STERNA or RUNNING BIRDS. 371 



which, from their habits, the birds have to find their 

 way. The narrowness of the sternum in proportion 

 to its length, and the accordant general shape of the 

 bird, also conduce to the same purpose, and the 

 straightness of the sternum enables it better to resist 

 pressure in the direction of its length than the curved 

 sterna of other birds, though these support much 

 more powerfully the flying motion of the wdng. 



Many of the birds which have the sternum of this 

 form are swift and smooth runners, and all, or at least 

 most of them, run with the head advanced nearly in 

 a line with the axis of the body, which is carried 

 horizontally. The head, too, is small and pointed, 

 and the shoulders are narrow and tapering. This 

 form of the body enables the birds to make their way 

 among the tall herbage of humid places, and especially 

 the banks of brooks and streams, and the reedy 

 margins of pools and shallow lakes in rich and flat 

 countries, where they seek the greater part of their 

 food. Many, if not all of them, can at times enter 

 the water, and, when they are immersed to a sufficient 

 depth, they use their wings for motion in that element. 

 The narrow sternum, and the consequently short and 

 rapid motion of the wings, are much better adapted 

 for action in the water than wings which take longer 

 strokes, and are more powerful and efficient in the 

 air. This can be readily understood when it is borne 

 in mind that the raising or recovering of the wing is 

 a much more laborious operation in water than in air, 

 and that the power requisite for working long wings, 

 almost to the full circle, as is done by air birds, would 

 in water be greater than any ordinary structure of a 

 bird could be supposed to possess. Besides, when 

 wings are used under water, they must keep time 

 with the feet, and hence they are never moved far 



B B 2 



