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Migration of Birds

The changing picture of bird populations throughout the year intrigues those who are observant and who wish to know the source and destination of these birds. While many species of fish, mammals, and even insects undertake amazing migratory journeys, birds as a group are the most mobile creatures on Earth. Even humans with their many vehicles of locomotion do not equal some birds in mobility. No human population moves each year as far as from the Arctic to the Antarctic with subsequent return, yet the Arctic Terns do.

Birds are adapted in their body structure and physiology to life in the air. Their feathered wings and tails, bones, lungs and air sacs, and their metabolic abilities all contribute to this amazing faculty. These adaptations make it possible for birds to seek out environments most favorable to their needs at different times of the year. This results in the marvelous phenomenon we know as migration—the regular, recurrent, seasonal movement of populations from one geographic location to another and back again.

Throughout human experience, migratory birds have been important as a source of food after a lean winter and as the harbinger of a change in seasons. The arrival of certain species has been heralded with appropriate ceremonies in many lands. Among the eskimos and other tribes this phenomenon is the accepted sign of the imminence of spring, of warmer weather, and a reprieve from winter food shortages. The European fur traders in Alaska and Canada offered rewards to the Native American who saw the first flight of geese in the spring, and all joined in jubilant welcome to the newcomers.

As North American became more thickly settled, the large flocks of ducks and geese, as well as migratory rails, doves, and woodcock that had been hunted for food became objects of the enthusiastic attention of an increasing army of sportsmen. Most of the nongame species were also found to be valuable as allies of the farmer in his never-ending confrontation against insect pests and weed seeds. And in more recent years, all species have been of ever-increasing recreational and esthetic value for untold numbers of people who enjoy watching birds. We soon realized that our migratory bird resource was an international legacy that could not be managed alone by one state or country and that all nations were responsible for its well being. The need for laws protecting game and nongame birds, as well as the necessity to regulate the hunting of diminishing game species, followed as a natural consequence. In the management of this wildlife resource, it has become obvious that studies must be made of the species' habits, environmental needs, and travels. In the United States, the Department of the Interior recognized the value of this resource and is devoted to programs that will ensure sustainability for these populations as they are faced with the impacts of alteration in land use, loss of habitat, and contaminants from our technological society. Hence bird investigations are made by the U.S. Fish and Wildlife Service, the arm of the Department of Interior charged by Congress under the Migratory Bird Treaty Act with the duty of protecting those avian species that in their yearly journeys pass back and forth between United States and other countries. In addition, the federal government through the activities of the Biological Resources Division of the U.S. Geological Survey also promotes basic research on migration. Federal agencies cooperate with their counterparts in other countries as well as with state agencies, academic institutions, and non-governmental groups to gain understanding and for the protection of migratory species through such endeavors as Partners in Flight, a broadly-based international cooperative effort in the Western Hemisphere.

For almost a century the Fish and Wildlife Service and its predecessor, the Biological Survey, have been collecting data on the important details of bird migration. Scientists have gathered information concerning the distribution and seasonal movements of many species throughout the Western Hemisphere, from the Arctic archipelago south to Tierra del Fuego.

Supplementing these investigations is the work of hundreds of U.S., Latin American, and Canadian university personnel and volunteer birdwatchers, who report on the migrations and status of birds as observed in their respective localities. These data, stored in field notes, computer files, and scientific journals, constitute an enormous reservoir of information pertaining to the distribution and movements of North American birds.


Early Ideas About Migration
The migrations of birds probably attracted the attention and aroused the imagination of humans since our African genesis. Recorded observations on the subject date back nearly 3,000 years to the times of Hesiod, Homer, Herodotus, and Aristotle. 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?" The author of Jeremiah (8:7) wrote: "The stork in the heavens knoweth her appointed time; and the turtledove, and the crane, and the swallow, observe the time of their coming." The flight of Migratory Quail that saved the Israelites from starvation in their wanderings through the Sinai wilderness is now recognized as a vast migration between their breeding grounds in eastern Europe and western Asia and their winter home in Africa.

Aristotle, naturalist and philosopher of ancient Greece, was one of the first observers whose writings are known to discuss the subject of bird migration. He noted cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile, and pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. Pliny the Elder, a Roman naturalist, in his "Historia Naturalis," repeated much of what Aristotle had written on migration and added comments of his own concerning the movements of European species of starlings, thrushes, and blackbirds.

