From Current Literature

Physical Foundations of Bird Navigation

In a recently published paper, Yeagley*) has convincingly shown the exceptional ability of the homing pigeon to return home from very remote points, and on an apparently unfamiliar road, has a physical explanation. Until now this circumstance, as well as the ability of many birds to return home after wintering in southern countries, has been “explained” by instinct. As the author of the paper rightly observes, to say this is all the same as to say “I do not know.”

The following experimental facts must form the basis of a physical explanation of the phenomenon. 1. A pigeon released from an unfamiliar place, before directing its flight in a straight line, describes several broad circles. 2. Homing pigeons cannot fly in dense fog and in complete darkness. 3. In the presence of a wind with a speed greater than 35 miles per hour, homing pigeons lose the ability to navigate. 4. The training of pigeons consists in the fact that, over a period of 6–8 weeks, they are released successively at distances of 1, 2, 4, 8, 16, and 32 miles from home. After this they acquire the ability to navigate from a distance of 100, and after several weeks of 200 and more miles. Year-old, experienced birds can cover distances of 400–600 miles. 5. Pigeons lose the ability to orient themselves if they are released near powerful radio-transmitting devices.

To explain all these facts the author advances the following hypothesis. The ability of homing pigeons to navigate is determined by three principal factors: 1. Sensitivity to the Earth’s magnetic field. 2. Sensitivity to the inertial forces arising from the rotation of the Earth and acting on masses moving along the Earth’s surface. 3. Visual sensitivity to their own speed relative to the Earth’s surface.

The first and third factors allow the bird to determine its magnetic latitude; the second and third factors allow it to determine geographic latitude. As a result of this the pigeon is able to determine the location of its home as the point of intersection of the characteristic lines of the vertical magnetic field of the Earth and of latitude. From this point of view, the secret of navigation lies in the displacement of the Earth’s magnetic poles from the poles of rotation, since it is precisely this circumstance that causes the magnetic-field lines to intersect the lines of latitude at an angle.

One of the first facts that led to the formulation of the above hypothesis was the observation of a complete failure in the training of homing pigeons near the city of Indianapolis (USA). Attention was drawn to the fact that in this particular region the equipotential line of the vertical component of the magnetic field runs almost parallel to the geographic latitude. Thus, in this place the “physical characteristic” of the home is absent, since identical values of the magnetic field and of the Coriolis force occur not at a given point, but along a long segment of a line.

) H. L. Yeagley, J. Appl. Phys., 18*, 1035, 1947 (December).

The author did not set himself the task of showing or investigating wherein lies the mechanism of action of the magnetic field and of inertial forces on a flying bird. His task consisted only in proving experimentally the validity of the hypotheses put forward. Therefore the author describes bird navigation in the following way. By means of some organ or organs the bird feels the effect of motion in the Earth’s vertical magnetic field. This effect depends on the speed of flight; however, at a given flight speed this effect will differ depending on proximity to the Earth’s magnetic pole. A pigeon released far from its home directs its flight toward that place where the magnetic effect has the value familiar to it. It is obvious that the magnetic effect alone enables the bird to find only one coordinate of its home. Therefore it must be assumed that, by means of some organ or organs, the pigeon feels the Coriolis force. This effect likewise depends on the speed of flight and, at a given speed, will be the same for a given geographical latitude. A bird released far from its home also continuously “feels” for the effect of the inertial force and directs its flight toward the side of its familiar geographical latitude.

The grids of curves given in the work show that, in the general case, the magnetic latitude and the geographical latitude intersect only at two “conjugate” points. For the most part the conjugate points are situated very far from one another. Thus, usually, in the region of many hundreds of miles from home, the direction of flight will be unambiguous. The matter should be otherwise if the bird is released not near its home but near a conjugate point having the same characteristic features as the home. In this case the pigeon should head just as surely toward a completely unfamiliar place as toward its home. In the region of the city of Kearney (USA) there is a conjugate point of the city of Pennsylvania. One of the most convincing proofs of the validity of the proposed hypotheses was precisely such a test, conducted near these cities, which we shall describe below.

