MIGRATION OF BIRDS
Circular 16 ^ Revised Edition - 1979
FISH & WILDLIFE SERVICE / UNITED STATES DEPARTMENT OF THE INTERIOR
From the collection of
in association with
San Francisco, California
MIGRATION OF BIRDS
By Frederick C. Lincoln
Revised By Steven R. Peterson
Associate Editor Peter A. Anastasi
Illustrated By Bob Hines
Circular 16 Revised Edition - 1979
FISH & WILDLIFE SERVICE / UNITED STATES DEPARTMENT OF THE INTERIOR
TABLE OF CONTENTS
THE HISTORY AND SCOPE OF MIGRATION 4
TECHNIQUES FOR STUDYING MIGRATION 7
Direct Observation 7
Preserved Specimens 8
Bands, Collars, Streamers 8
Radio Tracking 10
Radar Observation 10
Orientation and Navigation 11
Physiology of Migration 11
ADVANTAGES OF MIGRATION 13
STIMULUS FOR MIGRATION 15
WHEN BIRDS MIGRATE 17
Time of Year 17
Time of Day 20
SPEED OF FLIGHT AND MIGRATION 25
ALTITUDE OF FLIGHT AND MIGRATION 32
SEGREGATION DURING MIGRATION 35
By Individuals or Groups of Species 35
By Age 36
By Sex 38
By Kinds of Flocks 40
WHERE BIRDS MIGRATE 41
Migration by Populations Within Species 41
Fall Flights Not Far South of Breeding Range 42
Long Distance Migration 44
ORIENTATION AND NAVIGATION 47
INFLUENCE OF WEATHER 51
INFLUENCE OF TOPOGRAPHY 56
PERILS OF MIGRATION 58
Aerial Obstructions 58
ROUTES OF MIGRATION 61
General Considerations 61
Flyways and Corridors 62
Narrow Routes 65
Converging Routes 65
Principal Routes From North America 69
Atlantic Oceanic Route 69
Atlantic Coast Route and Tributaries 70
Mackenzie Valley-Great Lakes-Mississippi Valley Route
and Tributaries 73
Great Plains-Rocky Mountain Routes 75
Pacific Coast Route 76
Pacific Oceanic Route 80
Arctic Routes 80
PATTERNS OF MIGRATION 82
Pelagic Wandering 90
Vertical Migration 91
Pre-migratory Movements 91
Vagrant Migration 92
ORIGIN AND EVOLUTION OF MIGRATION 95
WHERE WE STAND 100
LIST OF BIRD SPECIES MENTIONED IN TEXT . , . 115
Frederick C. Lincoln's classic work on the "Migration of Birds"
first appeared in 1935. It was revised in 1950 and has been out of print
for several years, after selling over 140,000 copies. Unfilled requests
by many individuals, clubs, and institutions prompted the Office of
Conservation Education (now the Office of Public Affairs) in the U.S.
Fish and Wildlife Service to petition another update for reissue. This
publication incorporates the results gathered by research biologists
in the U.S. Fish and Wildlife Service to meet these requests.
Lincoln's original intent was to present to the American public a
summary of the facts on bird migration as they existed in the early
1930's. He wrote with a style that made the topic fascinating to the
young and old, to the educated and uninformed, and to the ardent
observer as well as the backyard watcher. An attempt has been made
to retain this style, while incorporating material from often highly
technical research efforts. Much of the content and organization of
the original publication has been maintained, but new sections were
added to incorporate recent concepts and techniques. Other concepts,
known to be inconsistent with present knowledge, have been deleted.
Because graphics are of utmost importance in this type of
publication, most of the original figures were preserved and, where
appropriate, new illustrations have been added.
Since the previous edition, tremendous progress has been made in
researching and understanding bird migration; along with this
increased effort has come a substantial increase in the literature
devoted to the subject. Emphasis was given to reviewing literature
pertaining to migration studies conducted in North America after
1950, but a number of examples from the European literature have
been included to emphasize similarities and differences in migration
throughout the world. Because extensive author citations tend to
disrupt the flow of thought, they were kept to a minimum in the text.
However, this publication is essentially a review of the literature on
the subject as it existed in the early 1970's, and a rather extensive
bibliography has been included to cover all the papers quoted in the
text as well as the many used but not specifically cited. The
bibliography, then, is primarily intended for those interested in
pursuing the subject further.
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. Birds are the most mobile creatures on
Earth. Even man with his many vehicles of locomotion does not equal
some birds in mobility. No human population moves each year as far
as from the Arctic to the Antarctic and return. Yet the Arctic terns
do and without the aid of aircraft or compass.
Birds are adapted in their body structure, as no other creatures, to
life in the air. Their wings, tails, hollow bones, and internal air sacs
all contribute to this great 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, seasonal
movement of entire populations of birds from one geographic
location to another.
Throughout the ages, 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; among the Eskimos and
other tribes, the phenomenon to this day is the accepted sign of the
imminence of spring, of warmer weather, and a change from winter
food shortages. The pioneer fur traders in Alaska and Canada offered
rewards to the Indian or Eskimo who saw the first flight of geese in
the spring, and all joined in jubilant welcome to the newcomers.
As the North American Continent became more thickly settled, the
large flocks of ducks and geese, rails, doves, and woodcock 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 insect pests. All species have been of
ever-increasing recreational and esthetic value for untold numbers
of people who enjoy watching birds. We began to realize our
migratory bird resource was an international legacy (that cannot be
managed alone by one state or country) and 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 course. In the
management of this wildlife resource, it has become obvious that
continuous studies must be made of the species' habits, environ-
mental needs, and travels. In the United States, the Department of
the Interior recognizes the value of this resource and is devoted to
programs that will ensure its preservation and wise use. Hence bird
investigations are made by the U.S. Fish and Wildlife Service, an
arm of the Interior Department, 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 other countries.
For more than three-quarters of 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 New World, from the
Canadian archipelago south to the Argentine pampas. Supplement-
ing these investigations is the work of hundreds of U.S. and
Canadian university personnel and volunteer birdwatchers, who
report on the migrations and status of birds as observed in their
respective localities; while others place numbered bands on the legs
of birds to determine their movements from one place to another.
These data, stored in field notes, computer cards, scientific journals,
and on magnetic tape constitute an enormous reservoir of
information pertaining to the distribution and movements of North
American birds. It is the purpose of this publication to summarize
these data and present the more important facts about that little
understood but universally fascinating subject of bird migration.
The U.S. Fish and Wildlife Service is grateful to the many persons
who have contributed their knowledge so that other people, be they
bird study classes, conservation organizations, or just individuals
interested in the welfare of the birds, may understand and enjoy this
precious resource as well as preserve it for generations to come.
THE HISTORY AND SCOPE OF MIGRATION
The migrations of birds were probably among the first natural
phenomena to attract the attention and arouse 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?" 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 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.
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 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. In the earliest years of the
Christian era, Pliny the Elder, 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
starlings, thrushes, and European 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, Dr. Elliott Coues
(1878), * an eminent American ornithologist, listed the titles of no less
than 182 papers dealing with the hibernation of swallows. In fact the
hibernation theory survived for more than 2,000 years, and it was not
until early in the nineteenth century that its acceptance as an
explanation for the winter disappearance of birds was almost
completely abandoned. Even after this, a few credulous persons
suggested this idea as an explanation for the disappearance of
chimney swifts in the fall before bands from wintering swifts were
finally reported as taken by Indians in Peru (Coffey 1944).
The followers 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
'Publications referred to parenthetically by date are listed in the Bibliography, p. 102
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 allegedly seen congregating in 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
Although the idea of hibernation as a regular method of spending
the winter is no longer accepted for any species of bird, certain
hummingbirds, swifts, and poorwills have been known to go into an
extremely torpid condition in cold weather (Jaeger 1948, 1949). Thus
Aristotle was at least partially vindicated.
Aristotle also was the originator of the theory of transmutation, or
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, "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," mentioned by Clarke (1912: v. 1,
9-11) 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. Astronauts have so far failed to
Some people, who easily accepted the migratory travels of larger
birds, were unable to understand how smaller species, some of them
notoriously poor fliers, 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 North American Indians.
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 United States, 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. One of the
fundamental differences in migration patterns of the Northern and
Southern Hemispheres is that no land species nesting in the South
Temperate Zone migrates into the North Temperate Zone, but a few
seabirds, such as the sooty shearwater, Wilson's storm-petrel, and
others, migrate north across the Equator over the vast ocean
expanses after nesting in the South.
TECHNIQUES FOR STUDYING MIGRATION
Before we discuss the many intricacies of how, when, and where
birds migrate, one should have a general idea of how migration data
are collected and what methods are currently being used to increase
our knowledge. Since this publication first appeared in 1935, many
new procedures have been used in the study of bird migration. One of
these, radar, has been an invaluable adaptation of a technique
developed for a quite different, but related, purpose.
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 begun by Wells W.
Cooke and his collaborators (Cooke 1888-1915) 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 oberservations on
whether a species was or was not seen in a given locality at a
particular time of the year. More recently, "The Changing Seasons"
reports by many amateur bird observers in Audubon Field Notes
(now American Birds) have been a most important source of
information on direct observation of migration. In the agregate,
direct observation has contributed much to our knowledge of
migration, but, as will be pointed out in other sections, until a few
years ago, observers were not aware of some of the biases in this
The "moon watch" is a modification of the direct observation
method. It has long been known that many species of birds migrate at
night. Until recently, it was not apparent just how important
nocturnal migration really is. Significant information has been
derived from watching, through telescopes, the passage of migrating
birds across the face of a full moon. 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. Large-scale, cooperative moon-watching studies
have been organized and interpreted by George H. Lowery, Jr. (1951;
Lowery and Newman 1966).
Another specialized direct observation approach which has yielded
important information on the spatial and altitudinal distribution of
night migrating birds has been the use of small aircraft equipped
with auxiliary landing lights (Bellrose 1971). Major disadvantages of
night observation are that species cannot be identified and that birds
continue to migrate without a full moon. However, these techniques
do give information on the nocturnal migration movements that
could not be obtained by other methods.
An adjunct to the previously described nocturnal observation
methods, which has potential for species identification, is the use of a
parabolic reflector with attached microphone to amplify call (chip)
notes (Ball 1952; Graber and Cochrane 1959). 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
and there is considerable difficulty in identifying the chip notes made
by night migrants. In addition, the bird may not call when it is
directly over the reflector and consequently it would not be recorded.
These calls are quite different from the notes we hear given by
familiar birds during the daytime while they are scolding an
intruder or advertising their territory.
Reference material consisting of preserved bird skins with data on
time and place of collection exist 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
determined. Such properly identified breeding specimens may be
used for comparison with individuals collected during migration to
associate them with their breeding areas (Aldrich 1952; Aldrich,
Duvall, and Geis 1958). This supplies a convenient way of recognizing
and referring to individuals representative of known populations
wherever they may be encountered.
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. A few of the general methods are summarized in this
Bands, Collars, Streamers
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 in cooperation with the Canadian Wildlife Service.
Every year professional biologists and voluntary cooperators,
working under permit, place bands on thousands of birds, game and
nongame, large and small, migratory and nonmigratory, with each
band carrying a serial number and the legend, NOTIFY FISH AND
WILDLIFE SERVICE, WASHINGTON, D.C., or on the smaller
sizes, an abbreviation. When a banded bird is reported from a second
locality, a definite fact relative to its movements becomes known, and
a study of many such cases develops more and more complete
knowledge of the details of migration.
The records of banded birds are also yielding other pertinent
information relative to their 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, the degree of regularity with which individual
birds return to the summer or winter quarters used in former years,
and many other details. 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
where particular populations of birds are purposely banded to
produce certain types of information when they are recovered.
Examples of such planned banding 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 in "Operation Recovery," the cooperative program
of banding small landbirds along the Atlantic Coast (Baird et al.
