Spotted fly- 

 catcher hovering. The 

 wings are passing back- 

 wards and there are spaces 

 between the feathers. 

 (Drawn from a photo- 

 graph by E. Hosking, 

 with permission.) 



16. Soaring on up-currents 



Up-currents of air arise in the neighbourhood of large objects on 

 the ground (cliffs or even a ship) and particularly from variations 

 in the rate of heating of the earth's surface in the sun, over rocks, 

 vegetation, mountain shadows, &c. Birds using such currents usually 

 proceed upwards in a series of small circles, a behaviour seen in 

 buzzards and other hawks and especially characteristic of vultures, 

 which may ascend in this way above 1,000 ft (Figs. 270-2). The 

 characteristic features of such thermal soarers are large wing area, 



