28 INTRODUCTION. 



Wonderful as is the soaring of many birds, a still more remark- 

 able feat is that of hovering. This consists in the bird itself 

 remaining stock-still, while its wings quiver with great velocity. 

 Humming birds do this regularly when searching blossoms for 

 their insect food. When thus poised the body is in a vertical 

 position, maintained by the action of the wings, which are in 

 such rapid motion that the eye cannot follow the strokes. There 

 are other kinds of hovering, however, among birds of prey — 

 such as the kestrel falcon or wind-hover, as it is aptly called, 

 in which all motion, that of the wings included, is suspended 

 for a time. Various naturalists account for this in various ways. 

 For instance, " in a mountain gorge, swept by a stiff breeze, a 

 hawk was seen hovering in search of prey. In the rapid current 

 of air it remained apparently fixed in space, without the slightest 

 flutter, for at least two minutes. It changed its position a few 

 feet with a slight motion of its wings, and came to rest again 

 apparently as motionless as the rocks around it." Mr Hubert 

 Airy, who has been studying this question for years, says he 

 always found, "First, a fresh wind blowing; second, the bird 

 was facing the wind ; and third, beneath it there was a steep 

 slope facing the wind." These, he says, "confirm his .theory 

 that the bird, in hovering, is upheld by a slant upward current 

 of air sufficiently strong to neutralise the force of gravity." The 

 Duke of Argyll, on the other hand, says " he has never seen a 

 kestrel's wings motionless when hovering, these usually having 

 a rapid and tremulous action ; and that, even when this action 

 seemed suspended for a moment or two during a stiff breeze, he 

 could still detect the quivering of the quills." This sort of 

 hovering, he maintains, is perpetually accomplished, under the 

 ordinary conditions of horizontal currents of air, by terns or sea- 

 swallows over the surface of the sea. 



I have repeatedly watched both kestrels and terns hovering, 

 and come to the conclusion that both Mr Airy and the Duke of 

 Argyll are right according to their view of the question. For, 

 as the albatross can only sail with motionless wings so long as 

 it has a sufficient breeze, failing which it must have recourse to 

 the flapping of its wings — in other words, by taking to its oars — 

 so the same with hovering as with sailing or soaring, the feat 

 being done by the long-winged birds without any motion of the 

 wings when the exceptionally favourable conditions prevail, 

 while, under a less favourable condition, the bird, if it is to 

 hover at all, must have recourse to a greater or less extent of 

 wing and tail motion. 