1958). When these banded birds are recovered, information
concerning movements 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
We have learned about the migratory habits of some species
through banding, but the method does have shortcomings. If one
wishes to study the migration of a particular species through
banding, the band 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, found dead, etc. 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 is a
nonhunted species that 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 unknown breeding or wintering
areas may be prohibitive. One problem in interpretation of all
banding results is the fact that recoveries often reflect the
distribution of people rather than migration pathways of the birds.
Other methods used to mark individuals in migration studies
include clipping the tip end off a feather (not a major flight feather)
with a fingernail clipper or touching the feather with colored paint or
dye. This marking technique is obviously good for only as long as the
bird retains the feather (usually less than one year), but allows the
investigator to recognize whether the bird has been handled
previously or not.
One of the most promising methods of tracking the movements of
individual birds in migration has been developed in recent years. It is
called radio tracking, or telemetry, and consists of attaching to a
migrating bird a small radio transmitter that gives off periodic
signals or "beeps". With a radio receiving set mounted on a truck 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 Graber in 1965. He captured a
grey-cheeked thrush on the University of Illinois campus and
attached a 2.5-gram transmitter to it (a penny weighs 3 grams). The
bird was followed successfully for over 8 hours on a course straight
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 flew 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 receivers can be mounted
on orbiting satellites (Graber 1965; Bray and Corner 1972; Southern
One of the developments of our modern age of electronics has been
the discovery that migrating birds show up on radar screens used in
monitoring aircraft. At first, the screen images caused by flying
birds were a mystery to radar operators, and they designated the dots
"angels." Later when their nature was understood, students of bird
migration seized on the unique opportunity to obtain information on
movements of birds over extensive areas (Sutter 1957; Drury 1960;
Lacke 1963a, b; Bellrose 1967; Graber 1968; and Gauthreaux 1972a,
Three types of radar have been used for studing 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) a 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) a Doppler
radar similar to those operated by law enforcement agencies for
measuring the speed of a passing automobile. The latter radar set is
useful in determining the speed of flying birds.
The use of radar in migration studies has been invaluable in
determining direction of mass movement, 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 land
birds migrating over the seas rather than along the coastlines and in
directions observers were completely unaware of a few years ago.
Orientation and Navigation __
Studies on how migrating birds orient (travel in one compass
direction) or navigate (travel toward a specific goal) have received
increasing emphasis in the past 20 years. These studies have focused
on the ability of birds to orient themselves by the position of the sun
and stars. Outstanding in this facet of research have been the works
of Matthews (1951, 1955), Kramer (1952, 1959, and 1961), Sauer and
Sauer (1960), Mewaldt and Rose (1960), Sauer (1961), Hamilton
(1962a, b), Schmidt-Koenig (1963, 1964), and Emlen (1969). The basic
method used in the experiments is to observe the direction in which
confined birds attempt to move during the period of migratory
restlessness. The birds are not permitted to have any view of the
landscape but only the sky above them. In some cases the positions of
the celestial bodies are changed by the use of mirrors to see the effect
on the orientation of the experimental birds. In other cases the
experiments are performed in plantetariums so positions of the stars
in the artificial heavens can be manipulated and the effect observed.
Physiology of Migration
The physiological basis for bird migration has received
considerable attention, particularly the effects of seasonal increases
and decreases in daylight and the seasonal rhythms occurring within
animals and referred to as "biological clocks." Investigations in this
field include the pioneering work on the relationship of photoperiod
(daylength) to migration by Rowan (1925, 1926) and many
subsequent studies (Wolfson 1940, 1945; Marshall 1961; King,
Barker and Farner 1963; King and Earner 1963; King 1963; Farner
1955, 1960; and Farner and Mewaldt 1953). These studies have
become ever more deeply involved in the intricate relationships
between photoperiod, endocrine interactions, gonad development,
fat deposition, and migratory unrest. They add to our knowledge of
the mechanisms that regulate the migratory behavior we observe.
ADVANTAGES OF MIGRATION
Why should a bird subject itself to the rigors of a long migratory
journey twice a year if it can find all the requirements suitable for
existence in one locality? It seems well to consider briefly the ends
that are served by this annual round trip between breeding grounds
and winter quarters. Obviously, the migratory habit enables a
species to enjoy the summer of northern latitudes and to avoid the
severity of winter. In other words, migration makes it possible for
some species to inhabit two different areas during the seasons when
each presents favorable conditions. If it was not advantageous to
make the trip twice a year, natural selection would have eliminated
the tendency, but bird migration has become the rule over much of
the world rather than the exception.
By withdrawing in the spring to regions uninhabitable earlier in
the year, migrant species are generally assured of adequate space
and ample food upon their arrival in the winter-freed North, and
those nonmigratory kinds, which stay behind to nest, are also assured
of ample space for these activities.
Every pair of birds requires a certain amount of territory for the
performance of its reproductive duties, the extent of which varies
greatly between different species. This territory must be large
enough to provide adequate food, not only for the parent birds but
also for the lusty appetites of their young. In the Arctic summer, 24
hours of daylight allow the young to feed or be fed almost con-
tinuously and rapid growth is apparent. The short breeding season in
northern latitudes exposes the vulnerable young to predation for a
brief period and prevents a build up of predator populations.
It cannot be said that the winter or summer area of every species is
entirely unsuited to the requirements of all of its members at other
seasons, because some individuals pass the winter season in areas
that are frequented only in summer by other individuals of their
species. Such species may have extensive breeding ranges with wide
climatic variations so that some individuals may actually be
permanently resident in a region where others of their kind are
present only in winter. Also, some individual song sparrows and blue
jays, for example, have been known to change their migratory status
(e.g., a particular bird may migrate one year and not the next or vice
versa). Thus, different individuals or populations within these
species appear to have different tolerances for climatic conditions.
The tendency of some birds to move southward at the approach of
winter is not always due to seasonal low temperatures. Experiments
have demonstrated many of our summer insect feeders, when
confined in outdoor aviaries, comfortably withstand temperatures
far below zero as long as abundant food is provided. The main
consideration then, is depletion of the food supply, caused by either
the disappearance or hibernation of insects or the mantle of snow or
ice that prevents access to seeds and other food found on or close to the
ground or submerged in water. Also, shortened hours of daylight
may restrict the ability of birds to obtain sufficient food at a time
when low temperatures require increased energy to maintain body
heat. It is noteworthy that some of our smaller birds, such as the
chickadees, can withstand a cold winter because their food supplies
are always available above ground on trees. When there is a good
supply of pine and spruce seeds, red-breasted nuthatches and
crossbills will remain through the winter in Canadian woods, but
when these birds appear abundantly in winter at southern latitudes,
it may be concluded there is a shortage of these foods in the North.
STIMULUS FOR MIGRATION
Modern views based on studies of bird behavior and physiology
indicate migration is a regular, annually induced movement,
modified by local weather conditions, but largely independent of
them. Migration is a phenomenon far too regular to be created anew
each season merely under stress of circumstances, such as need for
food; and it begins before the necessity for a change in latitude
becomes at all pressing. Swallows, nighthawks, shorebirds, and
others may start their southward movement while the summer food
supply in the North is at peak abundance. American robins and
bluebirds may leave abundant food in the South and press northward
when food supplies there are almost entirely lacking and severe cold
and storms are likely to cause their wholesale destruction. Regularity
of arrival and departure is one of the most impressive features of
migration, and since birds travel in a rather strict accordance with
the calendar, we might ask: "What phenomena, other than the
regular changes in length of day, occur with sufficient precision to
act as a stimulus for migration?"
Experimental work has abundantly demonstrated the effect of
increased light upon the growth, flowering, and fruiting of plants.
Similarly, Rowan's (1925) experiments with slate-colored juncos and
the work of numerous subsequent investigators showed, at least in
some temperate zone species of migratory birds, increasing periods
of daylight triggered sex organs to develop, fat to be deposited, and
migration restlessness to begin (King and Farner, 1963). When these
conditions develop to a certain level, the bird enters a "disposition to
migrate" and takes off for its breeding or wintering grounds. There is
reason to believe certain weather conditions influence the actual time
of departure and especially the rate of progress to the breeding area.
This explanation of the stimulus for migration may apply very
broadly to birds that winter in temperate parts of the world and nest
in the same hemisphere but fails in those birds wintering in the
tropics, where little change in length of day occurs and even
decreases during the spring in regions south of the Equator. It might
be asked: "If the lengthening day is the stimulating factor, why
should our summer birds, wintering in the tropics, ever start north?"
In addition, if daylength influences when birds are stimulated to
migrate, why should they not all leave the same locality at the same
time? Or, if weather controls the departure of birds from a given
area, should not all the migrants leave when conditions are optimal
and refrain from departing when conditions are not so? Actually, the
conditions that place a bird in a disposition to migrate are probably
the result of a combination of factors affecting different species
differently. Thus not all birds arrive at this condition at the same
It has been demonstrated experimentally that Andean sparrows,
resident in equatorial regions, come into breeding condition twice
annually entirely independent of changing light periods (Miller
1963); evidently the breeding cycle is controlled by periodic internal
stimuli. Probably northern migrants that winter in equatorial
regions and beyond have their migratory urges controlled by similar
rhythms or biological clocks. Also, no evidence suggests that the
southward migration of birds is controlled by changing periods of
light even among species such as white-crowned sparrows, for which
this is a controlling factor in the spring. The fall stimulus is probably
an innate cyclic occurrence brought on by a biological mechanism of
unknown nature (King, Barker, and Farner 1963).
It is pertinent to point out that the migratory instinct appears to be
more or less transitory and not persistent over an extended period.
Migratory birds may be delayed en route, either by natural
conditions such as unusually abundant food supplies or forcibly by
man. If detained until the end of the migratory season, migrants may
not attempt to finish the journey because they apparently lose the
migratory impulse. In the fall and early winter of 1929, abundant
food and open water caused an unusual number of mallards to arrest
their migration and remain in western Montana and northern Idaho.
Later, however, when a heavy snowfall with subzero temperatures
suddenly cut off the food supply, great numbers of the birds
subsequently starved to death; a flight of a few hours could have
carried them to a region of open water and abundant food.
WHEN BIRDS MIGRATE
One ordinarily thinks of the world of birds as sedentary during two
periods each year, at nesting time, and in winter. For individuals this
is obviously the case, but when the entire avifauna of North America
or the world is considered, it is found that at almost all periods there
are some latitudinal movements of birds. A few of these movements
reoccur year after year with calendar-like regularity. Each species,
or group of species, migrates at a particular time of the year and some
at a particular time of the day. In this section some of the interesting
differences will be discussed as to when birds migrate.
Time of Year
Some species begin their fall migrations early in July, and in other
species distinct southward movements can be detected late into the
winter. While some migrants are still traveling south, some early
spring migrants can be observed returning north through the same
locality. For example, many shorebirds start south in the early part
of July, while the goshawks, snowy owls, 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 procession 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, the northern movement may continue well into June. In
some species 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 actually start south while others of the
species are still on their way north.
A study of these facts indicates the existence of northern and
southern populations of the same species that 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 (Fig. 1). It spends the winter in southern Florida, the West
Indies, southern and eastern Mexico, Central America, and
northwestern 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. (Lines that
connect points where birds arrive at the same line are called
isochronal lines. Fig. 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 southward before the
middle of July. Then with a return 50-day trip south, the earliest
migrants from the northern areas would reach the Gulf Coast in
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.
Figure 1. Summer and winter homes of the black-and-white warbler. A very slow
migrant, these birds nesting in the northern part of the country take 50 days to cross
the breeding range. The speed of migration is shown in Fig. 2.
Many smiliar 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. Redstarts and yellow
warblers, evidently the more southern breeders, are seen returning
southward on the northern coast of South America just about the
time the earliest of those breeding in the North reach Florida on their
way to winter quarters. Examples of the Alaska race of the yellow
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.
