xvi. 1 8 



SPEED OF FLIGHT 



461 



can take advantage of variations in horizontal wind velocity, including 

 gusts at any one level. The bird moves upwards as it meets an accele- 

 rating gust and turns when the wind decelerates. Gulls also use up- 

 currents at cliff faces and, no doubt, air movements of all sorts are 



Fig. 274. Heron (Ardea) leaving its perch. The legs have been used to make a jump 

 and the wings are fully spread. (Drawn from photograph by Aymar.) 



widely used, especially by large birds. However, it is evident that the 

 wing equipment only allows the bird a limited range of choice and 

 probably even the slightly different wing shapes of related species 

 depend on the differing conditions they are called upon to meet. A 

 vulture could no more zoom backwards and forwards over the waves 

 than an albatross could circle slowly on a gentle thermal up-current. 

 A pigeon cannot equal a gull at steady gliding and soaring, but the 

 pigeon can rise more steeply or descend more rapidly without stalling. 



18. Speed of flight 



Estimation of the speed of flight involves distinguishing between air 

 and ground speed. The speed relative to the ground may be very high ; 

 there are records of birds covering more than 100 land miles in an 



