DISPERSAL AND MIGRATIONS 175 



where they flutter to and fro, or drift by in an 

 aimless manner utterly bewildered, or strike 

 against the glass with such force as to kill them- 

 selves. Large numbers are known to perish in 

 this manner every season. It is only on over- 

 cast nights, however, that such scenes are 

 witnessed, and even then as soon as the heavens 

 become clear the migrants disappear like magic, 

 passing on their way again. Many birds also 

 lose their way when on migration, especially in 

 autumn. A very large percentage of these lost 

 birds are young ones. This fact in itself should 

 at once do away with any theory of instinct, for 

 instinct must be held to be infallible. The 

 various cries uttered by birds when on migration 

 are also very interesting. These cries are uttered 

 most probably to keep the members of the flock 

 together as they fly across the night sky, and 

 often sound singularly weird and imposing from 

 the darkened air. It is also somewhat remark- 

 able that the noisy birds are those that chiefly 

 migrate by night : birds that travel by day 

 generally do so in silence. 



In conclusion, we ought also to allude to the 

 various kinds of migration that birds undertake. 

 The best known type of migration is that of 

 which the Swallow, for instance, is a good 



