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Migration of Birds

Circular 16

U.S. Fish and Wildlife Service

Migration of Birds

Circular 16

by Frederick C. Lincoln, 1935

revised by Steven R. Peterson, 1979

revised by John L. Zimmerman, 1998

Division of Biology, Kansas State University, Manhattan, KS

Associate editor Peter A. Anatasi

Illustrated by Bob Hines

U.S. Department of the Interior U.S. Fish and Wildlife Service

FISH &WILDLIFE^ U.S. SERVICE

D E P A R TMENTOFTHEINTERIOR

TABLE OF CONTENTS

• PREFACE.............................................................................................................. Page
• INTRODUCTION ................................................................................................
• EARLY IDEAS ABOUT MIGRATION............................................................
• TECHNIQUES FOR STUDYING MIGRATION..........................................
  • Direct Observation ....................................................................................
  • Aural ............................................................................................................
  • Preserved Specimens ................................................................................
  • Marking ......................................................................................................
  • Radio Tracking ..........................................................................................
  • Radar Observation ....................................................................................
• EVOLUTION OF MIGRATION......................................................................
• STIMULUS FOR MIGRATION......................................................................
• WHEN BIRDS MIGRATE ..............................................................................
  • Time of Year ..............................................................................................
  • Time of Day ..............................................................................................
• FLIGHT SPEED AND RATE OF MIGRATION ........................................
• MIGRATORY FLIGHT ALTITUDE..............................................................
• SEGREGATION DURING MIGRATION ....................................................
  • As Individuals or Groups of Species......................................................
  • By Age ......................................................................................................
  • By Sex ........................................................................................................
• GEOGRAPHIC PATTERNS OF MIGRATION ..........................................
  • Populations Within Species ....................................................................
  • Short Distance Migration ......................................................................
  • Long Distance Migration ........................................................................
• ORIENTATION AND NAVIGATION ..........................................................
  • Vector Navigation ....................................................................................
  • Orientation Cues ......................................................................................
• INFLUENCE OF WEATHER........................................................................
• INFLUENCE OF TOPOGRAPHY ................................................................
• PERILS OF MIGRATION ..............................................................................
  • Storms........................................................................................................
  • Aerial Obstructions ..................................................................................
  • Exhaustion ................................................................................................
• ROUTES OF MIGRATION..............................................................................
  • General Considerations ..........................................................................
  • Flyways and Corridors............................................................................
  • Narrow Routes ........................................................................................
  • Converging Routes ..................................................................................
  • Principal Routes From North America ................................................
  • Atlantic Oceanic Route ............................................................................
  • Atlantic Coast Route and Tributaries ..................................................
    • and Tributaries ................................................................................ Mackenzie Valley-Great Lakes-Mississippi Valley Routes
  • Great Plains-Rocky Mountain Routes ..................................................
  • Pacific Coast Route ..................................................................................
  • Pacific Oceanic Route ..............................................................................
  • Arctic Routes ............................................................................................
• PATTERNS OF MIGRATION ........................................................................
  • Loops..........................................................................................................
  • Dog-legs ....................................................................................................
  • Pelagic Wandering....................................................................................
  • Leap-frogging ..........................................................................................
  • Vertical Migration ....................................................................................
  • Pre-migratory Movements ....................................................................
  • Vagrant Migration....................................................................................
• FUTURE DIRECTIONS ................................................................................
• BIBLIOGRAPHY ..............................................................................................
• LIST OF BIRD SPECIES MENTIONED IN TEXT ..............................

PREFACE TO THE 1998 EDITION

Frederick C. Lincoln’s “Migration of Birds” was published in 1935. Lincoln’s writing style effectively communicated the wonders of bird migration to a wide audience, both young and old, experienced observers of birds as well as the simply curious. Indeed the demand for this little book was so great that it was revised in 1950 and soon was out of print again. In 1979, Steven R. Peterson developed a second revision, adding additional examples and presenting an understanding of bird migration that reflected current research. The style, figures, and most of the content of the origi- nal publication were retained, but new illustrations were added where appropriate.

