Popular Science Monthly/Volume 30/April 1887/Bird-Migration
| BIRD-MIGRATION. |
By BARTON W. EVERMANN.
NINE hundred and forty-one species and sub-species of birds are now recognized by ornithologists as belonging to the avi-fauna of North America. Eighty-two of these may be regarded as stragglers from other countries, and their occurrence in North America as purely accidental. Of these eighty-two species, about twenty-two have been found in Greenland, fourteen in Alaska, fourteen in Florida, thirteen in Texas, and the remaining ones, about a score, in various other parts of the United States—only one or two in anyone place, however. We thus have left about eight hundred and fifty-nine species, of which this continent may properly be called the habitat.
About two hundred of these have been identified as birds of the county[1] in which the writer lives. Twenty-six species of these two hundred are permanently resident here—that is, they rear their young here, and they or other individuals of their species remain with us throughout the year. Fourteen other species visit us from the North, and only in the winter. Besides the twenty-six permanent residents, about seventy-five other species breed within our borders, while the remaining eighty-five species are seen here only for a few days in spring, and again for a short time in the fall. Twice a year they flit by us like an apparition, and we ask: Whence come these birds in spring; where do they spend the summer; and whither do they go when winter comes? Since life of bird and man began, these questions have been asked and studied, but not always have the results been satisfactory.
It had long been observed that, as the sun comes to our side of the equator, the buds swell, the leaves and flowers unfold, and a sheet of verdure expands and spreads toward the pole; and closely following this there comes a mighty army of bird-life—orioles, tanagers, and warblers of brightest hue—filling every orchard, grove, and woodland with life, and song, and joy. And again, when autumn comes, the wave returns to us from the North, bringing with it the black snow-birds and other species which make their winter home with us, while the great bulk of the species pass on to the southward as mysteriously as they came.
The flight of storks has given trouble to the Germans and the Chinese, while the disappearance and the reappearance of the swallows have caused untold trouble everywhere. Learned bodies, like the French Academy and the Royal Society of London, have gravely asserted that, in the fall, swallows plunge into the mud of marshes and mill-ponds, become torpid, and hibernate like frogs and snakes. I have seen a list of nearly two hundred articles written all along from the middle of the seventeenth century down to 1877, for the purpose of proving or disproving the hibernation of swallows and other birds! And Dr. Coues says he can lay his hand upon papers of that period which discuss the migration of swallows to the moon, the falling of the little quadrupeds called lemmings in showers from the clouds, and the origin of brant-geese from barnacles that grew on trees. Indeed, not a year ago I was assured by a gentleman of more than average intelligence that this last is undoubtedly the correct theory as to the origin of the barnacle-goose! And it was not a decade ago that I read, in one of the leading newspapers of this State, an article of as curious a character. Its purpose was to explain the sudden appearance in fall of the black snow-birds, and their as sudden disappearance in spring, and the explanation given was that our common sparrows change color in fall, becoming snow-birds, which they remain until spring, when they put on their other dress and become sparrows again! And I find that, among the common people of the country, there are many who have this belief.
We have long known in a general way that the birds go southward to winter, and return to spend the summer at the North. But just where in the South do they go? Why do they go there? By what routes do they travel? At what rate of speed? Do they travel by night, or day, or both? What species migrate first, which last, and why? How are they guided in their course? What is the winter as well as the summer habitat of each particular species, when does it get there, and when does it leave the one for the other? In what way and to what extent are their movements dependent upon or influenced by vegetable and meteorological phenomena?
These are but a few of the questions which rnithologists have sought in various ways to answer. The limits of this paper will permit any discussion of but few of these interesting questions. From their very nature it is evident that futile must prove the labors of him who would attempt to solve these problems alone. Only through the concerted action and labors of many observers in different and widely separated regions can any reliable conclusions be reached. Not until recently has the subject of bird-migration been studied systematically in this country or elsewhere.
Three years ago a gentleman[2] in the Mississippi Valley addressed personal letters to several hundred naturalists, teachers, ministers, farmers, and others in the different Valley States, asking them to cooperate with him in studying the movements of the birds of this region. They were requested to note only the more common phenomena of migration, such as the time of arrival and departure of each species, the time of breeding, and the comparative abundance of the various species. They were asked to record such observations as they could, and send in reports to him of what they had seen. More than one hundred observers were thus secured, who reported to him the results of their observations, and this, if I mistake not, was the first corps of migration observers in America.
A little later, in September, 1883, the American Ornithologists' Union was organized. Among the committees appointed at its first meeting was one on "The Migration and Geographical Distribution of North American Birds." Dr. C. Hart Merriam, of Locust Grove, New York, was made chairman of this committee. It has been and is the purpose of this committee "to investigate in all its bearings, and to the fullest extent possible, the subject of the migration of birds in the United States and British North America, Its work is not limited to the accumulation of records of the time of arrival and departure of the different species, but it embraces as well the collection of all data that may aid in determining the causes which influence the progress of migration from season to season." For example, severe storms, gales of wind, long periods of unusally high or low temperature, are some of the atmospheric conditions which are known to exert marked effects upon the movements of birds.
