FLIGHT 269 



Gull, oil which the observations have for the most part been made, 

 but there is at present no reason for supposing that the active 

 flight of other birds dift'ers from that described in any essential 



Fig. 8. — Photographic trajectciky of the tip of a Crow's Wing. The little arrows shew 

 the direction of the tip's movement. (From Marey.) 



particulars, though there is still a good deal to be learned about the 

 mechanics of active flight, and data sufficient, for example, to enable 

 us to calculate the work done by a bird in flying through the air 

 are still wanting. AVe have, however, in the above given only a 

 very incomplete sketch of what has already been learned on the 

 subject. 



III. "We now come to the interesting subject of soaring or sailinff. 

 The typical soaring birds come under Miillenhoff's ^ " Vulture-type," 

 ■whose sail-area in relation to the total weight of the bird is a large 

 one. In this category come such birds as the larger Corvidx, viz. 

 the Raven and Crow, the Falcons and Vultures, the Owls, Pelicans, 

 and Storks. Those of them which can soar are mostly large birds, 

 A\ath a relatively large wing-area, and few of them are commonly 

 visible in this country, Avhich is possibly one reason why the whole 

 subject of soaring flight is still so obscure. 



The main characteristics of the soaring flight of such birds 

 have, however, been recorded by a number of trustworthy 

 observers, and are no longer seriously disputed. A certain 

 amount of wind appears to be essential, soaring flight not being 

 observed in a dead calm. Observers seem to agree also in this, 

 that the soaring bird, with motionless outstretched wings (having 

 raised itself some distance from the ground or sea b}' active wing- 

 strokes), describes in its flight, curves or circles which lead it to 

 alternately sail up the wind and down the wind. It describes 

 wide curves, and loses in vertical position while it is directed down 



^ Tom. cit. p. 425. 