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 magnitude of which varies with populations, species, weather,
and time of year. 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 takes the form of a bell. 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, warblers, and the like. Each of
these species in turn has its own "curve" of migration in the major
Time of Day
Because most birds appear to be creatures of daylight, it seems
remarkable that many should select the night for extended travel.
Among the many nocturnal migrants are the smaller birds such as
rails, flycatchers, orioles, most of the sparrows, the warblers, vireos,
thrushes, and shorebirds. It is common to find woods and fields on one
day almost barren of bird life and on the following day filled with
sparrows, warblers, and thrushes, which indicates the arrival of
migrants 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 a flock 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 indication
of the numbers of birds in the air at night during peaks of migration.
At such times radar observations have shown that nocturnal
migration begins about an hour after sundown, reaches a peak
shortly before midnight, and then gradually tapers off until
daybreak. Unless special curcuits are installed in radar sets, bird
echoes during peak migration periods may cover a radar screen.
It has been suggested that small birds migrate by night to avoid
their enemies. To a certain extent this may be true because the
group includes not only weak fliers, such as the rails, but also the
small song and insectivorous birds, such as wrens, small woodland
flycatchers, and other species that habitually 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 include also the snipe,
sandpipers, and plovers. Most shorebirds are usually found in the
open and are among the more powerful fliers, as some of them make
annual migratory flights over 2,000 miles nonstop across the 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 one 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 migration. If the smaller migrants were to make
protracted flights by day they would arrive at their destination at
nightfall almost exhausted, but since they are entirely daylight
feeders, they would be unable to obtain food until the following
morning. Unless reserve energy was carried in the form of fat, the
inability to feed would delay further flights and result in great
exhaustion or possibly even death should their evening arrival
coincide with cold or stormy weather. By traveling at night, they can
pause at daybreak and devote the entire period of daylight to
alternate feeding and resting. This schedule permits complete
recuperation and resumption of the journey on a subsequent evening
after sufficient energy has been restored.
The day migrants include, in addition to some of the ducks and
geese, the loons, cranes, gulls, pelicans, hawks, swallows,
nighthawks, and swifts. Soaring birds, including broad-winged
hawks, storks, and vultures, can only migrate during the day because
their mode of flight makes them dependent on up-drafts created by
heat from the sun for their long distance travels. On the other hand,
swifts and swallows feed entirely on diurnal flying insects. The
circling flocks are frequently seen in late summer feeding as they
travel while working gradually southward. Formerly, great flocks of
red-tailed, Swainson's, and rough-legged hawks could be seen
wheeling majestically across the sky in the Plains States. In the East,
good flights of broad-winged, Cooper's, and sharp-shinned hawks are
still often seen, particularly along the Appalachian ridges.
Because many species of wading and swimming birds are able to
feed at all hours, they migrate either by day or night and are not
accustomed to seek safety in concealment. Some diving birds,
including ducks that submerge when in danger, often travel over
water by day and over land at night. Strong flyers like the snow geese
can make the entire trip from their staging area in James Bay,
Canada, to the wintering grounds on the Louisiana Gulf coast in one
continuous flight. These birds are seldom shot by hunters enroute
between these two points but are often observed, when migrating, by
aircraft pilots. Graham Cooch of the Canadian Wildlife Service
tracked a flight of the blue phase of this species in 1955. The birds left
James Bay on October 17 and arrived on the Gulf coast 60 hours later
after an apparent continuous flight over the 1,700-mile route at an
average speed of 28 miles per hour. Golden plovers, likewise,
probably make the southward flight from the Arctic to the South
American coast in one giant leap. Other Arctic species on their
northward flight in the spring might prefer to fly at night in lower
altitudes, but must necessarily fly during the day at higher altitudes
because of the length of the days. Many warblers that normally fly at
night may find themselves over water at daybreak and be forced to
keep flying during the day until landfall is made.
An interesting comparison of the flights of day and night migrants
may be made through a consideration of the spring migrations of the
blackpoll warbler and the cliff swallow. Both spend the winter as
neighbors in South America, but when the impulse comes to start
northward toward their respective breeding grounds, the warblers
Figure 3. Migration of the blackpoll warbler. As the birds move northward, the iso-
chronal lines become farther apart, which indicates that the warblers move faster irith
the advance of spring. From April 30 to May 1 the average speed is about 30 miles per
day, while from May 25 to May 30 it increases to more than 200 miles.
strike straight across the Caribbean Sea to Florida (Fig. 3), while the
swallows begin their journey by a westward flight of several hundred
miles to Panama (Fig. 4). From there they move leisurely along the
western shore of the Caribbean Sea to Mexico, and, continuing to
avoid a long trip over water, go completely around the western end of
the Gulf of Mexico. This circuitous route adds more than 2,000 miles
to the journey of the swallows that nest in Nova Scotia. The question
may be asked: "Why should the swallow select a route so much longer
_ Isochronal Migration Lines
Figure 4. Migration of the cliff swallow. A day migrant that, instead of flying across
the Caribbean Sea as does the black-poll warbler (see Fig. 3), follows the coast
of Central America, where food is readily obtained.
and more roundabout than that taken by the blackpoll warbler?" The
explanation is simple. The swallow is a day migrant while the
warbler travels at night. The migration of the warbler is made up of a
series of long nocturnal flights alternated with days of rest and
feeding in favorable localities. The swallow, on the other hand, starts
its migration several weeks earlier and catches each day's ration of
flying insects during its aerial evolutions, while slowly migrating.
The 2,000 extra miles flown along the insect-teeming shores of the
Gulf of Mexico are exceeded by the great distances covered by these
birds in normal pursuit of food.
Although most of our smaller birds make their longest flights at
night, close observation shows travel is continued to some extent by
day. During the latter half of a migratory season birds may show
evidence of an overpowering desire to hasten to their breeding
grounds. At this time flocks of birds maintain a movement in the
general direction of the seasonal journey while feeding on or near the
ground. Sometimes they travel hurriedly, and while their flights
may be short, they can cover an appreciable distance in the course of a
SPEED OF FLIGHT AND MIGRATION
There is a widespread misconception among people concerning the
speed at which birds can fly. One often hears stories of birds fly ing "a
mile a minute." While undoubtedly some birds can and do attain this
speed, such cases are exceptional, and it is safe to say that, even when
pressed, few can develop an air speed of 60 miles per hour. Birds
generally have two greatly differing speeds, one being the normal
rate for ordinary purposes, and an accelerated speed for escape or
pursuit. All birds, except the heavy-bodied, small-winged species
such as auks, grebes, and other divers, have a reserve speed that may
be double the normal rate.
Although it was thought for a long time that migratory flights
were made at normal cruising speeds, Harrison (1931) and
Meinertzhagen (1955) showed that migration speeds were in between
cruising speeds and escape speeds. The theory that migrating birds
attain high speeds received encouragement from the German
ornithologist Gatke (1895) who, for many years, observed birds at the
island of Heligoland. He postulated that the bluethroat, a species of
thrush smaller than the American hermit thrush, could leave
African winter quarters at dusk and reach Heligoland at dawn; this
flight would mean a sustained speed of 200 miles per hour! He also
thought the American golden plover flew from the coast of Labrador
to Brazil in 15 hours at the tremendous speed of 250 miles per hour.
Most ornithologists now consider these conclusions to be unwarrant-
Reliable data on the speed of birds are accumulating slowly.
Accurate measurements are difficult to obtain unless the bird travels
over a measured course and wind conditions at the level of flight are
known. Several subtle factors, besides wind and pursuit, can
influence the speed of a flying bird. For instance, species that have a
courtship flight often reach their maximum speeds then. Small
woodland birds often fly faster across an open area where they might
be attacked by a bird of prey than under cover where there is less
danger. Birds in flocks generally fly faster than when flying alone. A
thermal draft may induce an almost imperceptible air movement at
the Earth's surface, but a good glider with motionless wings may
make 35 miles per hour on a current of air that is rising vertically at
less than 2 miles per hour. If the bird coasts downhill at a slight angle
in still air, it can attain a similar speed.
For sustained flight, it may be generally concluded that larger
birds fly faster than smaller birds. A common flying speed of ducks
and geese is between 40 and 50 miles per hour, but among the smaller
birds it is much less. Herons, hawks, horned larks, ravens, and
shrikes, timed with the speedometer of an automobile, have been
found to fly 22 to 28 miles per hour, whereas some of the flycatchers
fly at only 10 to 17 miles per hour. Even such fast-flying birds as the
mourning dove rarely exceed 35 miles per hour. A peregrine falcon
will have difficulty catching a pigeon during a level chase at 60 miles
per hour, but this predator can probably exceed 200 miles per hour
during a swoop from a greater height onto its prey.
The speed of migration is quite different from that attained in
forced flights for short distances. A sustained flight of 10 hours per
day would carry herons, hawks, crows, and smaller birds from 100 to
250 miles, while ducks and geese might travel as much as 400 to 500
miles in the same period (without the aid of a tail wind). Measured as
straight line distances, these journeys are impressive and indicate
birds could travel from the northern United States or even from
northern Canada to winter quarters in the West Indies, Central, or
South America in a relatively short time. It is probable that
individual birds do make flights of the length indicated and that barn
swallows seen in May on Beata Island, off the southern coast of the
Dominican Republic, may have reached that point after a nonstop
flight of 350 miles across the Caribbean Sea from the coast of
Radar has given us some of our best estimates of ground speeds for
migrating flocks, especially at night. Radar echoes, identified as
shorebirds migrating off the New England coast, moved steadily
about 45 miles per hour for several hours; songbird echoes typically
traveled around 30 miles per hour (Drury 1960). Some birds appear
to reduce flight speed in proportion to the degree of assistance or
resistance. The literature is in some disagreement on the flight speed
of birds and the influence of wind, but good radar observations
coupled with accurate measurements of winds aloft will help give us
a more accurate estimate of migrating speeds for different species
under varying wind conditions.
The intensity of migration depends on circumstances including the
need for haste. In fall the flights are more likely to be performed in a
leisurely manner, so that after a flight of a few hours the birds often
pause to feed and rest for one or several days, particularly if they find
themselves in congenial surroundings. Some indication of this is
found in the recoveries of banded birds, particularly waterfowl. If we
consider only the shortest intervals between banding in the North
and subsequent recovery in the South, it is found that usually a month
or more is taken to cover straight-line distance of a thousand miles.
For example, a black duck banded at Lake Scugog, Ontario, was
killed 12 days later at Vicksburg, Mississippi. If the bird was taken
shortly after its arrival, the record would indicate an average daily
flight of 83 miles, a distance that could have been covered in about 2
hours' flying time. Among the thousands of banding records of ducks
and geese, evidences of rapid migrations are decidedly scarce, for
with few exceptions, all thousand-mile flights have required 2 to 4
weeks or more. Among sportsmen, the blue-winged teal is well
known as a fast-flying duck and quite a few of these banded on
Canadian breeding grounds have covered 2,300 to 3,000 miles in a
30day period. Nevertheless, the majority of those that have traveled
to South America were not recovered in that region until 2 or 3
months after they were banded. Probably the fastest flight over a
long distance for one of these little ducks was one made by a young
male that traveled 3,800 miles from the delta of the Athabaska River,
northern Alberta, Canada, to Maracaibo, Venezuela, in exactly 1
month. This flight was at an average speed of 125 miles per day. A
very rapid migration speed was maintained by a lesser yellowlegs
banded at North Eastham, Cape Cod, Massachusetts, on 28 August
1935 and killed 6 days later, 1,900 miles away, at Lamentin,
Martinique, French West Indies. This bird traveled an average daily
distance of more than 316 miles.
It seems probable that most migratory journeys are performed at
little more than the normal, unforced rate of flight, as this would best
conserve the strength of the birds. Migrating birds passing
lightships and lighthouses or crossing the face of the moon have been
observed to fly without hurry or evidence of straining to attain high
speed. The speed or rate of migration would therefore depend chiefly
on the duration of flights and tail wind velocity.