Aristotle also must be credited with the origin of some superstitious beliefs that persisted for several centuries. One of these, that birds hibernated, became so firmly rooted that the eminent nineteenth century American ornithologist, Dr. Elliott Coues, listed in 1878 the titles of no less than 182 papers dealing with the hibernation of swallows. The students of Aristotle believed the disappearance of many species of birds in the fall was accounted for by their passing into a torpid state where they remained during the cold season, hidden in hollow trees, eaves, or in the mud of marshes. Aristotle ascribed hibernation not only to swallows, but also to storks, kites, and doves. Some early naturalists wrote fantastic accounts of flocks of swallows allegedly seen congregating in marshes until their accumulated weight bent the reeds into the water, submerging the birds, which apparently then settled down for a long winter's nap. 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. Olaus Magnus, Archbishop of Upsala, published a work in 1555 entitled "Historia de Gentibus Septentrionalis et Natura" observing 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.

The idea of hibernation as a regular method of spending the winter is no longer broadly accepted for birds, although the Common Poorwill is a possible exception. Many species, however, such as chickadees, swallows, hummingbirds, swifts, and nightjars regularly go into torpor under cold stress on winter nights but also even during the breeding season.

Aristotle also was the originator of the theory of transmutation, the seasonal change of one species into another. Frequently one species would arrive from the north just as another species departed for more southerly latitudes. From this he reasoned the two different species were actually one and assumed different plumages to correspond to the summer and winter seasons.

Probably the most remarkable theory advanced to account for migration is contained in a pamphlet titled, "An Essay toward the Probable Solution of this Question: Whence come the Stork and the Turtledove, the Crane, and the Swallow, when they Know and Observe the Appointed Time of their Coming," published in 1703. It is written "By a Person of Learning and Piety," whose "probable solution" stated migratory birds flew to the moon and there spent the winter.

Some people who easily accepted the migratory travels of larger birds were unable to understand how smaller species, some of them notoriously poor flyers, could make similar journeys. They accordingly conceived the idea that larger species (e.g., storks and cranes) carried their smaller companions as living freight. In some southern European countries, it is still believed these broad-pinioned birds serve as aerial transports for hosts of small birds that congregate upon the Mediterranean shore awaiting the opportunity for passage to winter homes in Africa. Similar beliefs, such as hummingbirds riding on the backs of geese, have been found among some tribes of Native Americans in the Western Hemisphere. Such fantasies, however, are not without some empirical basis, such as the observation of an Eastern Kingbird harassing a Great Horned Owl that actually perched on the shoulder of the owl's outstretched wing as the owl glided toward wooded cover.

Today we realize that birds do not migrate by "hitching" rides with other birds and that the scope of the migration phenomenon is worldwide, not simply limited to the Northern Hemisphere or the world's land masses. The migration heritage is developed just as extensively in Old World warblers migrating to and from Europe and Africa as in our wood warblers traveling from Canada and the United States to South America and back. Although South Temperate Zone species migrate northward to the tropics during the austral winter, no land species nesting in the South Temperate Zone migrates into the North Temperate Zone. Some seabirds like the Sooty Shearwater and Wilson's Storm-petrel, however, migrate to North Temperate seas after nesting on shores south of the equator.


Techniques for Studying Migration

Direct Observation
The oldest, simplest, and most frequently used method of studying migration is by direct observation. Size, color, song, and flight of different species all aid the amateur as well as the professional in determining when birds are migrating. Studies by Wells W. Cooke and his collaborators from 1888 to 1915 and continued by his successors in the U.S. Bureau of Biological Survey (later U.S. Fish and Wildlife Service) were of particular importance in the earlier years of these investigations in North America. Some of the largest and most interesting routes and patterns were sorted out by tediously compiling and comparing literally thousands of observations of species in a given locality at a particular time of the year.

More recently, the National Audubon Society and many state Audubon and ornithological societies publish information in their bulletins and newsletters on direct observation of migration. In the aggregate, direct observation has contributed much to our knowledge of migration, but this method is limited by its being largely restricted to daytime, ground-based data on birds either before or after a period of actual migratory flight.

The "moon watch" is a modification of the direct observation method. Many species of birds migrate at night. Until mid-century, it was not apparent just how prevalent nocturnal migration really was. Significant information has been derived from watching the passage of migrating birds across the face of a full moon through telescopes, noting both the numbers and directions of flight. Since the actual percent of the sky observed by looking through a telescope at the moon is extremely small (approximately one-hundred thousandth of the observable sky), the volume of birds recorded is small. On a night of heavy migration, about 30 birds per hour can be seen. The fact that any birds are observed at all is testimony to the tremendous numbers passing overhead. A large-scale, cooperative moon-watching study was organized and interpreted by George H. Lowery, Jr. of Louisiana State University in the 1960's.

