MIGRATION OF BIRDS 321 



ing it arrange themselves in one continuous file, the 

 beak of each following bird touching the tail of the 

 preceding, in order that the passage opened through 

 the air may not have time to close again. But if 

 the flock is a large one, two files of equal length are 

 formed, which meet at an acute angle, the front of 

 the moving mass. 



"This angular arrangement, of which we find ex- 

 amples in the ship's prow, the plowshare, the thin 

 edge of a wedge, and a multitude of utensils designed 

 for cleavage, is the most favorable for pushing 

 through the mass of the air with the least fatigue. 

 If in marshaling their flying battalions the goose 

 and the duck had taken counsel of the engineer's 

 science, they could not have managed better. But 

 they have no need of others' advice: instructed by 

 their own instinct, they utilized long before we did 

 the principle of the wedge. 



"Moreover, to divide among all the members of 

 the flock the excess of fatigue incurred by the file- 

 leader in opening a passage through the air by 

 strength of wing, each in turn takes the post of 

 honor, the forward end of the single file or the point 

 of the angle formed by the double file. Its term of 

 service ended, the bird at the head retires to the 

 rear to recuperate, and another leader takes its place. 

 By this equitable division of labor the fatigue does 

 not prove excessive for any one bird, and the flock 

 leaves no stragglers behind." 