The speed of migration is demonstrated by the dates of arrival,
particularly during the spring movement. The Canada goose affords
a typical example of regular but slow migration. Its advance
northward is at the same rate as the advance of the season (Fig. 5). In
fact, the isotherm of 35 F appears to be a governing factor in the
speed at which these geese move north. (An isotherm is a line that
connects points that have the same temperature at the same time.)
From an evolutionary viewpoint we might expect this. If the geese
continually advanced ahead of the 32 F isotherm, they would always
find food and water frozen and unavailable. By migrating north just
behind the advance of this isotherm, birds that breed in the far north
will find food and open water available and have as long a breeding
season as the climate will allow.
Few species perform such leisurely migrations; many wait in their
winter homes until spring is well advanced, then move rapidly to
their breeding grounds. Sometimes this advance is so rapid, late
migrants actually catch up with species that may have been pressing
slowly but steadily northward for a month or more. The following
several examples of well-known migrants illustrate this.
The grey-cheeked thrush, which winters in the Colombia-
Ecuador-Peru-Venezuela-British Guiana area, does not start its
northward journey until many other species are well on their way. It
does not appear in the United States until the last of April 25 April
near the mouth of the Mississippi and 30 April in northern Florida
(Fig. 6). A month later, or by the last week in May, the bird is seen in
northwestern Alaska. Therefore, the 4,000-mile trip from Louisiana
was made at an average distance of about 130 miles per day.
Another example of rapid migration is furnished by the yellow
Isotherm of 35F
Isochronal Migration Lines
Figure 5. Migration of the Canada goose. The northward movement keeps pace with the
progress of spring, because the advance of the isotherm of 35 F agrees with that of the
Figure 6. Isochronal migration lines of the gray-cheeked thrush, an example of rapid
migration. The distance from Louisiana to A laska is about 4, 000 miles and is covered
at an average speed of about 130 miles per day. The last part of the journey is covered
at a speed several times what it is in the Mississippi Valley.
warbler. This species winters in the Tropics and reaches New
Orleans about April 5, when the average temperature is 65 F. By
traveling north much faster than the spring season progresses, this
warbler reaches its breeding grounds in Manitoba the latter part of
May, when the average temperature is only 47 F. They encounter
progressively colder weather over their entire route and cross a strip
of country in the 15 days from May 1 1 to 25 that spring temperatures
normally take 35 days to cross. This "catching up" with spring is
habitual in species that winter south of the United States as well as in
most northern species that winter in the Gulf States. There appears
to be only six exceptions to this rule: the Canada goose, the mallard,
the pintail, the common crow, the red-winged blackbird, and the
The snow goose presents a striking example of a late but very rapid
spring migration. Most all of these geese winter in the great coastal
marshes of Louisiana, where every year over 400,000 spend the
winter and congregations of 50,000 or more may be seen grazing in
the "pastures" or flying overhead in flocks of various sizes. Their
breeding grounds are chiefly on Baffin and Southampton Islands in
the northern part of Hudson Bay where conditions of severe cold
prevail except for a few weeks each year. The birds are not
stimulated to migrate even though the season in their winter
quarters is advancing rapidly while their nesting grounds are still
covered with a heavy blanket of ice and snow. This suggests the
stimulus for spring departure is regulated by an internal
mechanism, such as development of the gonads. Accordingly, blue
geese remain in the coastal marshes until the last of March or the first
of April, when the local birds are already busily engaged in
reproduction. The flight northward is rapid, almost nonstop so far as
the United States is concerned; although the birds are sometimes
recorded in large numbers in the Mississippi Valley, eastern South
Dakota, and southeastern Manitoba, there are few records anywhere
along the route of the great flocks that winter in Louisiana. When the
birds arrive in the James Bay region, they apparently enjoy a
prolonged period of rest because they are not seen in the vicinity of
their breeding grounds until the first of June. During the first 2
weeks of that month, they pour onto the Arctic tundra by the
thousands, and each pair immediately sets about the business of
rearing a brood.
The American robin has been mentioned as a slow migrant, and, as
a species, it takes 78 days to make the 3,000-mile trip from Iowa to
Alaska, a stretch of country that is crossed by advancing spring in 68
days. In this case, however, it does not necessarily mean that
individual robins are slow. The northward movement of the species
probably depends upon the continual advance of birds from the rear,
so that the first individuals arriving in a suitable locality are the ones
that nest in that area, while the northward movement of the species is
continued by those still to come.
There is great variation in the speed of migration at different
latitudes of the broad region between the Gulf of Mexico and the
Arctic Ocean. The blackpoll warbler again furnishes an excellent
example (Fig. 3). This species winters in northwestern South
America and starts to migrate north in April. When the birds reach
the southern United States, some individuals fly northwest to the
Mississippi Valley, north to Manitoba, northwest to the Mackenzie
River, and then almost due west to western Alaska. A fairly uniform
average distance of 30 to 35 miles per day is maintained from the Gulf
to Minnesota, but a week later this species has reached the central
part of the Mackenzie Valley, and by the following week it is observed
in northwestern Alaska. During the latter part of the journey,
therefore, many individuals must average more than 200 miles per
day. Thirty days are spent traveling from Florida to southern
Minnesota, a distance of about 1,000 miles, but scarcely half that time
is used to cover the remaining 2,500 miles to Alaska. Increased speed
across western Canada to Alaska is also shown by many other birds
(Figs. 2,4,6). A study of all species traveling up the Mississippi Valley
indicates an average speed of about 23 miles per day. From southern
Minnesota to southern Manitoba 16 species maintain an average
speed of about 40 miles per day. From that point to Lake Athabaska,
12 species travel at an average speed of 72 miles per day, while 5
others travel to Great Slave Lake at 116 miles per day, and another 5
species cover 150 miles per day to reach Alaska. This change is in
correlation with a corresponding variation in the isothermal lines,
which turn northwestward west of the Great Lakes.
As has been previously indicated, the advance of spring in the
northern interior is much more rapid than in the Mississippi Valley
and on the Gulf coast. In other words, in the North spring comes with
a rush, and, during the height of migration season in Saskatchewan,
the temperature in the southern part of the Mackenzie Valley just
about equals that in the Lake Superior area, 700 miles farther south.
Such conditions, coupled with the diagonal course of the birds across
this region of fast-moving spring, exert a great influence on
migration and are probably factors in the acceleration of travel
speed. However, it should be remembered that the birds are getting
closer to the breeding season and may be stimulated to travel faster
for this reason.
Thus it has been shown that the rate of migration varies greatly
under varying circumstances. Radar investigations along the
eastern coasts of the United States and England indicate spring
migration is several miles per hour faster than in the fall. Also,
directions of migrations in spring were much less diverse than in the
fall, which suggests less time lost in passage (Tedd and Lack 1958;
Nisbet and Drury 1967a). King and Farner (1963) found the same
species put on more fat preparatory to migration in the spring. This
would give the migrants greater energy reserves for longer flights at
ALTITUDE OF FLIGHT AND MIGRATION
The factors regulating the heights of bird migration are not clear.
High-altitude flight may be used to locate familiar landmarks, fly
over fog or clouds, surmount physical barriers, gain advantage of a
following wind, or maintain a better physiological balance.
Meteorological conditions probably account for most of the high-
altitude records. Wind conditions at ground level are usually quite
different in direction and velocity than at points higher up.
In general, human estimates of bird heights are quite unreliable
except under special conditions, and these estimates will vary with
the eyesight of the observer. Lucanus (1911) found a European
sparrow hawk could be distinguished at 800 feet but disappeared
from sight at 2,800 feet. A rook (a European member of the crow
family) could be recognized at 1,000 feet but disappeared from sight
at 3,300 feet. Meinertzhagen (1955) did an interesting experiment
with an inflated model of a vulture painted black; it had a wing
expanse of 7 feet 10 inches. When released from an airplane at 4,700
feet, it was barely visible and invisible without binoculars at 5,800
feet. At 7,000 feet it was not picked up even when x!2 binoculars were
At one time students of bird migration believed normal migratory
movements took place at heights above 15,000 feet. They reasoned,
somewhat uncertainly, that flying became easier as altitude was
gained. It has now been shown, through comprehensive radar
studies, that 95 percent of the migratory movements occur at less
than 10,000 feet, and the bulk of the movements occur under 3,000
feet. However, birds can and do fly well over 15,000 feet without
apparent ill effects. The physiology of long-distance flight at high
altitudes is of great interest but can only be touched on briefly in this
Bird flight at 20,000 feet, where less than half the oxygen is present
than at sea level, is impressive if only because the work is achieved by
living muscle tissue. A Himalayan mountain climber at 16,000 feet
was rather amazed when a flock of geese flew north 2 miles over his
head honking as they went (Swan 1970). At 20,000 feet a man has a
hard time talking and running or other rapid movements are out of
the question; but those geese were probably flying at 27,000 feet and
even calling while they traveled at this tremendous height.
Accurate observations on the altitude of migratory flights is
scanty, although altimeter observations from airplanes and radar
are becoming more frequent in the literature. An example is the
report of a mallard struck by a commercial airliner at 21,000 feet
over the Nevada desert (Manville 1963). It is, of course, obvious that
some birds must cross mountain ranges during migration and attain
great altitudes. Numerous observations have come from the
Himalayas (Geroudet 1954; Swan 1970). Observers at 14,000 feet
recorded storks and cranes flying so high that they could be seen only
through field glasses. In the same area large vultures were seen
soaring at 25,000 feet and an eagle carcass was found at 26,000 feet.
The expedition to Mt. Everest in 1952 found skeletons of a pintail and
a black-tailed godwit at 16,400 feet on Khumbu Glacier (Geroudet
1954). Bar-headed geese have been observed flying over the highest
peaks (29,000+ feet) even though a 10,000-foot pass was nearby.
Probably 30 or more species regularly cross these high passes (Swan
Except to fly over high mountain ranges, birds rarely fly as high as
those traveling down the western Atlantic (Richardson 1972). Many
of these birds are making long-distance flights to eastern South
America and beyond. Therefore, flight at high altitudes in this region
is probably advantageous for them. Richardson postulated stronger
advantageous tail winds were found higher up and the cooler air
minimized evaporative water losses. This investigator found air
temperatures averaged 35 F at 10,000 feet over Nova Scotia in
September. The lower the ambient temperature, the more heat can
be lost by convection and the less water is required for cooling. Also, a
bird flying high can achieve the same range as one flying at sea level
but must cruise at a higher speed with a corresponding increase in
power output and oxygen consumption. But the increased cruising
speed results in shorter flight time and less interference from wind
Another postulate favoring the high-altitude flying theory was
that the wonderful vision of birds was their sole guidance during
migratory flights. To keep landmarks in view, birds were obliged to
fly high, particularly when crossing wide areas of water. This will be
considered in greater detail in the section, "Orientation and
Navigation," so here it will be sufficient to say that birds rely only in
part upon landmarks to guide them on migration. Also, it must be
remembered that definite physical limitations to the range of
visibility exist even under perfect atmospheric conditions. Chief of
these is the curvature of the earth's surface. Thus, if birds crossing
the Gulf of Mexico to Louisiana and Florida flew at a height of 5
miles, they would still be unable to see a third of the way across
(during daylight hours). And yet this trip is made twice each year,
much of the distance probably at night, by thousands of thrushes,
warblers, and others.
The altitude of migration depends upon the species of bird,
weather, time of day or year, and geographical features. Nocturnal
migrants, studied by radar, appear to fly at different altitudes at
different times during the night. Birds generally take off shortly
after sundown and rapidly gain maximum altitude. This peak is
maintained until around midnight, then the travelers gradually
descend until daylight. For most small birds the favored altitude
appears to be between 500 and 1,000 feet (Bellrose 1971), but radar
studies have found some nocturnal migrants (probably shorebirds)
over the ocean were at 15,000 or even 20,000 feet (Lack 1960b; Nisbet
1963b; Richardson 1972). Observations made from lighthouses and
other vantage points indicate that certain migrants commonly travel
at altitudes of a very few feet to a few hundred feet above sea or land.