The first experiment, set up to test the theory, consisted in checking the first part of the hypothesis—the magnetic effect. The theory assumes that the influence of the magnetic field appears as an induction effect (correlation with the speed of flight). It is not difficult to estimate the order of magnitude of this effect. At a flight speed of 18 m/sec and an intensity of the vertical component of the field equal to 0.6 oersted, the electromotive force of induction is equal to 10 microvolts. However, what is essential is not this value but its change during flight. In different places on the terrestrial globe the field changes differently. In the region where the experiments were carried out, this change is equal to 0.007 oersted per degree. Hence it is calculated that the change in electromotive force of induction per 1 degree (70 miles of flight) should be 0.13 microvolt.

The experiment to test the possibility of the bird’s sensitivity to so small an effect was set up as follows. Permanent magnets were tied to the bird’s wings. Their magnetic flux and the distance between them were selected so that the induction effect arising during the motion of the wings would have the same order of magnitude. With the horizontal arrangement of the wings, these two magnets produced in the plane between them the exact field equal to 0.17 oersted (which, incidentally, is equal to the magnitude of the horizontal component of the magnetic field). Assuming that the pigeon makes 180 wing beats per minute, and estimating the amplitude of the sweep at one-eighth of a foot, we obtain a value of the electromotive force equal to 0.12 microvolt, and the current, as in the case arising from the bird’s movement, will be directed perpendicular to the line of flight. It is obvious that, if the point of view expressed above is correct, the alternating induction effect arising from the work of the wings (from plus 0.12 to minus 0.12 microvolt), having the same order of magnitude as the orienting effect, should make navigation impossible.

The stated assumptions were confirmed by comparing the behavior of pigeons released at some distance from home: of the first group, to which magnets had been tied under the wings, and of the second group, to which

copper plates of the same weight (to create equality of conditions) were tied under the wings. The experiment showed that all the birds of the first batch did not return home, while all the birds of the second batch returned home in the first two days after release (this is considered normal). Only one bird, under whose wings magnets had been tied, returned home, but even then on the fourth day—therefore after a long search. Also very interesting were observations of the direction of flight of the newly released birds. In none of the birds kept under normal conditions did the line of initial flight deviate from the homeward line by more than 50 degrees. Among the birds equipped with magnets, fewer than half flew in directions differing from the correct one by 45–90 degrees, while the greater half flew in the direction opposite to home.

Over the course of three years, under the author’s direction, experiments were carried out to test his hypothesis regarding the existence of “conjugate” points. In all these experiments (the author has taken into account data from approximately 500 bird flights), the sensitivity of the pigeon to the network of curves of the magnetic field—the field of Coriolis forces—was investigated. The experiments may be divided into two groups. In the first series of experiments, birds trained with respect to a certain home were brought to a place close to the conjugate point of this home, and the results of the birds’ flight under these conditions were determined. In the second series of experiments, the result of the birds’ flight was investigated near a certain region in which the line of the constant vertical magnetic field coincides with the latitudinal line. In this case, obviously, the birds should lose their orientation, since the fields acting on the birds do not determine a point in space unambiguously. It must be borne in mind that a series of random flights complicates the result of the experiments. Naturally, one may suppose that a number of pigeons arriving at the conjugate point do not recognize their home and fly off in an arbitrary direction. It should be emphasized that the pigeons were trained in a wooded locality (their true home), while the flights to the conjugate point took place in a mountainous locality of an entirely different landscape. Despite all these circumstances, the results of the experiments should be considered very convincing.

The experimental data are processed in the following way: the flight of each bird is characterized by a vector of definite length and direction (the vector connects the release point with the place where the flight ended). For each series of experiments, the resultant vector is determined, whose direction and length (divided by the number of flights included in the sum) are regarded as the principal characteristic of the experiment. Studies of this type have shown that, in different series of experiments, the resultant flight vector deviates by only a few degrees from the desired direction, i.e., from the direction leading to the conjugate point. For very well-trained birds, one series of experiments gave a deviation of the resultant vector by an amount less than one degree.

The author believes that the hypotheses he has advanced may be considered confirmed by these experiments. It is evident that biologists should search for other phenomena in which motion in the Earth’s magnetic field, as well as Coriolis acceleration, exert an influence on the nervous system.

A. K.