MIGRATION of BIRDS
When the birds that have nested in our dooryards and those that have frequented the neighboring woods, hills, and marshes leave us in the fall, the question naturally comes to mind: Where do they go? This, however, is only one small part of the question as we also wonder: Will the same ones return next spring to their former haunts? What dangers will they face on their round-trip flight and while in their winter homes? These and other questions on the migratory habits of most species of Northern Hemisphere birds puzzle all who are interested in them, whether it be the farmer who profits by their tireless warfare against the weed and insect pests of his crops, the bird student who enjoys an abundance and variety of feathered inhabitants about him, or the hunter who wants a continuation from year to year of the sport of wildfowling. Lack of information on the subject may mean the loss of an important resource by unconsciously letting it slip from us, as ignorance might be responsible for inadequate legal protection for such species as might urgently need it. More general knowledge on the subject will aid in the perpetuation of the various migrants, the seasonal habitats of some of which are in grave danger from man's utilization, sometimes unwisely, of the marsh, water and other areas that were formerly homes for birds.
The migrations of birds were probably among the first natural phenomena to attract the attention and intrigue the imagination of man. Recorded observations on the subject date back nearly 3,000 years, to the times of Hesiod, Homer, Herodotus, Aristotle, and others. In the Bible there are several references to the periodic movements of birds, as in the Book of Job (39:26), where the inquiry is made: "Doth the hawk fly by Thy wisdom and stretch her wings toward the south?" Jeremiah (8:7), wrote: "The stork in the heavens knoweth her appointed time; and the turtle [dove], and the crane, and the swallow, observe the time of their coming." And the flight of quail that saved the Israelites from starvation in their wanderings in the wilderness of Sinai is now recognized as a vast movement of migratory quail between their breeding grounds and their winter home in Africa.
Throughout the ages the return flights of migratory birds have been important as a source of food after a lean winter and as the harbinger of a change in season. The arrival of certain species has been heralded with appropriate ceremonies in many lands, and among the Eskimos and other tribes the phenomenon to this day is the accepted sign of the imminence of spring and of warmer weather. The pioneer fur traders in Alaska and Canada offered rewards to the Indian or Eskimo who saw the first goose of the spring, and all joined in jubilant welcome to the newcomer.
As the North American Continent became more thickly settled, the large flocks of ducks and geese that always had been hunted for food became objects of the enthusiastic attention of an increasing army of sportsmen. Most of the nongame species were found to be valuable also as allies of the farmer in his never-ending warfare against weed and insect pests. The need for laws protecting the valuable game and nongame birds and for regulating the hunting of the diminishing game species followed as a natural course. In the management of this wildlife resource it has become obvious that continuous studies must be made of the food habits of the various species, their environmental needs, and their travels. Hence bird investigations are made by the Fish and Wildlife Service, the bureau charged by Congress under the Migratory Bird Treaty Act with the duty of protecting those species that in their yearly journeys pass back and forth between the United States and Canada, and between the United States and Mexico.
For more than half a century the Fish and Wildlife Service and its predecessor, the Biological Survey, have been collecting data on the interesting and important phenomenon of the migration of North American birds. The field men of the Service have gathered information concerning the distribution and seasonal movements of the different species in many extended areas, from the Arctic coast south to the pampas of Argentina. Supplementing these investigations is the work of hundreds of volunteer ornithologists and bird students throughout the United States and Canada, who each year, spring and fall, forward to the Service reports on the migrations as observed in their respective localities. Added to the mass of data thus assembled is a rapidly growing recovery file of marked individuals. These data, together with other carded records gleaned by the Fish and Wildlife Service from a vast literature, constitute a series of files that now contain well over 3,000,000 entries, easily the greatest existing accumulation of information pertaining to the distribution and movements of North American birds. Not only do the facts thus assembled form the basis of regulatory action for the protection of the birds, but they also make it possible to publish scientific accounts of the ranges and migrations of the different species. They furnish the basis of this bulletin.