Sandpipers, northern phalaropes, and various sea ducks have been
seen flying so low they were visible only as they topped a wave.
Observers stationed at lighthouses and lightships off the English
coast have similarly recorded the passage of landbirds flying just
above the surface of the water and rarely above 200 feet. During the
World Wars, broad areas in the air were under constant surveillance,
and many airplane pilots and observers took more than a casual
interest in birds. Of the several hundred records resulting from their
observations, only 36 were of birds flying above 5,000 feet and only 7
above 8,500 feet. Cranes were once recorded at an altitude of 15,000
feet, while the lapwing was the bird most frequently seen at high
levels, 8,500 feet being its greatest recorded altitude. Records of the
U.S. Civil Aeronautics Administration show that over two-thirds of
all the bird-aircraft collisions occur below 2,000 feet and practically
none occur above 6,000 feet (Williams 1950).
Recently, radar has aided greatly in determining differences in the
altitude of bird flight. Nocturnal migrants appear to fly slightly
higher, on the average, than diurnal migrants, but daytime flights
may be influenced more by cloud cover (Lack 1960a; Eastwood and
Rider 1965). Bellrose (1971) found little difference in the altitudinal
distribution of small nocturnal migrants under clear or overcast
skies. Many night migrating birds are killed each year by striking
lighthouses, television towers or other man-made illuminated
obstructions, but this does not furnish proof that low altitudes are
generally used during nocturnal flight because these accidents occur
chiefly in foggy weather. Under such conditions, migrating birds
seem to be attracted to and confused by lights. Seabirds, such as
loons, eiders, and scoters, generally fly very low over the water but
gain altitude when land is crossed. The reverse is true for landbirds
(Dorst 1963; Bergman and Donner 1964; Eastwood and Rider 1965).
There may be a seasonal difference in the altitude of migration, but
the evidence is conflicting. Radar echoes studied by Bellrose and
Graber in Illinois (1963) showed fall migrants flew higher than
spring migrants. They speculated this difference was related to the
winds during the fall being more favorable for southerly migration
at higher altitudes, while winds at these altitudes in the spring would
be less favorable for northerly migration. Eastwood and Rider (1965)
studied seasonal migration patterns in England and found the
reverse to be true. They suggested one reason for this seasonal
difference was that flocks of fall migrants included many young
birds whose flight capabilities are inferior to adults and
consequently are unable to achieve the higher altitudes in the fall.
SEGREGATION DURING MIGRATION
By Individuals or Groups of Species
During the height of northward movement in spring, the woods
and thickets may suddenly be filled with several species of wood
warblers, thrushes, sparrows, flycatchers, and other birds. It is
natural to conclude they traveled together and arrived simultaneous-
ly. Probably they did, but such combined migration is by no means
the rule for all species.
As a group, the wood warblers probably travel more in mixed
companies than do any other single family of North American birds.
In spring and fall, the flocks are likely to be made up of the adults and
young of several species. Sometimes swallows, sparrows, blackbirds,
and some of the shorebirds also migrate in mixed flocks. In the fall,
great flocks of blackbirds frequently sweep south across the Plains
States, with common grackles, red-winged blackbirds, yellow-
headed blackbirds, and Brewer's blackbirds included in the same
On the other hand many species keep strictly to themselves. It
would be difficult for any other kind of bird to keep company with the
rapid movements of the chimney swift. Besides flight speed, feeding
habits or roosting preferences can be so individual as to make
traveling with other species incompatible. Nighthawks also fly in
separate companies, as do crows, waxwings, crossbills, bobolinks, and
kingbirds. Occasionally, a flock of ducks will be observed to contain
several species, but generally when they are actually migrating,
individuals of each species separate and travel with others of their
Although different species generally do not migrate together, we
often find many species passing through an area at the same time. If
the different kinds of birds observed in a specific area are counted
every day throughout the entire migration season, this count often
rises and falls much like the bell-shaped curve exhibited when the
number of individuals of a given species are counted through the
same time period. Figure 7 shows two peaks in the number of species
passing through the desert at the north end of the Gulf of Eilat
(=Akaba) in the Red Sea. These two peaks happen to coincide with
peaks in the numbers of individuals (mostly from the order of
perching birds) traveling through the area. Therefore, in the latter
part of March and again in April, one notices not only more birds in
the area but also more different kinds.
Closely related species or species that eat the same food organisms
are not often found migrating through the same area at the same
time. Ornithologists call this species replacement. In North America,
peaks in the migration of the five kinds of spotted thrushes generally
do not coincide. Dates of departure in these species have evolved so all
the individuals of these closely related birds do not converge on one
area at the same time and subsequently exhaust the food supply. By
selection of staggered peak migration dates, evolution has distrib-
uted the members of this family more or less evenly throughout the
entire season. Likewise, in the eastern Mediterranean area, we find a
similar situtation in spring migration for three closely related
buntings; Cretzschmar's bunting comes through first, followed a few
weeks later by the Ortolan bunting and, at the tail end of the
migration period, the blackheaded bunting appears (Fig. 8).
The adults of most birds leave the young when they are grown. This
gives the parents an opportunity to rest and renew their plumage
before starting for winter quarters. The young are likely to move
south together ahead of their parents. This has been documented in a
number of species including our mourning dove, the common swift of
Europe, and storks. Mueller and Berger (1967) found an age-specific
migration pattern in sharp-shinned hawks passing through
Wisconsin. The immatures were much in evidence during
mid-September while the adults came through a month later. Far to
Figure 7. Average number of species captured daily in mist nets during spring migra-
tion at Eilat, Israel, in 1968. The number of species passing through an area on
migration will rise and fall similar to the number of birds counted in the area. In this
case two major movements came through about 1 month apart.
the south in the Antarctic, young Adelie penguins depart for
northern wintering grounds much earlier than adults.
In a few species, adults depart south before the young. Adult
golden plovers, Hudsonian godwits, and probably most of the Arctic
breeding shorebirds leave the young as soon as they are capable of
caring for themselves and set out for South America ahead of the
juveniles. Likewise, data for the least flycatcher indicate adults
migrate before the young, but Johnson (1963) did not find this
segregation in the Hammond's flycatcher. In Europe, adult
Average Number Captured Daily
Figure 8. Average number of three species of buntings captured daily in mist nets
during spring migration atEilat, Israel, in 1968. Closely related species that migrate
through the same area often appear at different times. Thus species that may eat the
same foods do not compete with each other.
red-backed shrikes are known to migrate ahead of their young.
In contrast to this loss of parental concern, geese, swans, and cranes
remain in family groups throughout migration. The parent birds
undergo a wing molt that renders them flightless during the period
of growth of their young so that both the adults and immatures
acquire their flight capabilities at the same time and are able to start
south together. Large flocks of Canada geese, for example, are
composed of many families banded together. When these flocks
separate into small V-shaped units it is probably correct to assume an
old goose or gander is leading the family. After female ducks start to
incubate their eggs, the males of most species of ducks flock by
themselves and remain together until fall. When segregation of the
sexes such as this occurs the young birds often accompany their
mothers south. Murray and Jehl (1964) concluded from mist-netting
many thousands of migrant passerines at Island Beach, New Jersey,
that adults and juveniles travel at approximately the same time.
Males and females of some species may migrate either
simultaneously or separately. In the latter case it is usually the males,
rarely the females, that arrive first. Sometimes great flocks of male
red-winged blackbirds reach a locality several days before any
females; this is particularly the rule in spring. The first robins are
usually found to be males, as are also the first song sparrows,
rose-breasted grosbeaks, and scarlet tanagers. In Europe, the three
buntings mentioned previously are also segregated as to sex during
migration. Figure 8 shows two prominant peaks for both the
Cretzschmar's and Ortolan buntings; during passage the first peak
was primarily males while the second peak consisted mostly of
females. This early arrival of males on the breeding grounds is
associated with territorial possession whereby the male selects the
area where it intends to breed and each individual attempts to
protect a definite territory from trespass by other males of his own
kind, while announcing his presence to rival males and later arriving
females by song or other display. The female then selects the site
where she wishes to nest. The long-billed marsh wren is a noteworthy
example; the males may enthusiastically build several nests before
the females arrive. In the fall, common and king eiders are sexually
segregated during migration. During July, flocks crossing Point
Barrow are composed almost entirely of males, while after the
middle of August the flocks are almost all females (Thompson and
Person 1963). In the Chicago area, Annan (1962) reported that some
males, such as the hermit thrush, Swainson's thrush, gray-cheeked
thrush, and veery, arrive before any females and predominate
during the first week of occurrence.
In a few species the males and females apparently arrive at the
breeding grounds together and proceed at once to nest building. In
fact, among shorebirds, ducks, and geese, courtship and mating
often takes place in whole or in part while the birds are in the South or
on their way north, so that when they arrive at the northern nesting
grounds they are paired and ready to proceed at once with raising
their families. Mallards and black ducks may be observed in pairs as
early as December, the female leading and the male following when
they take flight. Naturally, these mated pairs migrate north in
company, and it was largely to protect such pairings that duck
shooting in spring was abolished by Federal law.
In the coastal subspecies of the western flycatcher, the sexes
appear to migrate in synchrony during the spring in contrast to
migration of Hammond's flycatcher in which the adult males usually
precede the females (Johnson 1973). Both sexes of the common
blackcap of Europe appear to migrate together at least across the
eastern end of the Mediterranean during the spring (Fig. 9).
Figure 9. Numbers of male and female blackcaps captured daily in mist nets during
spring migration at Eilat, Israel, in 1968. At this point in their migration the sexes
are passing through the area at the same time. In other species (e.g., the buntings in
Fig. 8), the males often preceed the females.
By Kinds of Flocks
Migratory flights are frequently accomplished in close flock
formation, as with shorebirds, blackbirds, waxwings, and especially
some of the buntings, longspurs, juncos, and tree sparrows. Other
species maintain a very loose flock formation; examples are turkey
vultures, hawks, swifts, blue jays, swallows, warblers, and bluebirds.
Still others, the grebes, snowy owls, winter wrens, shrikes, and belted
kingfishers, ordinarily travel alone, and when several are found in
close proximity it is an indication they have been drawn together by
unusual conditions, such as abundant food.
Just as flocking among resident birds provides group protection
against predation, flocking in migration greatly facilitates the
attainment of destination (Pettingill 1970). The V-shaped flocks often
associated with Canada geese have a definite energy conserving
function by creating favorable air currents for every member of the
flock but the leader; when the leader becomes tired, it will often
change places with a member behind. Night migrating flocks
generally fly in looser formations than do day migrating flocks.
WHERE BIRDS MIGRATE
Migration by Populations Within Species
Both length and duration of migratory journeys vary greatly
between families, species, or populations within a species. Bobwhite,
western quails, cardinals, Carolina wrens, and probably some of the
titmice and woodpeckers are apparently almost or entirely
nonmigratory. These species may live out their entire existence
without going more than 10 miles from the nest where they were
Many song sparrows, meadowlarks, blue jays, and other species
make such short migrations that the movement is difficult to detect
because individuals, possibly not the same ones, may be found in one
area throughout the year while other individuals that move south
may be replaced by individuals from the north. Information on
different movements of this type, within a species, can be gained by
observing birds marked with numbered bands, colored materials, or
identification of racially distinct museum specimens.
The American robin is a good example of this type of movement.
