FLOCKING HABIT OF MIGRATING BIRDS 



213 



helpful in migration, it is to be expected that the 

 flocking together for the migration has slowly devel- 

 oped into a well-formed habit. Of course the desire 

 for companionship and also the following of a leader 

 must be additional reasons for the flocking of birds, 

 but taken together or alone they do not seem to be a 

 complete explanation. 



The Night Migratory Call of Birds 

 During the spring and autumn migrations at any 

 time during the night in regions traversed by large 

 and small wild fowl one can hear the curious night- 

 call of the passing birds. This call is generally re- 

 garded by ornithologists as a signal by which the 

 birds may communicate from one to another. The 

 night call, in the case of many species, is apparently 

 only used while migrating, as it is also apparently 



Fig. 4. Single-file 

 Flock Formation, 

 which is seldom observ- 



nearly alike tor certain species, although the matter e d. Only the first one 

 has been little investigated. There are, however, or tw0 in the line can 

 sufficient reasons for the belief that the call has a 

 and that 



see ahead. The field of 

 view becomes more ob- 

 scured as the number of 

 birds increase. Non- 

 protective. 



special purpose, and that it may be a protective 

 measure which aids in preventing the individual 

 birds of small flocks from deviating from the cor- 

 rect migratory course. The call can be heard almost any time during 

 the nights of April and May and also of August and September, the 

 months when the greater number of birds make their migration in the 

 temperate zone. The night call note appears to be different in many 

 cases from the usual day cry of the birds and is peculiarly short and 



Fig. 5. Echelon Flock Formation. The usual flight formation of large birds. 

 All birds can see ahead, and towards one side, making the best arrangement for pro- 

 tection. The protective efficiency of the formation is little affected by an increase 

 in the number of birds of the flock. 