The several important bird-protective measures adopted by State and Federal Governments, particularly those having as their objectives the conservation of the migratory song, insectivorous, and game species, can be effective only if they have intelligent public support. To increase such support, information must be more generally available on that little understood but universally fascinating subject of bird migration. A brief presentation of facts on the migratory habits of the birds scientifically gathered by the Fish and Wildlife Service over many years, will be helpful to bird-study classes, to conservation organizations, and to farmers and others individually interested in the welfare of the birds.
In addition to his original investigations in the field and in the files of the Fish and Wildlife Service, the author has made free use of the writings of many other students of the subject. To all of these grateful acknowledgment is made.
The Mystery of Migration
Of observers whose writings are extant, Aristotle, naturalist and philosopher of ancient Greece, was one of the first to discuss the subject of bird migration. He noted that cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile, and that pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. In the earliest years of the Christian era, the elder Pliny, Roman naturalist, in his Historia Naturalis, repeated much of what Aristotle had said on migration and added comments of his own concerning the movements of the European blackbird, the starling, and the thrushes.
In spite of the keen perception shown in some of his statements Aristotle also must be credited with the origin of some superstitious beliefs that persisted for several centuries. One of these, that of hibernation, became so firmly rooted that Dr. Elliott Coues (1878),[1] one of America's greatest ornithologists, listed the titles of no less than 182 papers dealing with the hibernation of swallows. The hibernation theory accounted for the autumnal disappearance of certain species of birds by having them pass into a torpid state and so remain during the cold season, hidden in hollow trees, caves, or in the mud of marshes. Aristotle ascribed hibernation not only to swallows, but also to storks, kites, doves, and others. Some early naturalists wrote fantastic accounts of the flocks of swallows that allegedly were seen congregating in the marshes until their accumulated weight bent into the water the reeds on which they clung and thus submerged the birds. It was even recorded that when fishermen in northern waters drew up their nets they sometimes had a mixed "catch" of fish and hibernating swallows. Clarke (1912) quotes Olaus Magnus, Archbishop of Upsala, who in 1555 published a work entitled "Historia de Gentibus Septentrionalis et Natura," wherein he observed that if swallows so caught were taken into a warm room they would soon begin to fly about but would live only a short time.
[1] Publications referred to parenthetically by date are listed in the [Bibliography, p. 94].
The hibernation theory survived for more than 2,000 years and, until the winter home of the chimney swift was discovered in 1944 through the recovery of banded individuals, it was occasionally repeated by credulous persons to account for the sudden disappearance of the immense flocks that each autumn gather in southern Georgia and northern Florida. Although the winter range is still unknown in fullest detail, Lincoln (1944b) has shown that some of these birds spend the winter season in northeastern Peru.
Although the idea that hibernation is a regular feature of the life cycle of birds is no longer accepted for any species, recognition must be accorded the observations of Edmund C. Jaeger of Riverside College, Riverside, Calif. (1949). Earlier (1948), he had given a brief account of the behavior of a poor-will found during the winter of 1946-47 in the Chukawalla Mountains of the Colorado Desert, Calif., and which was in a state of profound torpidity.
What was presumably the same individual was found in the same rock niche in a comatose condition on November 26, 1947. Beginning on December 30, 1947, rectal temperatures were taken every 2 weeks, the last on February 14, 1948. The temperature dropped from 67.6° on the first date to 64.4° on January 18 and February 1, recovering to 65.8° on the late date of record. The weight decreased from 45.61 grams on January 4 to 44.56 grams on February 14. An attempt to detect heart beat by the use of a medical stethoscope was negative. No movement of the chest walls could be detected and no moisture could be collected on a cold mirror placed in front of the nostrils. Strong light aimed directly into the pupil resulted in no response, not even an attempt to close the eyelid. No waste matter was passed during the entire period of observation and all evidence indicated that the bird was in an exceedingly low state of metabolism.
