PLIGHT OF SEA GULLS — MILLER 399 



Plate 3, Figure 1. These gulls were hovering with almost no forward 

 motion, picking up bits of food from the water without alighting. 

 The advanced wings, depressed tail, and lowered feet indicate the 

 efforts to check forward flight. 



The feet are ordinarily held close against the under tail coverts in 

 flight (pi. 2, A, C, and E), but may be lowered and even the webs 

 spread out to act as " brakes " in retarding flight. The coordinated 

 use of feet and tail for this purpose is admirably shown in Plate 2, F, 

 especially in the bird only partly included at the top of the photo- 

 graph. 



In rising from the water a further use of the feet becomes evident 

 (pi. 3, fig. 2). A certain forward momentum is necessary before the 

 bird can rise, and a gull may often be seen contributing to the efforts 

 of its wings by kicking vigorously as it leaves the water. 



However complicated may be the process of flapping flight, so 

 long as a bird's wings are in motion we are able to understand, at 

 least in a measure, how it keeps aloft; but what are we to say when 

 we witness a large bird sailing for great distances on almost motion- 

 less pinions without loss of altitude, or even steadily gaining altitude 

 with no more effort than the occasional twitch of a wing in making 

 an adjustment to some sudden gust? This is the phenomenon re- 

 ferred to as soaring flight, which has ever been a source of wonder- 

 ment to layman and scientist alike. 



While the gulls are not masters of this type of aerial navigation 

 to quite the same extent as the larger hawks and vultures, neverthe- 

 less they often give remarkable exhibitions of their powers along this 

 line. It is a common sight to observe a gull travel several miles at 

 a speed of from 12 to 18 knots per hour without a single flap of the 

 wings; and I think it probable that much higher speeds than this 

 would be recorded if there were faster steamers on the bay to 

 serve as a basis of comparison. 



Various theories have been proposed from time to time to account 

 for soaring flight, some of which are plausible, while others are 

 rather obviously at variance with the facts. 



It has been commonly urged that a soaring bird has gotten into an 

 upward current of air, in which it has only to maintain itself by 

 proper adjustments, retaining its height or ascending according to 

 the force of the rising current and the angle of its wings. In other 

 words, soaring flight is simply a downward glide in an ascending 

 column of air. 



It has been objected to this that birds are often seen to soar in the 

 absence of any ascending current, so far as can be detected, and 

 even that they studiously avoid such currents (Hankin, 1913, pp. 

 19, 63, etc.). 



