WIND AND FLIGHT 



131 



exertion on the bird's part unnecessary, any more 

 than it does in the case of the Snipe that faces the 

 wind in rising. 



There is no reason why a bird should not advance 

 in the same way in the teeth of a wind when the 

 conditions are favourable, and occasionally Gulls 

 may be seen employing this method behind a steamer 

 when the wind is directly or almost directly ahead. 

 As the vessel moves onward, there is a down-current 

 behind the stern, since the air rushes down to fill 

 the space just vacated by the advancing ship. The 

 down-rush of air strikes the water and, rebounding 



A W 



Fig. 25. 



Flight at right angles to the wind, with motionless wings, 

 the direction of the wind. 



W: 



from it, forms an up-current a little way farther 

 back. Here, then, is an up-current extending only 

 over a very small area of water, but, since it moves 

 on with the steamer, it opens up great possibilities. 

 To start with, imagine the Gull flying a little in rear 

 of the up -current. He glides swiftly downward and 

 onward, and when near the water finds himself in 

 the convenient up -draught, which lifts him to his 

 former level, so that he is able again to glide down- 

 ward and make much headway. At the end of his 



K 2 