In order to secure as many observers as possible, and that the material collected by this great array of observers be speedily elaborated, the United States was divided into a dozen or more districts, each of which was placed in charge of a competent superintendent. The superintendent's duties are to secure as many observers in his district as he can, to give them all needed instructions concerning the work, to act as a means of communication between the observers and the chairman of the committee, to collect, at stated times, the results of their observations, and submit them to the committee. The chairman will arrange, condense, and systematize these reports, and present to the Union the fruits of the joint labors of this great corps of observers, together with such deductions or generalizations as he may be able to base upon them.
By noting the species which are permanent residents, which winter residents or visitants, which summer residents, and which spring and fall migrants, together with the relative abundance of each species in each locality where found, the geographical distribution of our birds has already been pretty well made out.
A like series of observations upon the time of arrival and departure of each species, the manner of its coming and going, the period of its stay, the place and time of its nesting, enables us to learn much of its life-history. And a study of meteorological phenomena, such as the direction of the wind, the temperature from day to day, the occurrence of storms, together with data regarding contemporaneous phenomena, such as the appearance in spring of the first frog, toad, or snake; the end of the period of hibernation of certain mammals; the leafing of different trees and the flowering of various plants; and the breaking up of the ice in the rivers and lakes—all these throw much light upon the causes wbich induce, and the conditions which influence or control, migration.
More than a thousand observers are now at work gathering data for the solution of these problems. Never before have so many persons worked together and systematically for the solution of any great question in science. Observers are at work in every State and Territory in the Union, besides a number in the West Indies, Mexico, and Canada. The interest of the lighthouse-keepers has been enlisted, and many of them are doing excellent service.
The most southern station in the United States is at Sombrero Key, at the southern end of Florida, while the most northern is at Point Barrow, Alaska, more than four thousand miles away. From New Brunswick and Maine on the east, the stations extend across the continent to California, Oregon, and Vancouver's Island. And from the time a bird crosses our southern border in early spring-time until it reaches its breeding-ground, wherever that may be, it is under the careful surveillance of these inquiring spies. Its every movement is watched and recorded, and by the time it has reached its summer home, reared its brood, and returned again to its winter resort, few, indeed, are the important facts in its life-history whicb have not been made a subject of note by one or another of these observers. Thus the records are made from year to year, and even now hundreds of note-books all over our country contain thousands of entries to the credit or debit of our birds for the last four years. All this, and the Union has just begun its work!
As an illustration, let me ask your attention to the record of a beautiful and familiar bird—the Baltimore Oriole. After spending the winter within the tropics, it returns to our Southern borders in early April. The record for 1884 shows that it appeared at Rodney, Mississippi, April 7th; at Oxford, April 15th; at Anna, Illinois, April 18th; at St. Louis, April 19th; at Glasgow, Missouri, April 23d; Jacksonville, Illinois, April 27th; Liter, Illinois, and Coralville, Iowa, May 2d; Des Moines, May 4th; Racine and Jefferson, Wisconsin, May 6th; Pine Bend, Minnesota, May 13th; Elk River, May 14th; and Oak Point, Manitoba, May 25th. East of the Mississippi, the record shows that it appeared in Jessamine County, Kentucky, April 18th; at Bloomington, Indiana, April 21st; at Camden, April 24th; Petersburg, Michigan, April 30th; Battle Creek, May 1st; Sing Sing, New York, May 2d; Lockport, May 4th; Painted Rock, May 5th; Locust Grove and Auburn, May 6th; Watertown, May 11th; Lake George, May 13th; Brewer, Maine, May 16th; and Montreal, Canada, May 24th.
The average rate of speed from Rodney to St. Louis was twenty-five miles per day, while the average daily rate for the entire distance—Rodney to Oak Point—was twenty-seven miles. The species seemed to move in greatest numbers about April 29th and 30th, filling up the whole country already reached by the vanguard.
The various reports show that the rate was remarkably uniform throughout the thirteen hundred miles, and that, though it increased very slightly to the northward, it nowhere varied greatly from the average rate—twenty-seven miles per day.