Aural
Another nocturnal observation method which has potential for species identification during the study of migration is the use of a parabolic reflector with attached microphone to amplify call (chip) notes. This device, when equipped with a tape recorder, can record night migrants up to 11,000 feet on nights with or without a full moon. A primary disadvantage is that one cannot tell the direction a bird is traveling. Furthermore, there may be some difficulty in identifying the chip notes made by night migrants, since these calls are often different from the notes heard during the daytime. Unfortunately, the bird may not call when it is directly over the reflector and consequently it would not be recorded.

Preserved Specimens
Reference material consisting of preserved bird skins with data on time and place of collection exists in many natural history museums. The essential ingredient in studying migration by this method is to have an adequate series of specimens taken during the breeding season so differences in appearance between geographically separated breeding populations of the same species can be discerned. Such properly identified breeding specimens may be used for comparison with individuals collected during migration to associate them with their breeding areas. This provides a convenient way of recognizing and referring to individuals representative of known populations wherever they may be encountered.

Marking
If birds can be captured, marked, and released unharmed, a great deal of information can be learned about their movements. Many different marking methods have been developed to identify particular individuals when they are observed or recaptured at a later date. Since 1920, the marking of birds with numbered leg bands in North America has been under the direction of the U.S. Fish and Wildlife Service (and more recently the Biological Resources Division of the U.S. Geological Survey) in cooperation with the Canadian Wildlife Service. Every year professional biologists and volunteers, working under permit, place bands on thousands of birds, both game and nongame, large and small, migratory and nonmigratory. Each band carries a serial number on the outside and an address where recovered bands can be sent on the inside. When a banded bird is reported from a second locality, a definite fact relative to its movements becomes known. The study of many such cases leads to a more complete knowledge of the details of migration.

The records of banded birds have also yielded other important information relative to migrations, such as arrival and departure dates, the length of time different birds pause on their migratory journeys to feed and rest, the relation between weather conditions and starting times for migration, the rates of travel for individual birds, and the degree of regularity with which individual birds return to the summer or winter quarters used in former years. Many banding stations are operated systematically throughout the year and supply much information concerning the movements of migratory birds that heretofore could only be surmised. The most informative banding studies are those that focus on particular populations of birds. Examples of such planned banding programs are the extensive marking of specific populations of ducks and geese on their breeding grounds by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service, as well as "Operation Recovery," the cooperative program of banding small land birds along the Atlantic Coast. When these banded birds are recovered, information concerning movements and survival rates of specific populations or the vulnerability to hunting is gained. Colored leg bands, neck collars, or streamers can be used to identify populations or specific individuals, and birds marked with easily observed tags can be studied without having to kill or recapture individuals, thus making it a particularly useful technique.

We have learned about the migratory habits of some species through banding, but the method does have shortcomings. To study the migration of a particular species through banding, the banded bird must be encountered again at some later date. If the species is hunted, such as ducks or geese, the number of returns per 100 birds banded is considerably greater than if one must rely on a bird being retrapped or found dead. For example, in Mallards banded throughout North America the average number of bands returned the first year is about 12 percent. In most species that are not hunted, less than 1 percent of the bands are ever seen again.

In 1935, Lincoln commented that with enough banding some of the winter ranges and migration routes of more poorly understood species would become better known. A case in point is the Chimney Swift, a common bird in the eastern United States. This species winters in South America. Over 500,000 Chimney Swifts have been banded, but only 21 have been recovered outside the United States (13 from Peru, 1 from Haiti, and the rest from Mexico). The conclusion is simply this: whereas banding is very useful for securing certain information, the volume of birds that need to be banded to obtain a meaningful number of recoveries for determining migratory pathways or breeding or wintering areas may be prohibitive. One problem in interpretation of many banding results is the fact that recoveries may often reflect the distribution of people rather than the distribution of birds.