In this present revision large sections of the text have remained unchanged from the previous revision or only slightly modified to make the discussion compatible with current understanding. The geographic emphasis of Lincoln and the wealth of pertinent examples added by Peterson have been maintained. I have made substantial changes, however, in sections dealing with the evolution of migration, stimulus for migration, orientation and navigation, and the influence of weather. I have also changed the emphasis of the final section to reflect current concerns. While some investigators are mentioned by name, specific studies are not cited in the text. An exten- sive bibliography has been included for those interested in pursuing the subject further. I have relied heavily upon the bibliographies on migration research prepared by Stanley H. Anderson and Loren W. Ayers, University of Wyoming Fish and Wildlife Coop Unit and Thomas S. Litwin, as well as the bibliography in Peterson’s revision. Additional citations have been suggested by Daniel R. Petit and Stephanie L. Jones, U.S. Fish and Wildlife Service. This edition was due to the support of the Fish and Wildlife Service Regional Nongame Migratory Bird Coordinators: Tara Zimmerman, Kent Wohl, Steve Lewis, Daniel Petit, Diane Pence, Stephanie Jones, Bill Howe, and Richard Coon. I am indebted to all these investiga- tors and most grateful for their assistance.

John Zimmerman, 1998

INTRODUCTION

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 loco- motion 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 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 meta- bolic 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 phenom- enon 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 sea- sons. The arrival of certain species has been heralded with appropriate ceremonies in many lands. Among the eskimos and other tribes this phe- nomenon 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 America 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 increas- ing 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, envi- ronmental 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 protect- ing those avian species that in their yearly journeys pass back and forth between the 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 interna- tional 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 distri- bution 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 observed in their respec- tive localities. These data, stored in field notes, computer files, and scien- tific journals constitute an enormous reservoir of information pertaining to the distribution and movements of North American birds. The purpose of this publication is to summarize these data and additional information from other parts of the world to present the more important facts about our cur- rent understanding of the fascinating subject of bird migration. The U.S. Fish and Wildlife Service is grateful to the many people who have con- tributed their knowledge so that others, whether in biology or ornithology classes, members of conservation organizations, or just individuals inter- ested in the welfare of the birds, may understand and enjoy this precious resource as well as preserve it for generations to come.

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 wander- ings 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 migra- tion. 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 account- ed 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 appar- ently 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 pos- sible 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 sea- sonal 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 win- ter 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 com- panions 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 war- blers 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

Since this publication first appeared in 1935, traditional methods as well as new procedures have been used in the study of bird migration. On occasion a method developed for a quite different but related purpose has become an invaluable innovative technique in our continuing exploration of the migration phenomenon.

Direct Observation

The oldest, simplest, and most frequently used method of studying migra- tion 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 observa- tions 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 newslet- ters on direct observation of migration. In the aggregate, direct observa- tion 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 reflec- tor 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 essen- tial 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 speci- mens may be used for comparison with individuals collected during migra- tion to associate them with their breeding areas. This provides a conve- nient 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 mark- ing 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 recov- ered 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 band- ing 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 popula- tions or the vulnerability to hunting is gained. Colored leg bands, neck col- lars, or streamers can be used to identify populations or specific individu- als, 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 band- ing, 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 dis- tribution 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 pos- sible 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 north- ern 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 sig- nals. 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 tech- nique 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 associ- ated 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 geo- graphic areas.

Three types of radar have been used for studying birds: 1) general surveil- lance 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 enforce- ment agencies for measuring the speed of a passing automobile or by mete- orologists for detecting tornadic winds. The data collected by radar can be electronically stored in the absence of a human observer and can be corre- lated 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 sea- sons 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 popula- tion 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 cli- matic variation is relatively low, these benefits are offset by lower repro- ductive 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, singu- lar 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 water- fowl, 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 physio- logical 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 winter- ing 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 south- ward. At lower latitudes, breeding seasons are longer, allowing multiple attempts to produce young. This longer breeding season, however, is relat- ed to a higher probability that nests will suffer losses to predators.

