FEET OF PETRELS. 299 



The petrels have the feet with the tarsi considera- 

 bly longer than those of the terns, and generally com- 

 pressed in the direction of their action. They have 

 three toes to the front, webbed like those of the terns, 

 and a small claw in place of the hind toe. These 

 feet are, like those of the others, only auxiliary to the 

 principal natural action which is performed by the 

 wings, and they are adapted to a different sort of 

 flight. The terns, though not birds of lofty flight, 

 fly moderately high, and catch those substances upon 

 which they feed by twitching down, snapping with 

 the bill, and instantly rising again. The petrels skim 

 along the surface, and pick up the lighter substances 

 without altering their motion. On these occasions 

 they often, but not always, tip the water with the 

 webs of their feet, to aid the motion of the wings, or 

 sometimes to render it unnecessary. The length and 

 compressed form of the tarsi fit them well for this 

 purpose, while their lightness of structure, in pro- 

 portion to the strength of the muscles by which they 

 are put in motion, enables them to move with great 

 celerity, and yet occasion little fatigue. At the same 

 time, the action of tipping is done in so clean a style, 

 that the water is not disturbed, and the motion of the 

 legs is not seen unless when the observer is very close 

 to the bird. Some of the larger species, which are 

 found on the Antarctic ocean, are nearly of the same 

 dimensions as a wild goose, and yet they will let 

 themselves down, with the wings fully expanded, till 

 the points of their webs touch the surface of the water, 

 and then they will glide along without a movement 

 of the wing till they are lost in the distance. The 

 cut represents the foot of the fulmar petrel, which is 

 very common on the seas to the northward of the 

 British islands. 