This species occurs in the southern United States throughout the
year, but in Canada and Alaska only during the summer. Its
movements are readily ascertained from study specimens. The
breeding robin of the southeastern states is the southern race. In
autumn most of its more northern nesters, such as those from
Maryland and Virginia move into the southern part of the breeding
range or slightly farther south. At about the same time the northern
American robin moves south and winters throughout the breeding
and wintering range of its smaller and paler southern relative. Thus
there is complete overlap of wintering ranges of northern and
southern American robin populations, although some individuals of
the northern race winter in areas vacated earlier by the southern
Among many migratory species there is considerable variation
among individuals and populations with respect to distances moved.
Certain populations may be quite sedentary while others are strongly
migratory, and certain individuals of the same population can be
more migratory than others. For example, red-winged blackbirds
nesting on the Gulf Coast are practically sedentary, but in winter
they are joined by other subspecies that nest as far north as the
Mackenzie Valley. In certain populations of the song sparrow and
other species, males remain all year on their northern breeding
grounds while the females and young migrate south.
Several species containing more than one distinguishable
population exhibit "leap-frog" migration patterns. The familiar
eastern fox sparrow breeds from northeastern Manitoba to
Labrador, but during the winter it is found concentrated in the
southeastern part of the United States. On the west coast of the
continent, however, a study of museum specimens by Swarth (1920),
indicated six subspecies of this bird breeding in rather sharply
delimited ranges extending from Puget Sound and Vancouver Island
to Unimak Island, at the end of the Alaskan Peninsula. One of these
subspecies, known as the sooty fox sparrow, breeds from the Puget
Sound-Vancouver Island area northward along part of the coast of
British Columbia. It hardly migrates at all, while the other races,
nesting on the coast of Alaska, are found in winter far to the south in
Oregon and California. Although much overlap exists, the races
breeding farthest north generally tend to winter farthest south. This
illustrates a tendency for those populations forced to migrate to pass
over those subspecies so favorably located as to be almost sedentary.
If the northern birds settled for the winter along with the sedentary
population, winter requirements may not be as sufficient as in the
unoccupied areas farther south (Fig. 10). Therefore, natural selection
has insured the different populations will survive the winter by
separating the subspecies into different wintering areas.
Another example of this "leap-frog" migration is illustrated by the
common yellowthroat of the Atlantic coast. Birds occupying the most
southern part of the general range are almost nonmigratory and
reside throughout the year in Florida, whereas the population that
breeds as far north as Newfoundland goes to the West Indies for the
winter. Thus the northern population literally "jumps" over the home
of the southern relatives during migratory journeys.
The palm warbler breeds from Nova Scotia and Maine west and
northwest to southern Mackenzie. The species has been separated
into two subspecies: those breeding in the interior of Canada and
those breeding in northeastern United States and Canada. The
northwestern subspecies makes a 3,000-mile journey from Great
Slave Lake to Cuba and passes through the Gulf States early in
October. After the bulk of these birds have passed, the eastern
subspecies, whose migratory journey is about half as long, drifts
slowly into the Gulf Coast region and remains for the winter.
Fall Flights Not Far South of Breeding Range
Some species have extensive summer ranges (e.g., the pine
warbler, rock wren, field sparrow, loggerhead shrike, and
blackheaded grosbeak) and concentrate during the winter season in
the southern part of the breeding range or occupy additional
territory only a short distance farther south. The entire species may
thus be confined within a restricted area during winter, but with the
return of warmer weather, the species spreads out to reoccupy the
much larger summer range.
Many species, including the tree sparrow, snow bunting, and
Lapland longspur, nest in the far north and winter in the eastern
Winter and Breeding Range
1 2 3
Figure 10. Migration of Pacific coast forms of the fox sparrow. The breeding ranges of
the different races are encircled by solid lines, while the winter ranges are dotted. The
numbers indicate the areas used by the different subspecies as follows: L Shumagin
fox sparrow; 2. Kodiakfox sparrow; 3. Valdezfox sparrow; 4. Yakutatfox sparrow;
5. Townsendfox sparrow; 6. sooty fox sparrow (After Swarth 1920).
United States, while others, including the vesper and chipping
sparrows, common grackle, red-winged blackbird, eastern bluebird,
American woodcock, and several species of ducks, nest much farther
south in the United States and Canada and move south a relatively
short distance for the winter to areas along the Gulf of Mexico. In a
few of the more hardy species, individuals may linger in protected
places well within reach of severe cold. The common snipe, for
example, is frequently found during subzero weather in parts of the
Rocky Mountain region where warm springs assure a food supply.
More than 100 summer birds leave the United States entirely and
spend the winter in the West Indies, Central America, or South
America. For example, the Cape May warbler breeds from northern
New England, northern Michigan, and northern Minnesota, north to
New Brunswick, Nova Scotia, and nearly to Great Slave Lake. In
winter it is concentrated chiefly in the West Indies on the island of
Long Distance Migration
Some of the common summer residents of North America are not
content with a trip to northern tropical areas of the West Indies and
Central America, but push on across the Equator and finally come to
rest for the winter in Patagonia or the pampas of Argentina. Species
such as nighthawks, some barn swallows, cliff swallows, and a few
thrushes may occupy the same general winter quarters in Brazil, but
other nighthawks and barn swallows go farther south. Of all North
American landbirds these species probably travel the farthest; they
are found north in summer to the Yukon Territory and Alaska, and
south in winter to Argentina, 7,000 miles away. Such seasonal flights
are exceeded in length, however, by the remarkable journeys of
several species of shorebirds including white-rumped and Baird's
sandpipers, greater yellowlegs, turnstones, red knots, and sander-
lings. In this group, 19 species breed north of the Arctic Circle and
winter in South America; six of these go as far south as Patagonia, a
distance of over 8,000 miles.
The Arctic tern is the champion "globe trotter" and long-distance
flier (Fig. 11). Its name "Arctic" is well earned, as its breeding range
is circumpolar and it nests as far north as the land extends in North
America. The first nest found in this region was only 7-1/2 (518
miles) from the North Pole and contained a downy chick surrounded
by a wall of newly fallen snow scooped out by the parent. In North
America the Arctic tern breeds south in the interior to Great Slave
Lake, and on the Atlantic coast to Massachusetts. After the young are
grown, the Arctic terns disappear from their North American
breeding grounds and turn up a few months later in the Antarctic
region, 1 1 ,000 miles away. For a long time the route followed by these
hardy fliers was a complete mystery; although a few scattered
individuals have been noted south as far as Long Island in the United
States, the species is otherwise practically unknown along the
Atlantic coasts of North America and northern South America. It is,
Figure 11. Distribution and migration of arctic terns. The route indicated for this bird
is unique, because no other species is known to breed abundantly in North America
and to cross the Atlantic Ocean to and from the Old World. The extreme summer and
winter homes are 11,000 miles apart.
however, known as a migrant on the west coast of Europe and Africa.
By means of numbered bands, a picture disclosed what is apparently
not only the longest, but also one of the most remarkable migratory
journeys (Austin 1928).
Few other animals in the world enjoy as many hours of daylight as
the Arctic tern. For these birds, the sun never sets during the nesting
season in the northern part of the range, and during their winter
sojourn to the south, daylight is continuous as well. In other months of
the year considerably more daylight than darkness is encountered.
ORIENTATION AND NAVIGATION
There probably is no single aspect of the entire subject of bird
migration that increases our admiration so much as the unerring
certainty with which birds cover thousands of miles of land and
water to come to rest in exactly the same spot where they spent the
previous summer or winter. Records from birds marked with
numbered bands offer abundant proof that the same individuals of
many species will return again and again to identical nesting or
winter feeding sites.
This ability to travel with precision over seemingly featureless
stretches of land or water is not limited to birds but is likewise
possessed by certain mammals, reptiles, fishes, and insects; the well-
known migrations of salmon and eels are notable examples.
For an animal to return to a specific spot after a lengthy migration,
it must use true navigation to get there. That is, it needs to not only
travel in a given compass heading and know where it is at any given
time so the course may be altered when necessary but also be able to
recognize its goal when it has arrived. It is dangerous to generalize on
the means of orientation and navigation in migration; different
groups of birds with different modes of existence have evolved
different means of finding their way from one place to another
(Pettingill 1970). We are only beginning to realize the complexities
involved in the many modes of bird orientation and navigation. All
we can do in this section is present a brief summary of some of the
more important principles involved and the studies that have
enhanced our knowledge in the area.
Ability to follow a more or less definite course to a definite goal is
evidently part of an inherited faculty. Both the direction and the goal
must have been implanted in the bird's genetic code when the
particular population became established at its present location. The
theory is sometimes advanced that older and more experienced birds
lead the way and thereby show the route to their younger
companions. This explanation may be acceptable for some species
such as geese, swans, and cranes because they stay in family groups,
but not for species in which adults and young are known to migrate at
different times, especially when young migrate ahead of the adults.
As indicated in a previous section on segregation, many North
American shorebirds as well as the cuckoos of New Zealand do this.
An inherited response to its surroundings, with a definite sense of the
goal to be reached and the direction to be followed, must be attributed
to these latter birds.
It is well known that birds possess wonderful vision. If they also
have retentive memories subsequent trips over the route may well be
steered in part by recognizable landmarks. Arguments against the
theory of landmark memory are chiefly that unescorted young birds,
without previous experience, can find their way to the winter
quarters of their species, even if the wintering area has a radically
different landscape and vegetation than the breeding grounds.
Experimental findings and field observations indicate landmarks
are used in navigation by certain birds, but the degree of use varies
considerably among the species (Bellrose 1972a).
To a land-dweller traveling the ocean, the vast expanse may seem
featureless but the reverse may be true for a seabird blown over land
by a storm. In the latter situation the differences in vegetation and
topography "obvious" to land-dwellers are completely foreign to a
seabird as it has had little previous experience to help interpret these
"strange objects." Griffin and Hock (1949) observed the flight
behavior of gannets displaced far inland away from their nests. The
bird appeared to search randomly until the coastline was met, then
the fliers pursued a much more direct course home. Herring gulls,
displaced about 250 miles from their nest in 2 consecutive years,
returned the second year in one-sixth the time required the first year
(Griffin 1943). To birds such as gannets, albatrosses, and
shearwaters, which spend almost their entire lives traveling
thousands of miles at sea and return to very specific nesting areas, the
"featureless ocean expanses" are probably very rich in visual cues. It
is difficult to believe a bird dependent on the sea for its livelihood
cannot help but be aware of wave direction, islands, reefs, atolls,
concentrations of floating flotsam, organisms, currents, clouds over
islands, fog belts, etc.
Much migration takes place at night and great stretches of the
open sea are crossed to reach destinations. Nights are rarely so dark
that all terrestrial objects are totally obscured, and features such as
coastlines and rivers are just those that are most likely to be seen in
the faintest light, particularly by the acute vision of birds from their
aerial points of observation. Even if terrestrial objects are completely
obscured on a very dark night, the migrants are still able to assess
their surroundings during the day before starting out again.
Some birds, especially colonial seabirds, seem to be able to fly
unerringly through the densest fog, particularly in the vicinity of
their nest site. Members of the Biological Survey, proceeding by
steamer through a dense fog from the island of Unalaska to Bogoslof
Island in the Bering Sea, recorded flocks of murres, returning to
Bogoslof after quests for food. The birds broke through the wall of fog
astern, flew by the vessel, and disappeared into the mists ahead on the
same course as the ship. On the other hand, radar observations of
migrating birds have indicated strong directional movements on
clear nights but often completely random movements in heavily
overcast or stormy weather. Possibly some birds can perceive the
position of the sun through an overcast as honey bees are known to do.
It is less likely the stars could be detected through overcast at night.
Careful studies have been made on the homing instinct in various
seabirds such as Laysan albatrosses, Manx shearwaters, and several
tropical species of terns. Sooty and noddy terns reach their most
northern breeding point on the Dry Tortugas, off the southwest coast
of Florida. They are not known to wander any appreciable distance
farther north. Displaced breeding birds returned to their nests on the
Dry Tortugas after they had been taken on board ship, confined in
cages below decks, and carried northward 400 to 800 miles before
being released in a region where they had had no previous
experience. Likewise, Laysan albatrosses and Manx shearwaters
have returned over 3,000 miles in similar homing experiments.
