2i 4 THE POPULAR SCIENCE MONTHLY 



sharp. If the call of any particular individual bird is listened for, it 

 appears to be repeated almost systematically every few hundred yards 

 as the bird travels on its journey. In the absence of some proof of the 

 true significance of the call note, an explanation can only be a hypo- 

 thetical one, but it certainly appears as if the relation in space of one 

 bird with its neighbors would be roughly maintained by this night call. 

 If we imagine the birds distributed here and there in the air through- 

 out the area through which the migration is taking place, the effect is 

 that of a great flock flying, for example, southward, the individuals of 

 which are widely separated. Each bird repeatedly signals to its neigh- 

 bors and thus learns from its fellow travelers the general direction of 

 migration. 



Of course where there are many species in migrating, the velocity of 

 flight of the various individuals would be different, and some birds 

 would relatively advance and others fall behind, but a movement by a 

 single bird diagonal in direction to the main movement of the migrating 

 birds would at once be made evident by means of the sentry-like calls of 

 the birds, both to the straying bird itself, and to the other alert indi- 

 viduals taking part in the migratory movement. 



The importance of the mutual reaction of individual birds, set 

 forth in a previous paragraph, as a means of preventing deviation from 

 the correct course of flight, may apply to the night migration of many 

 birds which are known to migrate singly, or in small flocks. That is, 

 the mutual reaction of the individuals and small flocks, would then be 

 communicated by the night call rather than by imitation through sight, 



v * ^ v v N N > 





V 



Fig. 6. Flock of Blue Geese in Echelon Formation, photographed at the 

 Mississippi delta by the Rev. H. K. Job. Note the acute angle of the flock and that 

 for each goose the view is unobstructed in front and on the side. 