Fall departure from higher latitudes removes individuals from climatic con- ditions 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 interspecif- ic competition with resident species in years when resource availability might be reduced. This cost, plus the hazards associated with the migrato- ry 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 suf- fered 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, cli- mates 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 per- manent 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 Yellow- throat 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 resi- dent, non-migratory population becomes proportionately smaller and smaller in numbers. If changing environmental conditions become increas- ingly disadvantageous for the resident population or interspecific competi- tion becomes more severe, the resident population could eventually disap- pear, 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 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 pat- terns 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 struc- tural correlate with migratory behavior. Migratory species typically have proportionally longer wings, with a higher aspect ratio, than related nonmi- gratory 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.

STIMULUS FOR MIGRATION

The environmental factors that have resulted in the evolution of migratory behavior are not the environmental factors that stimulate development of the migratory condition or actually cause birds to embark on migratory flights. If a bird would wait until food on its breeding range became abun- dant to begin its vernal migratory preparation, it would have insufficient time to migrate, establish a territory, mate, incubate eggs, and raise young to take advantage of this abundance. The timing of its entire annual cycle must result in young in the nest coincident with an optimal abundance of food or other environmental factor that has a critical effect on productivity. Similarly, if birds waited until the climate became no longer tolerable to begin preparations for fall departure from breeding areas, it would be too late to gain the necessary energy surplus above the demands of ther- moregulation to allow the required physiological changes associated with migration. The stimulus for development of the migratory state must be related to the eventual advent of suitable environmental conditions for reproduction or winter survival.

In the spring, the premigratory state is characterized by a change in neur- al centers in the lower part of the brain (the hypothalamus) controlling hunger and satiety so that the bird gains weight by overeating. This increased energy income, a food intake that is as much as 40% greater than during other times of the year, is stored as large fat deposits under the skin, in flight musculature, and in the abdominal cavity. Small perching birds like sparrows and warblers gain about 1 to 1.5 g per day, and this increased appetite continues over a period of about two weeks prior to migration. Furthermore, these birds retain the ability to rapidly gain weight during stopover periods in the course of their migratory journey. While during nonmigratory periods fat comprises about 3-5% of a bird's body weight, short and middle distance migrants increase their fat load to about 15% of their weight, while in long-distance migrants fat is 30-50% of their weight. They are literally obese. These fat stores fuel the aerobic contraction of flight muscles, permitting flights of long duration with mini- mal fatigue.

Experiments have demonstrated that day length is the environmental stimulus that results in vernal premigratory weight gain. Light not only directly affects the hypothalamic feeding centers but stimulates adjacent centers in the brain to affect a shift in the bird’s endocrine secretions, specifically increasing prolactin from the pituitary, corticosterone from the adrenal gland, and the sex steroids (e.g., testosterone) from the gonads. These hormonal changes facilitate the development of fat deposits result- ing from the greater food intake caused by increased appetite.

The premigratory state is also characterized by increased activity during the night, which is when most birds migrate. They become restless, per- haps in anticipation of the migratory flight. This behavior is seldom observed in the wild, but has been carefully evaluated in captive migrants. It has been shown, for example, that the intensity and duration of migrato- ry restlessness in captives are correlated with the distance and period of migration in the wild population. Like premigratory weight gain, migrato- ry restlessness is stimulated by long days through the effect of light on the hypothalamus, causing increased secretions of prolactin, corticosterone, and the sex steroids. Additionally, light stimulates the release of mela- tonin, a hormone produced in the pineal body on the top of the brain, which has also been shown to be necessary for the expression of this behavior.

It is important to emphasize that the light stimulus is a function of length of the light period rather than because of the change in daylengths. It is also clear that the absolute length of the daylight period that is considered “long” varies with species, not only in terms of the daylength characteris- tics of their environments but in the daily period when a species’ brain is receptive to the effects of light. Both the external and internal aspects of light stimulation reflect their geographic distributions. Thus, birds winter- ing in the tropics have evolved a response to that photoperiod which results in premigratory changes similar to that of birds wintering in the North Temperate zone under increasing daylength. Even birds wintering in South America initiate premigratory preparation in March and April under the decreasing daylengths of the austral fall.