Possibly the "homing instinct," as shown by pigeons, terns,
shearwaters, albatrosses, and by the frigatebirds trained as message
carriers in the South Pacific, may not be identical with the sense of
perceptive orientation that figures in the flights of migratory birds.
Nevertheless, it seems closely akin and is probably governed by the
same mechanisms. There are good reasons to assume that once we
know the processes governing displaced homing we will know, in
general, how birds navigate; this question is still far from being
answered (Wallraff 1967).
Some students have leaned toward the possible existence of a
"magnetic sense" as being the important factor in the power of
geographical orientation. The theory was conceived as early as 1855
and reported in 1882 by Viguier. Investigations of this have been
conducted by Yeagley (1947) and Gordon (1948) with contradictory
results. In 1951, Yeagley incorporated the idea that sensitivity to the
effect of the earth's rotary motion through the vertical component of
the magnetic field is the means of orientation. The basic question
asked of the theory is: "Can birds detect such minute differences in
the earth's magnetic field and can these forces affect bird behavior?"
Attempts to demonstrate the effect of radio waves on the
navigational ability of birds have produced contradictory results. In
some of these tests, homing pigeons released near broadcasting
stations have appeared to be hopelessly confused, whereas in others,
apparently conducted in the same manner, no effects could be
discerned. Before sensitivity of birds to electromagnetic stimuli of
any kind can be accepted or rejected, much additional experimental
work is necessary.
Human navigators have used the heavenly bodies in determining
their course and position for centuries. It would not be surprising
then to find other long-distance travelers using the same method. One
of the most constant visual cues a migrating bird could use would be
the sun's or moon's path and the location of the stars.
Some of the more recent experimental work on bird navigation has
been with astronomical (sun) and celestial (star) directional clues.
Studies by Kramer, Sauer, and others have indicated a phenomenal
inherited ability in birds to use the position of the sun by day and the
stars by night to chart their courses. This involves an intricate
compensation for daily, seasonal, and geographical changes in the
positions of these heavenly bodies. Kramer (1957, 1961) placed
diurnal migrants in circular cages and "changed" the position of the
sun with mirrors. The birds shifted their position to compensate for
these changes. Sauer (1957, 1958), in a fascinating study with
nocturnal migrant warblers, placed birds in a round cage open to the
sky. These birds oriented in the normal direction for that locality and
time of year. He next placed the cage and birds in a planetarium and
projected overhead the night sky star patterns for different seasons
and localities. The familiar star pattern produced a normal
orientation but an unfamiliar sky caused confusion and complete
disorientation. These experiments, begun in Germany, are still
continuing in other countries with other species. Emlen (1969) used
photoperiod manipulation to change the physiological states of
spring and fall migratory readiness in indigo buntings. Half the
sample of birds were in breeding condition whereas the other half
were already past the reproductive stage even though it was spring
"outside." When these birds were subjected to a spring star pattern in
a planetarium, the birds in spring condition oriented northward but
those in autumnal condition oriented southward. Although some
results have been negative, by and large the evidence supports the
original findings that the sun and stars are visual "landmarks" used
by at least some birds as well as bees and probably many other
creatures in finding their way home as well as to their winter and
In conclusion, then, we can say this about bird orientation and
navigation: 1) many cues are available to birds for migratory
guidance and one or several of these may be used by any migrant; 2)
different species and groups of birds use different cues, depending on
their migration traits; 3) visual cues probably play a predominant
role in migration (radar studies have indicated that some birds can
maintain their orientation even under completely overcast nights,
although they usually become disoriented under such conditions);
and 4) long-distance migrants and pelagic species have a much
higher developed sense of orientation than those species that migrate
only short distances or not at all.
INFLUENCE OF WEATHER
It is thought by some that the weather has little to do with the time
of arrival of migratory birds. It is assumed that if the bird is
physiologically prepared for migration it departs, irrespective of the
weather. Even if this were the case, weather can influence the
progress of migration by not only controlling the advance of the
seasons but also by helping, hindering, or even stopping bird flight
Some scientists believe that birds not only avoid bad weather at the
start of a journey but usually finish the journey in good weather
(Nesbit and Drury 1967b). Contrary to what many observers believe,
the arrival of birds in an area, whether they stop or continue on, is
more often controlled by the weather at the point of departure than at
the point of arrival. During the peak of migration, suitable weather
may occur at an observation site, but strong migratory movements
may be arrested before the birds arrive there because the weather
was not suitable at the point of departure or somewhere in between.
In addition, if there is good weather at the point of departure as well
as farther down the migration route, the migrants, once air-borne in
a favorable weather pattern, may continue on right over an expectant
observer and the whole flight will be missed. Nesbet and Drury's
(1967b) radar study on air-gound comparisons found, with few
exceptions, ground observers missed the largest movements observed
on radar. Observation of a large wave of arrivals indicated migrants
had been stopped by a meteorological barrier, and people were
actually not reporting maximum migration but an interruption to
migration. Therefore, when migration is proceeding normally under
safe conditions, very little movement is visible to the ground observer
but a large arrival of birds on the ground often indicates something is
not in order and the migrants have been forced to stop for one reason
The question is frequently asked: "How can I identify weather
conditions suitable or unsuitable for migration?" It is almost
impossible to discuss separately the effects of different weather
factors on migration because barometric pressure, temperature,
wind, and other meteorological phenomena are very closely related.
On the North American continent, air masses generally proceed
about 600 miles per day from the west to the east. These air masses
vary in pressure, temperature, humidity, and wind. The wind within
these masses travels in either a clockwise (anticyclonic) or
counterclockwise (cyclonic) direction. Cyclonic air masses contain
relatively moist warm air with low barometric pressure centers and
are designated "lows"; anticyclonic air masses are characterized by
dry cool air with high barometric pressure areas and are called
"highs." Where these air masses meet, a "front" is formed, and the
rapidity with which this front moves through an area depends on the
temperature and pressure gradient on either side of the front.
An understanding of frontal systems, with their associated wind,
temperature and humidity, is one of the keys to understanding when
birds migrate. You must not only watch the fronts in your area but
the progress of nearby air masses as well because the birds migrating
through your area have started their journey to the north or south of
you depending on the season. The weather conditions at point of
departure will dictate if and when birds will be passing through your
area in the near future.
During fall migration, the best passage of migrants usually occurs
2 days after a cold front has gone through. That is, the low has passed
and it is being followed by a high characterized by dropping
temperatures, a rising barometer, and clearing skies. The 24 hours
just after a low has passed are not always conducive to a good passage
of birds because winds are often too strong and turbulent in the
trough between the two air masses. Hochbaum (1955) correlated
mass movements of ducks through the prairies with weather systems
Figure 12. A hypothetical weather system that could be ideal for mass migrations of
waterfowl in the fall. The strong southerly flow of air created by counter-clockwise
winds about the lows and the clockwise rotation of air about the highs, aids the rapid
movement of waterfowl from their breeding grounds in the Canadian prairies to
wintering areas in southern United States.
and noted the combination of weather conditions described above
was ideal for mass migrations of ducks during November. During
this period, observers at Delta, Manitoba, south to Louisiana
recorded a tremendous flight of ducks as the proper conditions of
barometric pressure, temperature, wind, and cloud cover passed
across the central United States and Canada. An example of the type
of weather system that is often associated with mass movements is
illustrated in Fig. 12.
Records of lapwings on Newfoundland and the Gulf of St.
Lawrence appear to be the result of a particular series of
meteorological events (Bagg 1967). The lapwing is a European
species rarely found in the New World. If cold air moves into western
Europe from the east, lapwings move westward into England, Wales,
and Ireland. Occasionally, the development of an anomalous weather
pattern over the North Atlantic including an elongated low from
Europe to eastern Canada causes some birds to be literally "blown" in
the counter-clockwise airstream across the Atlantic to the Gulf of St.
During spring migration, weather conditions conducive to strong
movements of birds are somewhat the opposite from those in the fall.
Migrants will move north on the warm sector of an incoming low.
When a high pressure area has just passed, the influx of warm moist
tropical air is extended and intensified (Bagg et al. 1950). However,
during this time, cloudiness and rain associated with the low may
curtail migration or squeeze it into a narrow period proceeding along
the warm front. If a fast moving cold front approaches from the
northwest, the rapid movement of migrants will be sharply curtailed
or even grounded until more favorable conditions occur.
The incessant crescendo note of the ovenbird is ordinarily
associated with the full verdure of May woods, but this bird has been
known to reach its breeding grounds in a snowstorm, and the records
of its arrival in southern Minnesota show a temperature variation
from near freezing to full summer warmth. Temperatures at arrival
of several other common birds vary from 14 between highest and
lowest temperatures to 37, the average variation being about 24.
North American species spending the winter months in tropical
latitudes experience no marked changes in temperature conditions
from November to March or April, yet frequently they will start the
northward movement in January or February. This is in obedience to
physiological promptings and has no relation to the prevailing
weather conditions. For migratory birds the winter season is a period
of rest, a time when they have no cares other than those associated
with the daily search for food or escape from their natural enemies.
Their migrations, however, are a vital part of their life cycles, which
have become so well adjusted that the seasons of travel correspond in
general with the major seasonal changes on their breeding grounds.
With the approach of spring, therefore, the reproductive impulse
awakens, and each individual bird is irresistibly impelled to start the
journey that ends in its summer home.
In other words, the evidence indicates the urge to migrate is so
innate within a species or population that the individuals move north
in spring when the average weather is not unendurable. The word
"average" must be emphasized since it appears the migrations of
birds have evolved in synchrony with average climatic conditions.
More northern nesting populations of species such as American
robins and savannah sparrows, timed to arrive on their breeding
ground when the weather is suitable, pass through areas where their
more southern kin are already nesting. The hardy species travel
early, fearless of the blasts of retreating winter, while the more
delicate kinds come later when there is less danger of encountering
prolonged periods of inclement weather. Some of the hardy birds
pause in favorable areas and allow the spring season to advance.
Then, by rapid travel they again overtake it, or, as sometimes
happens, they actually outstrip it. Occasionally this results in some
hardship, but rarely in the destruction of large numbers of
individuals after arrival. Cases are known where early migrating
bluebirds have been overwhelmed by late winter storms.
Nevertheless, if such unfavorable conditions are not prolonged, no
serious effect on the species is noted. The soundness of the bird's
instincts is evidenced by the fact that natural catastrophes, great
though they may be, do not permanently diminish the avian
The spring flight of migrants, if interrupted by cold north winds, is
resumed when weather conditions again become favorable, and it is
probable that all instances of arrival in stormy weather can be
explained on the theory that the flight was begun while the weather
was auspicious. Even though major movements of migrants in spring
generally coincide with periods of warm weather and southerly
winds, observations on the beginning of nocturnal spring flights
from the coast of Louisiana failed to note any inhibiting factor other
than hard rain (Gauthreaux 1971).
Radar studies have indicated that migrant birds possess an
amazing understanding of wind patterns (Bellrose 1967). Birds can
recognize many characteristics and select for favorable patterns.
Head winds are as unfavorable to migration as is rain or snow
because they greatly increase the labor of flight and cut down the
speed of cross-country travel. If such winds have a particularly high
velocity, they may force down the weaker travelers, and when this
happens over water, large numbers of birds are lost. Moderate tail winds
and cross or quartering breezes appear to offer the best conditions for the
flight of migrants. Richardson (1971) found migrants traveling in
different directions at different altitudes, but each group of birds
was aided by a following wind. Thus we might expect natural
selection to operate in favor of those birds that could recognize and
respond to favorable wind patterns because it would reduce energy
consumption and flight time on long-distance flights (Hassler et al.
