Page:Popular Science Monthly Volume 84.djvu/216

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 which is seldom observed; only the end birds can see towards the side. The field of view becomes more obscured as the number of birds increase. Non-protective.

The influence of every bird in the flock will affect the flight direction of the flying flock to some extent, yet they each produce their reaction, although perhaps not proportionately. It is probable that the birds in the front of the main part of the flock affect the direction greatest.

It is a well-known fact that when man is lost on a prairie or in a snowstorm he will often travel in a circle. The reason for this is that there is a small constant deviation of his course to the right or to the left. Small as this deviation may be, it is certain to throw him completely off his bearings, and not infrequently results in circling with fatal results. It is possible that a bird is subject to a similar deviation of its course, owing to the inevitable production of a curved flight path in case one wing of the bird is stronger than the other, the effect being similar to a man in a boat pulling at the right oar more powerfully than the left. Now the error of individual birds caused by a constant deviation of path due to unequal wing power, as well as the error due to mistaken "bearings," would be corrected if the errors of the individual birds in the flock are averaged while in flight by mutual reaction. Fig. 2, A, is meant to illustrate these deviations due to unequal wing strength, or to some similar cause, when single birds are proceeding, for example, from the mainland to a far distant island destination D (Fig. 2), and Fig. 2, B, the result when the birds are flying in a flock and when these deviations are averaged. It is of course evident that the averaged deviations might give a flight direction that is not exactly the right one, and a flock of birds might fly in a wrong direction if much confused. This is exactly what takes place; for occasionally flocks of geese and other species of birds have been known to become completely confused in a fog or during a stormy night. It is a very common occurrence for birds that are alone to become lost, as shown by the fact that so frequently single land birds alight, utterly confused, on vessels far out at sea.

The averaging of the errors of the direction of flight of the individuals is of course subconsciously done by birds and is quite automatic.

The principle of mutual reaction and its advantageous effect may be advanced as a reason for the massing of birds into flocks prior to the migratory journey, for if it is true that the flock formation has proved