Radio Tracking
Radio tracking, or telemetry, is accomplished by attaching a small radio transmitter that gives off periodic signals or "beeps" from a migrating bird. With a radio receiving set mounted on a vehicle or airplane, it is possible to follow these radio signals and trace the progress of the migrating bird. One of the most dramatic examples of this technique was reported by Richard Graber in 1965. He captured a Gray-cheeked Thrush on the University of Illinois campus and attached a 2.5-gram transmitter (a penny weighs 3 grams). The bird was followed successfully for over 8 hours on a course straight north from Urbana, across Chicago, and up Lake Michigan on a continuous flight of nearly 400 miles at an average speed of 50 mph (there was a 27 mph tail wind aiding the bird). It is interesting to note that while the little thrush flew up the middle of Lake Michigan, the pursuing aircraft skirted the edge of the lake and terminated tracking at the northern end after running low on fuel while the bird continued to fly on. The limitations of radio telemetry, of course, are the size of the transmitter that can be placed on birds without interfering with flight and the ability of the receiving vehicle to keep close enough to the flying bird to detect the signals. Despite this difficulty, there has been considerable development in the technology, and encouraging results to date give promise for the future, particularly when birds can be tracked by orbiting satellites. Yet this technique should be used cautiously, since several studies have demonstrated that transmitter-equipped birds have significantly lower survival.

Radar Observation
Radar was developed to identify and track aircraft electronically and was an innovation that was critical to England's success in the Battle of Britain during the early years of the Second World War. Early radar observers noted, however, that they received moving returns that could not be associated with aircraft. These radar echos, whimsically termed "angels" by observers in England, were soon discovered to be birds. That bird flight could be monitored by radar was seized upon by students of migration after the end of the war as an opportunity to obtain information on the movements of birds during both day and night and over extensive geographic areas.

Three types of radar have been used for studying birds: 1) general surveillance radar, similar to ones located at airports, that scans a large area and indicates the general time and direction of broad movements of birds; 2) tracking radar that records the path of an airplane (or bird) across the sky by "locking on" to a designated "target" and continuously following only that object; and 3) Doppler radar similar to those operated by law enforcement agencies for measuring the speed of a passing automobile or by meteorologists for detecting tornadic winds. The data collected by radar can be electronically stored in the absence of a human observer and can be correlated with weather data sets.

The use of radar in migration studies has been invaluable in determining direction and speed of mass bird movements, dates and times of departure, height of travel, and general volume, especially at night. One interesting fact to come out of current radar work is the discovery of relatively large movements of warblers and other small land birds migrating over oceans rather than along coastlines and in directions about which ground-based observers were completely unaware.


Evolution of Migration
The rigors of the annual migratory journey are balanced by benefits derived from species being able to inhabit two different areas during seasons when each region provides favorable conditions. Upland Sandpipers breeding in the grasslands of North America and wintering on the pampas of Argentina never experience winter. If it were not advantageous to make the trip twice a year, the behavior would not have evolved or if once typical under one set of conditions, natural selection would have eliminated the tendency once the environment changed. An example of the latter case is the European Starling which is migratory on the continent, but the population isolated in the British Isles by the rise in sea level after the end of Pleistocene glaciation and now living in a moderate maritime climate has secondarily evolved nonmigratory behavior.

By departing in the spring from their wintering ranges to breeding areas, migrant species are probably assured of reduced interspecific competition for adequate space and resources such as ample food for themselves and their offspring. Permanent residents in temperate zones, whose wintering and breeding areas are in the same region, also gain a net benefit by being nonmigratory. Although not suffering the metabolic demands and hazards of migration, the energetic demands for survival and reproduction in an environment with a greater annual range of climactic variation, and the need to adapt to the seasonal changes in the availability and kinds of foods, are comparable. Even for permanent residents in the tropics where climatic variation is relatively low, these benefits are offset by lower reproductive success resulting from higher nest predation.

While the various kinds of wood warblers and flycatchers are wholly migratory, other species like most woodpeckers are permanent residents. Some populations of species have individuals that are migratory while other individuals breeding in the same area are not. These partial migrant species, like Blue Jays, exemplify the difficulty in suggesting simple, singular explanations for the origin of migration.

Birds require specific environmental resources for reproduction. Among both migratory and nonmigratory species alike, adequate food for the young appears to be primary in determining where, as well as when, a species will breed. American Goldfinches and Pine Siskins are closely related and winter together in gregarious flocks. With the emergence of abundant insect food in the spring, siskins disperse and begin nesting while goldfinches postpone their reproduction until late summer when thistle seeds become available for feeding young. For other species, like waterfowl, the availability of suitable nest sites rather than food for the young appears to determine the timing of breeding.

The evolution of migration also involves adaptations that affect the timing of this behavior so that the species is in the breeding or wintering habitat under the most propitious conditions. For most migrants, especially long-distance migrants, the evolution of migratory behavior demands a physiological response to environmental cues in preparation for migration that are different from the environmental factors that ultimately determine their reproductive success on the breeding range or survival on the wintering range. Thus, in the fall swallows and other insectivorous species depart southward long before food resources or weather become critical for their survival. Factors other than a decrease in food availability or cold stress, for example, must prompt their migratory departure.

