Fig. 4. Single-file Flock Formation, which is seldom observed. Only the first one or two in the line can see ahead. The field of view becomes more obscured as the number of birds increase. Non-protective. helpful in migration, it is to be expected that the flocking together for the migration has slowly developed 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 regarded 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 nearly alike tor certain species, although the matter has been little investigated. There are, however, sufficient reasons for the belief that the call has a special purpose, and that it may be a protective measure which aids in preventing the individual birds of small flocks from deviating from the correct 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 protection. The protective efficiency of the formation is little affected by an increase in the number of birds of the flock.