This bird is also very regular as to the time of its arrival at any given place from year to year. For the last four years the first arrivals have been seen here (Bloomington, Indiana) on April 20th, 21st, 20th and 21st. At Camden, one hundred miles north of this place, April 28th, 24th, 21st, 24th, and 25th, are the dates for the last five years. At Locke, Michigan, the record for twenty-five years (from 1856 to 1880), gives April 28th as the earliest date, and May 11th as the latest. The average date for the twenty-five years was May 5th, and it is interesting to know that this average date, May 5th, is the date upon which the first arrivals were seen for six of the twenty-five years. But the oriole is a late migrant, and therefore not so greatly influenced in its movements by the weather as are many other species. The late migrants, those which do not begin their northward flight until the weather has become in a measure settled, have been found to be the most regular in their movements. The early migrants are the ones which advance or retreat as the weather favors or prevents. The swallows and martins are excellent examples of this class, and their wide range has enabled them to be studied more, perhaps, than any other birds. As Dr. Coues has said, they are thoroughly cosmopolitan; their northward range reaches into the Arctic zone, and in the South the explorer has never gone so far that he did not find the swallows there. Insectivorous as to food, they must of necessity move with the appearance of insect-life, while their recession from the North is urged as well by their delicacy of organization and consequent susceptibility to cold, as by a failure in the food-supply. The prowess of their pinion has been the astonishment and admiration of all. The comings of the swallows have passed into proverb, and their leave-takings have been rehearsed in folk-lore among the signs of waning times. They have figured in augury; their flight is barometric, for they soar on clear, warm days, and skim the surface of the ground in heavy, falling weather, perhaps neither always nor entirely in the wake of insects upon which they feed. These birds cross our Southern border when the weather is yet cold and changeable. The record for the purple martin for 1884 shows that the first four degrees of latitude were passed at a rate of sixteen miles per day; the next two and one-half degrees at twelve miles; the next four and one-half degrees at sixty-three miles; and the last two and one-half degrees at but ten miles per day—making an average for the entire distance of eighteen miles per day. This record shows us a species very irregular in its rate of speed, and it is easily shown that this irregularity is due to the vicissitudes of the weather.
From observations such as these much has been learned regarding the rate at which various other species migrate. Data on fifty-eight species for 1883, for four hundred and twenty miles, show the average rate to have been twenty-three miles per day. Data on not quite so many species for 1884 show the average rate, for eight hundred and sixty-one miles, to have been exactly the same as for 1883. Twenty-five species gave an average daily rate of nineteen miles for March, twenty-three miles for April, and twenty-six miles for May, thus indicating—what I believe to be true—that the speed at which most species migrate increases toward the northern limit. This was one of the first important facts in migration observed and pointed out by Professor W. W. Cooke, and subsequent observations have all tended to prove the correctness of his views.
Were migration a steady movement, with the same individuals always in the lead, we might determine the exact rate of speed for many different species, but the movement resembles rather a game of leap-frog, and the leaders are constantly changing. Those individuals which arrive first at any given place are the birds of that species which will remain there to breed, while those in the rear pass on farther north. "The vanguard is thus constantly arresting itself, and the forward movement must await the arrival of a new corps, which may be near at hand or far behind. Migration is, then, a series of overlappings, and the real is evidently much greater than the apparent speed."
It has also been noticed that, as a rule, any given species migrates earlier up the Atlantic seaboard or the Mississippi Valley than it does across the more arid plains to the west; the first arrivals appear here from four to seven days earlier than in Kansas directly west of us. The cause, no doubt, lies in the difference in the character of the vegetation.
One of the most difficult, as well as most interesting, questions in bird-migration has been, How are birds guided in their flight? By instinct, has been the usual answer; but, thanks to the labors of such men as Cooke, Allen, Brewster, and Scott, the question is now better understood. Recent observations made at lighthouses and astronomical observatories go to show that many, if not most, of our smaller birds fly at very great heights while migrating—heights even as great as one to two miles. And we now well know that "there are certain definite routes or paths along which birds pass in especially great numbers. These are usually coast-lines, river-valleys, or continuous mountain-ranges. Toward these converge innumerable less-frequented paths, each of which in turn has still smaller tributaries of its own. Thus bird-streams, like brooks, flow into common channels, and each particular region may be said to have its bird- as well as water-shed."
Perhaps the greater part of our birds migrate almost exclusively by night, and it seems true that clear, pleasant nights are selected during which to perform their migrations. That most species are unable to foretell the weather, even for a few hours, seems to be true; for during the migrations, if the early part of the night be clear, and a storm or severe rain come up later in the night, the birds will be stopped in their flight. On mornings after such nights I have always found more birds than at any other time. If the rain brought with it any considerable lowering of temperature, the woods and groves would be full of arrested migrants during the next day. These birds had evidently started when the sky was clear and the weather favorable.
Keeping these various facts in mind, it becomes quite easy to see how birds are guided in their course. From the great height at which they fly the whole country appears as a map upon which, in the light of moon and stars, the coast-lines, river systems, and mountain-ranges are outlined in every direction for many miles. "Guided by such landmarks as these, the older birds can have little difficulty in following paths they have repeatedly traversed before, and these direct and lead the flight of the younger birds." Mr. William Brewster, in his recent excellent paper on this subject,[3] and from which I have freely quoted, shows that, while the birds often migrate in waves or flocks, the different flocks do not move independently of each other. He believes that the flocks do not fly in close order, but scatter so as to approach or mingle with the stragglers or advance guard of other flocks, "thus in effect forming a continuous but straggling army, often hundreds of miles in length, and varying in breadth with the character of the country over which it is passing."
But when all is said, a great part of the details concerning the facts upon which these conclusions are based, remains to be worked out. Much remains to be done before we can fully understand the forces which impel and enable the bird creation to perform those long and perilous journeys across the depths of air and tracts of ocean, to seek for warmth and food in distant lands, and to return in season to their winter or summer homes.