The verdant flush of regrowth in the spring is clearly associated with migratory movements of many species to higher latitudes where longer daylengths provide ample time for feeding young, permitting their rapid growth and shorter exposure in the nest to predation. But the higher the latitude the shorter the breeding season, so that while summer days may be long, the summer season is short and migrants in more northerly climes may have only one chance to breed before they must again travel southward. At lower latitudes, breeding seasons are longer, allowing multiple attempts to produce young. This longer breeding season, however, is related to a higher probability that nests will suffer losses to predators.

Fall departure from higher latitudes removes individuals from climatic conditions that will eventually exceed their physiological tolerance limits. The Dickcissel is a Neotropical migrant that breeds as far north as Winnipeg, but cannot survive environmental temperatures below freezing during the short days of winter at mid-temperate latitudes. The arrival of migrants on the winter range, however, increases the chances for greater interspecific competition with resident species in years when resource availability might be reduced. This cost, plus the hazards associated with the migratory journey, decreases adult survivorship. The evolution of migratory behavior must, on average, offer a favorable balance between these various costs and benefits.

Birds appear in the fossil record distinct from their reptilian ancestors about 150 million years ago. For the next 50 million years or so a relative uniform and benign maritime climate pervaded the Earth. Sometime around 65 million years ago, however, global climate abruptly changed, perhaps from impact by a large asteroid, and the biota of the planet suffered a major episode of extinction. But a remnant lineage of birds survived and gave rise to the modern groups of birds we see today. Yet with the slow, continuing drift of the continents into higher latitudes that began soon after the first appearance of birds, and the development of mountain ranges as a result of the collision between tectonic plates, climates became more latitudinally and often longitudinally differentiated. The resulting diversity in habitats provided the selective pressures that led to the evolution of migration again and again in different species.

The general model for the evolution of migratory behavior considers a permanent resident that expands its range due to intraspecific competition into an area that is seasonally variable, providing greater resources for reproduction but harsher climactic stress and reduced food availability in the non-breeding season. Individuals breeding in these new regions at the fringe of the species' distribution are more productive, but in order to increase non-breeding survival they return to the ancestral range. This results, however, in even greater intraspecific competition because of their higher productivity, so that survival is enhanced for individuals that winter in areas not inhabited by the resident population. The Common Yellowthroat of the Atlantic coast is a good example. Birds occupying the most southern part of the species' range in Florida are largely nonmigratory, whereas populations that breed as far north as Newfoundland migrate to the West Indies in the winter, well removed from the resident population in Florida. Because a migrant population gains an advantage on both its breeding and wintering range, it becomes more abundant, while the resident, non-migratory population becomes proportionately smaller and smaller in numbers. If changing environmental conditions become increasingly disadvantageous for the resident population or interspecific competition becomes more severe, the resident population could eventually disappear, leaving the migrant population as characteristic of the species. These stages in the evolution of migration are represented today by permanent resident populations, partial migrants, and fully migratory species. As for all adaptations, natural selection continues to mold and modify the migratory behavior of birds as environmental conditions perpetually change and species expand or retract their geographic ranges. Hence, the migratory patterns that we observe today will not be the migratory patterns of the future.

Migration involves not just the evolution of a specific behavioral pattern, but often morphological changes as well. The shape of the wing is a structural correlate with migratory behavior. Migratory species typically have proportionally longer wings, with a higher aspect ratio, than related nonmigratory species. This adaptation reduces the relative impact of wing-tip (induced) drag, resulting in greater effective lift as well as an often more efficient ratio between wing area and body weight. Furthermore, the outer primary feathers, which together with the inner primaries provide forward thrust in flapping flight, are often longer in migrants, giving the wing a pointed rather than a rounded shape. In Asia, the sedentary Black-headed Oriole has a rounded wing, whereas the closely related Black-naped Oriole with pointed wings is migratory between Siberia and India. Albatrosses, falcons, swifts, various shorebirds, and terns, many of which make long-distance journeys, have long, more pointed wings. Even among closely related migrants there is a difference. Thus the pointed wings of the Semipalmated Sandpiper, which migrates from the arctic to only northern South America has noticeably shorter wings than the Baird's and White-rumped sandpipers that fly from the arctic all the way to the southern tip of South America.

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