ALTITUDE OF THE MIGRATION FLIGHT 49 



this bird mounted higher and higher ; when it had reached an eleva- 

 tion of about four hundred feet, it performed two or three strokes 

 with its wings, and then soared upwards without an}^ further wing- 

 movements. There was a very hght south-east wind — in fact almost a 

 calm — at the time, and a light white stratum of cirrus clouds, miles 

 high, covered the sky uniformly ; the meteorological conditions 

 being, in fact, as favourable as possible for observations of this kind. 

 The bird's position in the air lay in a direction about south-south- 

 east — in fact, very nearly south. "Without changing the direction 

 of the axis of its body, or even its horizontal jDosition, it reached, 

 soaring vertically upwards, in the course of a minute, a height of at 

 least 1000 feet, and still continuing in its upward course, finally 

 disappeared from view in the clear noon-day sky in company of from 

 twenty to thirty other birds of the same species. 



The fact that these birds soar upwards without wing-movements, 

 and yet steadily and rapidly in unbroken lines, to heights where the 

 human eye can no longer reach them — in the present case at least 

 12,000 feet — renders this phenomenon still more remarkable, and 

 gives it a most striking likeness to the ascent of a balloon. 



An attentive observation of the flight of the large gulls, soaring 

 for hours at the same height in a perfectly calm atmosphere, with- 

 out the least motion of their wings, is indeed sufticient to convince 

 us that the expanded wing-surface alone could not be suthcient to 

 prevent the bird from sinking, in virtue of its weight, after the fashion 

 of a parachute. How much less, therefore, is it possible that an up- 

 ward movement like that of the Buzzards noted above could be 

 effected by an immovable expanded surface of this kind ? For 

 further remarks on this subject, see under Herring Gull, Part iii. 

 No. 358. 



By executing a number of wing-strokes, repeated at longer or 

 shorter intervals, birds may acquire a certain velocity of flight 

 which would enable them, by slightly raising the forepart of their 

 bodies, to glide, as it were, upwards supported by the resistance of the 

 air. In this manner they would be enabled to ascend in a spiral 

 path, as actually was the case with some Common Kestrels which were 

 accompanying the above-mentioned Buzzards in their migration. 

 Some birds, again, like many of the smaller species of Falcons, 

 when engaged in what is known here as 'Rtitteln' {i.e. shaking), 

 or Larks during their song, are able by means of a rapid, almost 

 trembling motion of the wings, to i-emain for a moment suspended 

 at a particular point in the air ; none, however, can, by the sole 

 help of their outspread wings, remain soaring for any length of 

 time in a calm atmosphere at the same height, — to say nothing 

 of rising upward. 



D 



