Page:Popular Science Monthly Volume 84.djvu/213



T is a noteworthy fact that many species of birds which take long migratory journeys make these movements in large flocks. The same is true to a less extent of the species which makes long daily flights for food. The origin of this flocking habit is not completely explained by the three ordinary theories, which neglect two most important considerations. These theories attribute the formation of the flock to the companionship which it affords, to the experience of old birds that know the sign posts of the journeys and hence lead the way, and to that protection against enemies, which may be secured by numerical strength; but two other protective features, of prime significance, are here overlooked. In the first place, the large flock automatically, by its numbers, maintains the proper direction of migration; secondly, and just as automatically, in the case of many large birds, the flock is warned and protected against its enemies by its form, or arrangement. The importance of these two automatic methods of protection is readily brought out by a consideration of the influences which tend to deviate the flock from its true course, on the one hand, and by a study of the actual groupings of birds in migratory flocks on the other.

The chief causes of errors relative to direction of flight to which a single migrating bird is subject, are: