Popular Science Monthly/Volume 45/August 1894/The Nocturnal Migration of Birds
By FRANK M. CHAPMAN.
NO branch of ornithology offers more attractions to the student of birds than the fascinating subject of migration. Birds come and go; absent to-day, to-morrow they greet us from every tree and hedgerow. Their departure and arrival are governed by as yet unknown laws; their journeys through the pathless sky are directed by an instinct or reason which enables them to travel thousands of miles to a winter home, and in the spring to return to the nest of the preceding year.
Volumes have been written to explain their mysterious appearances and disappearances.
Theories almost as numerous as the essays themselves have been advanced to account for the phenomena of migration. From the time of Jeremiah (viii, 7) to the present day we might cite a host of authors who have contributed to the literature of the subject. It is not our intention, however, to review the whole question of migration. The combined researches of ornithologists have placed it among the sciences, and its more prominent facts are common knowledge. We desire here to call attention to but one phase of the study, and more especially to outline some recent investigations in connection with the nocturnal migrations of birds.
From the nature of the case, our data concerning these night flights have long been meager and unsatisfactory. Even now our information has but reached a stage which permits us to intelligently direct further effort.
We know that the land birds which migrate by night include species of more or less retiring disposition, whose comparatively limited powers of flight would render them easy victims for birds of prey if they ventured far from the protection of their natural haunts during the day. Thus we find that the bush-or tree-loving thrushes, wrens, warblers, and vireos all choose the night as the most advantageous time in which to make their long semiannual pilgrimage, while such bold rovers as swallows, swifts, and hawks migrate exclusively by day.
The information we possess concerning the manner in which the first-mentioned class of birds accomplish a journey which leads them from boreal regions to the tropics, has been derived from three sources: First, through the birds which are killed by striking lighthouses or electric-light towers; second, through observations made at night from similar structures; and, third, through the use of the telescope.
It has long been known that lighthouses are most destructive to night-migrating birds. Probably no one artificial cause produces more disastrous results than these beacons which guide the mariner in safety, but prove fatal obstacles in the path of aerial voyagers.
The number of birds killed by striking lighthouses is incalculable. Over fifteen hundred have been found dead at the foot of the Bartholdi Statue in a single morning; while from Fire Island (Long Island) light we have a record of two hundred and thirty birds of one species—black-poll warblers—which met their fate on the night of September 30, 1883.
Reports from numerous lighthouses show (1) a great variation in avian mortality at different localities; (3) that as a rule no birds are killed during clear nights; and (3) that comparatively few birds strike the lights during the vernal migration. The fact that birds follow certain routes or highways of migration in their journeys to and from the South doubtless explains their absence or presence at a given locality; indeed, it has been definitely ascertained that lights which are situated in known lines of migration—as, for example, the Bartholdi Statue at the mouth of the Hudson River Valley—prove far more destructive than those which are placed far from the regular routes of migrating birds.
Through-telescopic observations, to be mentioned later, we have learned that when en route birds travel at an altitude of from one to three miles above the earth. It is obvious, then, that when their way is not obscured by low-hanging clouds they pass too far above us to be attracted by terrestrial objects. It has been noted that cloudy and especially rainy nights are most disastrous to migrants, evidently because the formation of moisture at the elevation at which they are flying must not only interfere with their progress, but in veiling the earth below robs them of their landmarks, while the condensation of this moisture into rain presents an effectual check to flight. The birds then descend to a lower altitude, and, should the storm be very severe, they are obliged to seek the nearest shelter, and even may be driven to earth wet, helpless, and dying.
The influence thus shown to be exerted by meteorological conditions is the best explanation of the comparatively small number of birds killed during the spring migration, when the inf requency of violent storms enables them to perform their journey with less danger from exposure to the elements.
The observations of Mr. William Brewster on the migration of birds at the Point Lepreaux (Bay of Fundy) lighthouse have never been exceeded in interest or value by the recorded experiences of any other observer of similar phenomena. Still, even his graphic account fails to produce the sensations which possess one when for the first time the air at night is actually seen to be filled with the tiny songsters which before were known only as timid haunters of woods and thickets.
On September 26, 1891, it was the writer's good fortune to pass the night with several ornithologists at the Bartholdi Statue in observing the nocturnal flight of birds. The weather was most favorable for our purpose. From the balcony at the base of the statue we saw the first bird enter the rays of light thrown out by the torch one hundred and fifty feet above us at eight o'clock. During the two succeeding hours birds were constantly heard and many were seen. At ten o'clock a light rain began to fall and for three hours it rained intermittently. Almost simultaneously there occurred a marked increase in the number of birds seen about the light, and within a few minutes there were hundreds where before there was one, while the air was filled with the calls and chirps of the passing host.
The birds presented a singular appearance. As they entered the limits of the divergent rays of light they became slightly luminous, but as their rapid wing-beats brought them into the glare of the torch they reflected the full splendor of the light, and resembled enormous fireflies or swarms of huge golden bees.
