Page:Popular Science Monthly Volume 84.djvu/214

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 In the left-hand diagram, A, the arrows are intended to represent birds and the directions in which each individual would fly if alone. Thus they indicate the amount of confusion of each bird with respect to the true destination D. In the right-hand diagram B, the birds are represented flying in a flock, the errors of orientation having been averaged by mutual reaction as indicated by the parallel arrows.

The large flock seems to eliminate these causes of error to a considerable extent, therefore it is probable that the origin of the flocking custom is largely due to the fact that it is protective.

The explanation suggested by the author to account for the flocking habit is as follows:

If, for example, in the case of a migrating flock, there are any individuals that are confused in their bearings with respect to the direction of their destination, and there must be many that are more or less confused, some erring toward the right, some toward the left, the errors of the individuals of the flock are automatically averaged and corrected by imitation, and by mutual reaction by those same individuals.

According to this hypothesis, birds flying in a flock will follow the direction which may be called the mean flight direction of the flock. The idea is illustrated by a diagram. In Fig. 1, A, the arrows attached to circles are meant to represent the directions that the individual birds of a migrating flock would take, for example, after resting on an open sea, and arising to fly to an intended destination, D, (Fig.), provided each individual was required to migrate alone from that moment. The assumption is made that most of the birds are confused with respect to