This bird was banded on January 5, 1948, with a Service band and was back in the same rock niche on November 24, 1948, certainly the second and probably the third season of return to this exact point. It was there on December 5, 1948, but 2 weeks later it had disappeared, probably the victim of some predator or an inquisitive human. Professor Jaeger reports that the Hopi Indians call the poor-will "Holchko," the sleeping one.
Aristotle also was the originator of the theory of transmutation, basing it upon the fact that frequently one species will arrive from the north just as another species departs for more southerly latitudes. From this he reasoned that although it was commonly believed that such birds were of two different species, there really was only one, and that this one assumed the different plumages to correspond with the summer and winter seasons.
Probably the most remarkable theory that has been advanced to account for migration is contained in a pamphlet mentioned by Clarke (1912: V. I, 9-11) as published in 1703 under the title: "An Essay Toward the Probable Solution of this Question: Whence come the Stork and the Turtle, the Crane, and the Swallow, when they Know and Observe the Appointed Time of their Coming." It was written "By a Person of Learning and Piety," whose "probable solution" was that migratory birds flew to the moon and there spent the winter.
Some peoples, who easily accepted the migratory travels of the larger birds, were unable to understand how the smaller species, some of them notoriously poor fliers, could make similar journeys. They accordingly conceived the idea that the larger species, as the storks and cranes, carried their smaller companions as living freight. In some of the Mediterranean countries, it is still believed that these broad-pinioned birds serve as aerial transports for the hosts of small birds that congregate upon the shores awaiting opportunity for this kind of passage to their winter homes in Africa. Similar beliefs have been found among some tribes of North American Indians.
Advantages of Migration
Before presenting some of the present theories concerning the origin of bird migration, it seems well to consider briefly the ends that are served by this annual round trip between breeding grounds and winter quarters. It is apparent that the migratory habit enables a species to enjoy the summers of northern latitudes while avoiding the severity of the winters. In other words, migration makes it possible for some species to inhabit two different areas during the seasons when each presents favorable conditions. In the performance of its reproductive duties, every pair of birds requires a certain amount of territory, the extent of which varies greatly in different species. Generally, however, this territory must be large enough to provide adequate food, not only for the parent birds, but also for the lusty appetites that come into being with the hatching of the eggs. Thus, if all birds were to remain constantly either in tropical or in temperate regions, there would be intolerable overcrowding during the breeding season. By the spring withdrawal to regions uninhabitable earlier in the year, the migrants are assured of adequate space and ample food upon their arrival in the winter-freed North, and it may be assumed that the nonmigratory species that are resident in the South are also benefited by the departure of the migrants.
Nevertheless, it cannot be said that the winter or summer area of every species is entirely unsuited to its requirements at other seasons, for some individuals pass the winter season in areas that are frequented only in summer by other individuals of their species. Such species have extensive breeding ranges, presenting wide climatic variations, so that some individuals may actually be resident in a region where others of their kind are present only in winter.
The tendency of some birds to move southward at the approach of winter is not always due to the seasonal low temperatures, since experiments have demonstrated that many of our summer insect feeders, when confined in outdoor aviaries, comfortably withstand temperatures far below zero. The main consideration is the depletion of the food supply, caused either by the disappearance or the hibernation of insects, or by the mantle of snow or ice that prevents access to the seeds and other forms of food found on or close to the ground or submerged in water. Possibly also the shortened hours of daylight materially restrict the ability of the birds to obtain sufficient food at a time when the cold requires an increased supply to maintain body heat. It is noteworthy that some of our smaller birds, such as the chickadees, have no fear of Arctic weather, as their food supplies are entirely arboreal and so are always available. Also, when there is a good supply of food in the form of pine and spruce seeds, nuthatches and crossbills will remain through the winter in Canadian woods. When these birds appear abundantly in winter at points in southern latitudes, it may be concluded that there is a shortage of their food in the North.
The Origin of Migration
Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimulus associated with the reproductive period. Its origin is locked in the ages of geologic time, but by study of the history of how birds came to occupy their present ranges, information becomes available from which reasonable theories may be developed and explored. The two that are most commonly accepted are diametrically opposed to each other.