The adaptation of migrants to the temporal control daylength is amazing. Consider the transequatorial migrant Bobolink. This species initiates pre- migratory preparation under decreasing daylengths in the South Temperate Zone, migrates northward toward the equator, experiencing lengthening daylengths but decreasing daily variation in daylength, then crosses the equator and experiences rapidly increasing daylength until it finally arrives on its previous year’s territory somewhere north of the forti- eth parallel. That birds, many plants, and other animals depend upon daylength to regulate their annual cycles is not surprising. Of all the vari- ables in the environment, only seasonal daylength variation has remained constant since the formation of the planet because of Earth’s rotation on an axis inclined to the plane of its revolution around the Sun.

Yet the development of the migratory state is not completely driven by daylength. Birds have evolved closer control of this process by responding to other environmental stimuli, either accelerating or inhibiting the rate of response to the primary daylength stimulus. Temperature is one of the environmental factors involved. Thus, when spring is late birds do not arrive too early; similarly, when spring is advanced the birds arrive early to take advantage of the precocious environmental resources. There is also evidence that development of vegetative cover can influence light- caused reproductive development. When songbirds normally nesting on Jan Mayen Island in the Arctic Ocean arrived during a late spring to find their breeding grounds still snowbound, gonadal development was immedi- ately truncated and the birds left, even though daylength was stimulatory.

The stimulus for autumnal premigratory preparation is not well under- stood. The current working hypothesis suggests that the spring photoperi- od sets an internal timer that allows the expression of fall premigratory preparation after the cessation of a reproductive period which has evolved to be commensurate with species-specific environmental resources. Perhaps hormonal changes following breeding release the expression of these preset events. In many species, the postnuptual (or pre-basic) molt may inhibit the development of the premigratory state. In other species, however, migration precedes the fall molt. And some species, like Barn Swallows, molt while migrating.

WHEN BIRDS MIGRATE

Individual birds are relatively sedentary during two periods each year, at nesting time and in winter. When the entire avifauna of a continent is con- sidered, however, during almost all periods there are some latitudinal movements of birds. Each species, or group of species, migrates at a par- ticular time of the year and some at a particular time of the day. Other species are more irregular in their migratory behavior. Red Crossbills, for example, are erratic wanderers and will settle down and breed any month of the year when and where an adequate supply of conifer seeds is avail- able.

Time of Year

Some species begin their fall migrations early in July, and in other species distinct southward movements cannot be detected until winter. For exam- ple, many shorebirds start south in the early part of July, while Northern Goshawks, Snowy Owls, Common Redpolls, and Bohemian Waxwings do not leave the north until forced to do so by the advent of severe winter weather or a lack of customary food. Thus, an observer in the northern part of the United States may record an almost unbroken southward pro- cession of birds from midsummer to winter and note some of the returning migrants as early as the middle of February. While on their way north, Purple Martins have been known to arrive in Florida late in January; and, among late migrants, like some wood warblers, the northern movement may continue well into June. In some species with a broad latitudinal range, the migration is so prolonged that the first arrivals in the southern part of the breeding range will have performed their parental duties and may complete nesting while others of the species are still on their way north. As you should expect, northern and southern populations of the same species can have quite different migration schedules.

In fall, migratory populations that nest farthest south migrate first to the winter range because they finish nesting first. For example, the breeding range of the Black-and-white Warbler covers much of the eastern United States and southern Canada northwest through the prairies to Great Bear Lake in Canada (Figure 1). It spends the winter in southern Florida, the West Indies, southern and eastern Mexico, Central America, and north- western South America. In the southern part of its breeding range, it nests in April, but those summering in New Brunswick do not reach their nesting grounds before the middle of May (Figure 2). Therefore, if 50 days are required to cross the breeding range, and if 60 days are allowed for reproductive activities and molting, they would not be ready to start south- ward before the middle of July. Then with an assumed return 50-day trip south, the earliest migrants from the northern areas would not reach the

Gulf Coast until September. Since adults and young have been observed on the northern coast of South America by August 21, it is very likely that they must have come from the southern part of the nesting area.

