A SHEAF OF NATURE NOTES 



to migrate or to commit suicide, for it amounts to 

 that. They leave their habitat in Norway and, 

 without being deflected by any obstacle, march 

 straight toward the sea, swimming lakes and rivers 

 that lie in their way. When the coast is reached, 

 they enter the water and continue on their course. 

 Ship captains report sailing for hours through 

 waters literally alive with them. This suicidal 

 act of the lemmings strikes one as a kind of insanity. 

 It is one of the most puzzling phenomena I know 

 of in animal life. But the migration of all animals 

 on a large scale shows the same unity of purpose. 

 The whole tribe shares in a single impulse. The 

 annual migration of the caribou in the North is an 

 illustration. In the flocking birds this unity of 

 mind is especially noticeable. The vast armies of 

 passenger pigeons which we of an older generation 

 saw in our youth moved like human armies under 

 orders. They formed a unit. They came in count- 

 less hordes like an army of invasion, and they 

 departed in the same way. Their orders were 

 written upon the air; their leaders were as intangi- 

 ble as the shadows of their wings. The same is 

 true of all our flocking birds; a flock of snow bunt- 

 ings, or of starlings, or of blackbirds, will act as 

 one body, performing their evolutions in the air 

 with astonishing precision. 



In Florida, in the spring when the mating-instinct 

 is strong, I have seen a flock of white ibises waltz- 



153 



