Popular Science Monthly/Volume 84/March 1914/On the Origin of the Flocking Habit of Migratory Birds
|ON THE ORIGIN OF THE FLOCKING HABIT OF MIGRATORY BIRDS|
By Professor C. C. TROWBRIDGE
IT 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.
Automatic Protection of a Large Flock
The chief causes of errors relative to direction of flight to which a single migrating bird is subject, are:
|(a) Confusion with respect to the proper direction of flight.|
|(b) Effect of heavy winds or thick fogs acting as a temporary confusing factor while a flock is migrating.|
|(c) Gradual deviation from the proper course due to unequal wing power.|
Fig. 1. 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:
The Direction Errors of the Individuals of the Flock are Averaged
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 the correct orientation to a greater or less extent, which is obviously the fact, although the confusion may be very slight in some cases. In Fig. 1, B, the direction in which the birds will actually move when in flight as a flock is indicated by the parallel arrows. Thus in Fig. 1, B, the mutually related influence of the individuals prevents the dispersion that would occur as indicated by the direction of the arrows in Fig. 1, A. The averaging of the errors that take place, as indicated in B, is approximately governed by mathematical certainty, and as a matter of fact, in the drawing Fig. 1, B, the flight direction was determined by taking the mean of all the directions indicated by the arrows in "A."
Each bird is affected by the averaging of flight directions due to the mutual reaction of the individuals, and the reaction prevents false starts. As an extreme case of this correcting influence, consider a flock of birds proceeding northward after resting on an open sea in a fog; and suppose that one bird was so confused that it would have flown in the opposite direction (to the south) if alone. Then, in all probability, in spite of its individual inclination to fly south, this bird would be carried northward with the flock by the powerful principle of imitation.
It must be distinctly understood that this theory does not give an explanation of the sense of direction, but it does provide a mechanism which will prevent individuals of the flock from getting lost. The only assumption is that there are all degrees of right and wrong "bearings" among the individual birds of the flock.
Fig. 2. The curved lines in the left-hand diagram A represent the paths which the birds would follow due to unequal unbalanced wing power, if each was alone. In the right-hand diagram, B, the arrows represent the birds flying together and the direction is the mean of all deviations and represents the flight direction towards D as corrected by mutual reaction.
Fig. 3. Platoon Flock Formation, 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.
Averaging of the Errors of Flight Due to Unequal Wing Power of Birds
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 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.
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.
or air pressure as in the closely formed flocks during the day. That countless numbers of birds migrate at night has been abundantly demonstrated and requires no proof.
The Protective Form of Certain Flight Formations
The flight formations of birds which fly in flocks vary greatly with different species, so varying is it that in many cases it is an identifying characteristic. Many small land birds fly in very compact groups, examples of which are the various species of blackbirds, grackels, cedar birds, finches, etc. Many water birds fly in compact flocks. Moreover, for many species there seems to be some general law regarding the size of the bird and the distance apart of the different individuals of the flock.
Probable Explanation of the Echelon Formation
The most distinctive formation is the echelon arrangement of flight and deserves special attention. Many of the larger water birds such as geese, many species of ducks, flamingoes, etc., practise this method of flight. The formation, according to one explanation, owes its origin to the fact that such an arrangement prevents swirls in the air produced by the wings of one bird from interfering with the next bird following. This may be true to some extent, but the explanation which the author holds as far more likely is that the formation is chiefly, if not wholly, a protective arrangement, and is, in fact, the only one in which the individual birds of the flock, if they are of considerable size, can see both forward and to the side at the same time, as demonstrated by the
Fig. 7. Flock of Blue Geese in Echelon Formation, photographed by the Rev. H. K. Job. As in Fig. 6, for each goose, the view is unobstructed in front and at right angles to the line of flight.
illustrations, Figs. 3, 4, and 5, and also the exceptionally fine photographs, Figs. 6, 7 and 8, taken by the Rev. H. K. Job, state ornithologist of Connecticut, who kindly provided them for illustrating the present discussion.
It is probable that the echelon formation has come about partially by the fact that birds naturally follow one another by imitation. For example, suppose a flock of geese arises from a surface of water in one of
Fig. 8. Flock of White-winged Scoters in Echelon Formation, migrating off Manomet, Mass. Photographed by the Rev. II. K. Job from a row-boat. The heads of all the birds can be seen from the boat. At the moment the camera appears as the danger point to the birds. Every eye on the boat side has an unobstructed view.
their daily flights to their feeding ground; at first the individuals will be grouped together indiscriminately, many following those that are leading. Then, for example, the second bird in line will shift to the right or left to be able to see ahead as well as to the side, and so on down the line. While the mode of flight is now a well-formed habit, yet the process of producing the formation goes on to some extent each time a flock (of geese, etc.) begins a flight. The echelon formation is thus the one that will permit the greatest number of birds to see towards the chief danger zones, the front and the sides. Perhaps this explanation has been given by others, but the author has not found it in other writings. Small birds that pursue an irregular flight (i. e., undulating or fluttering) have an opportunity to see into the danger zones even when in a large flock; hence the necessity is lacking for the echelon arrangement of the individuals of a flock in such cases.
Among certain species of ducks a reverse echelon flight formation is observed, namely, the directions of flight is in the opposite direction to the point of the V. The reverse V is nearly as protective as the V. Since in this form also the number of eyes of the birds in the flock which cover the "danger zone" is at a maximum. The above hypothesis does not in any way conflict with the theory that geese will follow a leader, a theory which may or may not be true.
The facts presented above are taken as strong arguments for the conclusion that the origin of the flocking habit is due to its protective effects, and that particularly the echelon arrangement of flight has been evolved from the protection against enemies which it affords.
This investigation has been aided by a grant from the Herman Fund of the New York Academy of Sciences.