Many similar cases might be mentioned, such as the Black-throated Blue Warblers still observed in the mountains of Haiti during the middle of May when others of this species are en route through North Carolina to New England breeding grounds. The more southerly breeding American

Figure 1. Summer and winter homes of the Black-and-white Warbler. A very slow migrant, warblers nesting in the northern part of the continent take 50 days to cross the breeding range. The speed of migration is shown in Fig. 2.

Black-and-white Warbler

Breeding Range

Winter Range

Redstarts and Yellow Warblers are seen returning southward on the north- ern coast of South America just about the time the earliest of those breed- ing in the north reach Florida on their way to winter quarters. Examples of the Alaska race of the Yellow Warbler have been collected in Mississippi, Florida, and the District of Columbia as late as October.

Students of migration know that birds generally travel in waves, the mag- nitude of which varies with populations, species, weather, and time of year.

Figure 2. Isochronal migration lines of the Black-and-white warbler, showing a very slow and uniform migration. The solid lines connect places at which these birds arrive at the same time. These birds apparently advance only about 20 miles per day in crossing the United States.

May 10 May 1

April 20 April 10

March 30

March 20

March 10

May 30

May 20

March 20

March 10

Black-and-white warbler

Isochronal Migration Lines

Characteristically, one will observe a few early individuals come into an area followed by a much larger volume of migrants. This peak will then gradually taper off to a few lingering stragglers. If we plot numbers observed against time, the rising and receding curve is bell-shaped. In the northern part of the United States there are two general migration waves. The first one in early spring consists of “hardy” birds, including many of our common seed eaters like the finches, sparrows, and others. The second wave occurs about a month later and consists primarily of insect-eating birds such as flycatchers, vireos, and warblers. Each of these species in turn has its own frequency curve of migration within the major wave.

Time of Day

Because most birds are creatures of daylight, it seems remarkable that many should select the night for extended travel. Smaller birds such as rails, shorebirds, flycatchers, orioles, most of the sparrows, the warblers, vireos, and thrushes are typical nocturnal migrants. It is common to find woods and fields on one day almost barren of bird life and on the following morning filled with newly arrived migrants that came during the night. Waterfowl hunters sitting in their blinds frequently observe the passage of flocks of ducks and geese, but great numbers of these birds also pass through at night; the calls of Canada Geese or the conversational gabbling of flocks of ducks are common night sounds in spring and fall in many parts of the country. Observations made with telescopes focused on the full moon have shown processions of birds, and one observer estimated their passage over his area at the rate of 9,000 per hour. This gives some indica- tion of the numbers of birds in the air at night during migratory peaks. Radar observations have shown that nocturnal migration begins about an hour after sundown, reaches a maximum shortly before midnight, and then gradually declines until daybreak. Bird echoes during peak migration peri- ods may cover a radar screen.

It has been suggested that small birds migrate by night to avoid their ene- mies. To a certain extent this may be true because the group includes not only weak flyers, such as the rails, but also the small insectivorous birds, such as wrens, small woodland flycatchers, and other species that habitual- ly live more or less in concealment. These birds are probably much safer making their flights under the protecting cloak of darkness. Nevertheless, it must be remembered that night migrants also include sandpipers and plovers. Most shorebirds are usually found in the open and are among the most powerful flyers, as some of them make annual nonstop migratory flights over 2,000 miles of open ocean.

Night travel is probably best for the majority of birds chiefly from the standpoint of feeding. Digestion is very rapid in birds, and yet the stomach of birds killed during the day almost always contains food. To replace the energy required for long flight, it is essential that either food be obtained at comparatively short intervals or stores of fat be laid on prior to migra- tion. If the smaller migrants were to make protracted flights by day, they would arrive at their destination at nightfall almost exhausted. Since they are entirely daylight feeders, they would be unable to obtain food until the following morning. The inability to feed would delay further flights and