At eleven o'clock we climbed to the torch and continued our observations from the balcony by which it is encircled. The scene was impressive beyond description; we seemed to have torn aside the veil which shrouds the mysteries of the night, and in the searching light reposed the secrets of Nature. As the tiny feathered wanderers emerged from the surrounding blackness, appeared for a moment in the brilliant halo about us, and continuing their journey were swallowed up in the gloom beyond, one marveled at the power which guided them thousands of miles through the trackless heavens. While by far the larger number hurried onward without pausing to inspect this strange apparition, others hovered before us like humming birds before a flower, then wheeling retreated for a short distance and returned to repeat the performance or pass us as did the first class mentioned, while others still, and the number was comparatively insignificant, struck some part of the torch either slightly or with sufficient force to cause them to fall stunned or dying. It was evidently by the merest accident that they struck at all; and so far as we could judge they were either dazzled by the rays of the light and thus unwittingly flew directly at the glass which protects it, or came in contact with some unilluminated part of the statue. During the two hours we were in the torch thousands of birds passed within sight, but less than twenty were killed.
This fact, in connection with the comparative or entire absence of birds on clear nights, very plainly shows that conclusions based solely on these casualties may be not only misleading but erroneous. In other words, the number of birds which strike a light is a poor index to the number which have flown by or above it in safety.
Throughout the evening there was a more or less regular fluctuation in the number of birds present; periods of abundance were followed by periods of scarcity, and the birds passed in well-defined flights, or loose companies, probably composed in the main of individuals which had started together.
The birds chirped and called incessantly. Frequently, when few could be seen, hundreds were heard passing in the darkness; the air was filled with the lisping notes of warblers and the mellow whistle of thrushes, and at no time during the night was there perfect silence. At daybreak a few stragglers were still winging their way southward, but before the sun rose the flights had ceased. The only birds identified were several species of warblers and thrushes, one red-eyed vireo, two golden-winged woodpeckers, one catbird, one whip-poor-will, and one bobolink. The most interesting and important results of the night's observations were, the immediate effect of rainfall in forcing birds to migrate at a lower level, the infrequency with which they struck the torch, the immense number which passed beyond its rays, and the constancy with which they called and chirped as they flew.
An almost virgin field awaits the investigator who will systematically observe night-migrating birds with the aid of a telescope. Messrs. Allen and Scott, at Princeton, and the writer, assisted by Mr. John Tatlock, Jr., at Tenafly, New Jersey, and at the Columbia College Observatory, have alone recorded the results of observations of this nature. Their labors, however, were too brief to more than show the possibilities which await more extended effort.
A comparatively low-power glass is focused upon the moon, the birds appearingupon its glowing surface as they cross the line of vision. Some idea of the multitude of feathered forms which people the upper regions of the air at night may be formed when it is stated that during three hours' observation at Tenafly no less than two hundred and sixty-four birds were seen crossing the restricted field included in the angle subtended by the full moon. Under proper focal conditions, birds were so plainly visible that in many instances marked character of flight or form rendered it possible to recognize the species. Thus ducks, snipe, and sora rail were distinguished with certainty.
The effect on the observer of this seeing of things unseen is not a little curious, and maybe likened to the startling disclosures which a high-power microscope presents in a drop of water.
From calculations based on an assumption that birds were not visible beyond a distance of five miles, we determined the greatest altitude at which birds migrate to be three miles above the earth's surface. Many, however, fly at a lower level; indeed, it is not improbable that certain species may, with more or less regularity, travel at a given altitude, and that this altitude may vary among birds of different families. With little doubt thrushes and warblers travel at a much lower level than do ducks and geese, a circumstance which may account for the great abundance of the first two named and the comparative absence of the last in the vicinity of lighthouses.
Such, in brief, are the sources and methods to which we owe our knowledge of the nocturnal flight of birds. It will be evident to the most casual reader how incomplete are our data. The time is still far distant when we can hope to conclusively account for the many perplexing phenomena of migration, but we may be pardoned if, in conclusion, we briefly review the bearing of our present information.
We need not discuss here the origin of migration or the causes which now induce birds to undertake a long and perilous journey twice each year. Bat the power and influences which guide a bird, in the darkness of the night, through space, and render a definite migration possible, are subjects kindred to our inquiry and worthy our attention.
Until we possess some exact knowledge of the distance to which birds can see we can not estimate the aid their vision is to them while migrating. We know, however, that the avian eye is far more powerful than ours, and it is fair to assume that to some extent their journeys are directed by a sight which enables them to follow mountain chains, river valleys, and coast lines, and to distinguish distant headlands or islands. At an altitude of two miles an object would be visible ninety miles and the horizon be separated by twice this distance. At no time, therefore, in their journey from North to South America are birds necessarily out of sight of land. But that they do venture beyond a point where land is visible is shown by the regular appearance of migrants in the Bermudas, six hundred miles from our coast, while Jamaica, four hundred miles north of the nearest point of South America, is a point of departure for many south-bound migrants. Here, with neither islet, shoal, nor reef to mark the way, it is evident that sight alone would prove an insufficient guide, and they must rely on some other sense. Primarily, this is the inherited habit which prompts birds to fly southward in the fall and to return in the spring. But, given the impulse of direction, there is little doubt that one of the best guides to night-flying birds is the sense of hearing. Birds' ears are exceedingly acute. They readily detect sounds which to us are inaudible. Almost invariably they will respond to an imitation of their notes. We have seen that when under way they constantly chirp and call, and when we take into consideration their aural power and their abundance in highways of migration, it is probable that at no time during the night is a bird out of hearing of its fellow-travelers. The line of flight once established, therefore, presumably by the older and more experienced birds, it becomes a comparatively easy matter for the novice to join the throng.