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 hypothetical 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 throughout 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 neighbors 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 individuals 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,
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.