Soaring birds such as hawks, vultures, and storks are very
dependent on proper wind conditions for migration. In the fall, often
the best day to observe hawk migration in the eastern United States
is on the second day after a cold front has passed providing there are
steady northwest to west winds and a sunny day for production of
thermals (Pettingill 1962). Considerable drifting may be observed in
this group of birds because they are literally carried along by the
wind or glide from one thermal to the next. Haugh and Cade (1966)
found most hawks migrated around Lake Ontario when winds were
10 to 25 miles per hour, but, if the wind exceeded 35 miles per hour,
most hawk migration stopped.
In conclusion then, we can say that the weather may be the impetus
for migration for many species, but it cannot stimulate a bird to
migrate unless it is physiologically prepared. Arrivals on the ground
are not necessarily indicative of the number of birds passing
overhead. During the fall, peak migrations usually follow the
passage of a cold front when the temperature is falling, the
barometer is rising, winds are from the west or northwest, and the
sky is clearing. In the spring, most migrants proceed north in the
warm sector of a low when winds are southerly, warm, and moist, but
rain, fog, or snow will often curtail the passage of migrants or
prevent the initiation of a migration. Evolution of migratory
behavior has probably resulted from the survival of birds capable of
selecting those wind conditions, which reduce flight time and energy
consumption, during their passage.
INFLUENCE OF TOPOGRAPHY
The relation of the world's land masses to each other and the
distribution and association of biotypes within these land masses
influence the direction birds migrate. Topography may aid, hinder,
or prevent the progress of a migrant depending on the bird's particular
requirements. Old World migrants must contend with east-west
tending mountain ranges and deserts, whereas New World travelers
can proceed north and south across a landscape with its major
mountain ranges and river systems oriented in the same direction as
the birds migrate.
When a distinct feature in the landscape, such as borders between
fields and forests, rivers, mountain ridges, desert rims, or
peninsulas, appears to influence migratory travel, we call these
formations "guiding lines," "diversion-lines," "leading lines," or in
German, "Leitlinie." It is an observed fact that some birds in a
migratory movement alter their course to travel along a leading line,
but whether this feature in the landscape caused the migrants to
change their course is only theory (Thomson 1960). Besides
topography, many other factors can influence this type of flight
behavior including weather, wind speed and direction, time of day,
species, age, and experience of the bird (Murray 1964).
Large bodies of water constitute real barriers to soaring birds
dependent on thermals and air currents. Good examples of these
barriers include the Mediterranean Sea between Europe and Africa
and the Great Lakes in North America. Because these water areas do
not create good thermals (generally a warm surface, such as a large
field on a sunny day, is needed to create the necessary rising air
currents for thermals to form) for birds to soar on, migrants are
forced to travel around them on updrafts created where land and
water meet. The shoreline, then, may appear to be the guiding line,
but more than likely the birds are simply following air currents
created by onshore winds replacing the rising air from the
surrounding warmer land surface and being deflected upward by
the shoreline. These conditions often concentrate our buteos (broad-
winged, rough-legged, red-shouldered, and red-tailed hawks) into
restricted areas where, on good days, numbers observed can be
spectacular. Similar conditions exist over the Bosphorus at the
eastern end of the Mediterranean Sea where literally thousands of
storks, eagles, and buzzards can be observed on a good day.
While extensive water areas may alter the migratory path of
soaring birds, mountain ridges, especially if parallel to the line of
flight, are often very conducive to migratory travel. Systematic
coverage of the Appalachian ridges indicates all of them aid the
migration of soaring birds. Apparently the highest and longest
ridges deflect the horizontal winds upward better than the shorter
ridges less than 1,000 feet high, and more birds are seen, on the
average, along the higher ridges (Robbins 1956).
In general, nocturnal migrants are not influenced by topography
as much as diurnal travellers. Radar observations have played an
important role in establishing this difference. Bellrose (1967) found
that waterfowl migrating at night through the Midwest were not
influenced by major river systems, but in the evening or after
daybreak ducks and geese tended to alter their course along the
rivers. Drury et al. (1961) recorded massive fall and spring
movements from the New England area out over the Atlantic Ocean
without any apparent regard for the coastline. Until nocturnal
migration could be "watched" on a radar screen, many bird observers
assumed the guiding effect of the coastline on migratory travel was
more restrictive than it really is.
In summary, topography may help or deter a migrant in its
passage. It affects different birds in different ways. In North
America, migratory movements are continent wide, and no evidence
has indicated any particular part of the landscape influences all
birds in the same manner. Certain bird populations may use general
areas in migration, but they are usually not rigidly restricted to them
because of topography.
PERILS OF MIGRATION
The migration season is full of peril for birds. Untold thousands of
smaller migrants are destroyed each year by storms and attacks by
predatory animals. These mortality factors, and others, help keep
bird populations in check. Perils of migration are among these
Of all the hazards confronting birds in migration, particularly the
smaller species, storms are the most dangerous. Birds that cross
broad stretches of water can be blown off course by a storm, become
exhausted, and fall into the waves. Such a catastrophe was once seen
from the deck of a vessel in the Gulf of Mexico, 30 miles off the mouth
of the Mississippi River. Great numbers of migrating birds, chiefly
warblers, were nearing land after having accomplished nearly 95
percent of their long flight when, caught by a "norther" against
which they were unable to make headway, hundreds were forced into
the waters of the Gulf and drowned. A sudden drop in temperature
accompanied by a snowfall can cause a similar affect.
Lighthouses, tall buildings, monuments, television towers, and
other aerial obstructions have been responsible for destruction of
migratory birds. Bright beams of lights on buildings and airport
ceilometers have a powerful attraction for nocturnal air travelers
that may be likened to the fascination for lights exhibited by many
insects, particularly night-flying moths. The attraction is most
noticeable on foggy nights when the rays have a dazzling effect that
not only lures the birds but confuses them and causes their death by
collision against high structures. The fixed, white, stationary light
located 180 feet above sea level at Ponce de Leon Inlet (formerly
Mosquito Inlet), Florida, has caused great destruction of bird life even
though the lens is shielded by wire netting. Two other lighthouses at
the southern end of Florida, Sombrero Key and Fowey Rocks, have
been the cause of a great number of bird tragedies, while heavy
mortality has been noted also at some of the lights on the Great Lakes
and on the coast of Quebec. Fixed white lights seem to be most
attractive to birds; lighthouses equipped with flashing or red lights
do not have the same attraction.
For many years in Washington, B.C., the illuminated Washington
Monument, towering more than 555 feet into the air, caused
destruction of large numbers of small birds. Batteries of brilliant
floodlights grouped on all four sides about the base illuminate the
Monument so brilliantly, airplane pilots noticed that it could be seen
for 40 miles on a clear night. It is certain there is an extensive area of
illumination, and on dark nights with gusty, northerly winds,
nocturnal migrants seem to fly at lower altitudes and are attacted to
the Monument. As they mill about the shaft, they are dashed against
it by eddies of wind, and hundreds have been killed in a single night.
In September 1948, bird students were startled by news of the
wholesale destruction of common yellowthroats, American redstarts,
ovenbirds, and others against the 1,250-foot-high Empire State
Building in New York City, the 491-foot-high Philadelphia Saving
Fund Society Building in Philadelphia, and the 450-foot-high WB AL
radio tower in Baltimore. In New York, the birds continued to crash
into the Empire State Building for 6 hours.
More recently, the television tower has become the chief hazard.
These structures are so tall, sometimes over 1,000 feet, they present
more of a menace than buildings or lighthouses. Their blinking lights
cause passing migrants to blunder into guy wires or the tower itself
while milling around like moths about a flame. Numerous instances
(e.g. Stoddard and Norris 1967) throughout the U.S. indicate this
peril to migration is widespread. The lethal qualities of airport
ceilometers have been effectively modified by conversion to
intermittent or rotating beams.
Both soaring and sailing birds are so proficient in aerial
transportation that only recently have the principles been
understood and imitated by aircraft pilots. The use of ascending air
currents, employed by all soaring birds and easily demonstrated by
observing gulls glide hour after hour along the windward side of a
ship, are now utilized by man in his operation of gliders. Moreover,
the whole structure of a bird makes it the most perfect machine for
extensive flight the world has ever known. Hollow, air-filled bones,
together with feathers, the lightest and toughest material known for
flight, have evolved in combination to produce a perfect flying
Mere consideration of a bird's economy of fuel or energy also is
enlightening. The golden plover probably travels over a 2,400-mile
oceanic route from Nova Scotia to South America in about 48 hours of
continous flight. This is accomplished with the consumption of less
than 2 ounces of body fat (fuel). In contrast, to be just as efficient in
operation, a 1 ,000-pound airplane would consume only a single pint of
fuel in a 20-mile flight rather than the gallon usually required.
Similarly, the tiny ruby- throated hummingbird weighing approx-
imately 4 grams, crosses the Gulf of Mexico in a single flight of more
than 500 miles while consuming less than 1 gram of fat.
One might expect the exertion incident to long migratory flights
would result in arrival of migrants at their destination near a state of
exhaustion. This is usually not the case. Birds that have recently
arrived from a protracted flight over land or sea sometimes show
evidences of being tired, but their condition is far from being in a
state of emaciation or exhaustion. The popular notion birds find long
ocean flights so excessively wearisome that they sink exhausted when
terra firma is reached generally does not coincide with the facts.
The truth is, even small landbirds are so little exhausted by ocean
voyages, they not only cross the Gulf of Mexico at its widest point but
may even proceed without pause many miles inland before stopping.
The sora, considered such a weak flyer that at least one writer was led
to infer most of its migration was made on foot, has one of the longest
migration routes of any member of the rail family and even crosses
the wide reaches of the Caribbean Sea. Observations indicate that
under favorable conditions birds can fly when and where they please
and the distance covered in a single flight is governed chiefly by the
amount of stored fat. Exhaustion, except as the result of unusual
factors such as strong adverse winds, cannot be said to be an
important peril of migration.
ROUTES OF MIGRATION
While it is beyond question that certain general directions of flight
are consistently followed by migratory birds, it is well to remember
the term "migration route" is to some extent a theoretical concept
referring to the lines of general advance or retreat of a species, rather
than the exact course followed by individual birds or a path followed
by a species with specific geographic or ecological boundaries. Even
the records of banded birds usually show no more than the place of
banding and recovery. One ought to have recourse to intermediate
records and reasoning based on probabilities to fill in details of the
route actually traversed between the two points. In determining
migration routes, one must constantly guard against the false
assumption that localities with many grounded migrants are on the
main path of migration and localities where no grounded migrants
are observed are off the main path.
There is also infinite variety in the routes covered during
migration by different species. In fact, the choice of migration
highways is so wide that is seems as if the routes of no two species
coincide. Differences in distance traveled, time of starting, speed of
flight, geographical position, latitudes of breeding and wintering
grounds, and other factors contribute to this great variation of
migration routes. Nevertheless, there are certain factors that serve to
guide individuals or groups of individuals along more or less definite
lines, and it is possible to define such lines of migration for many
Except in a few species, individuals probably do not follow
precisely the same route twice. This is especially true in the group of
soaring birds that utilize thermals. Mueller and Berger (1967b)
recaptured only three migrants in subsequent years at Cedar Grove,
Wisconsin, after banding over 50,000 birds there. In general, those
populations of species with very discernible breeding or wintering
grounds have readily discernible migration routes. However, even
the whole migration process of certain species may show great yearly
fluctuations (Rudebeck 1950).
Aldrich et al. (1949) showed from banding data great variation in
migration patterns between species of waterfowl. In some species
there was considerable diversity in direction of movement, not only of
different breeding populations within a species but also for different
individuals of the same breeding population. The impression is
inescapable; waterfowl migration is even more complicated than
originally supposed, and it is difficult to make generalizations with
regard to migration pathways for even a single species let alone
waterfowl in general.
Flyways and Corridors
Through plotting accumulated banding data in the 1930's,
investigators became impressed by what appeared to be four broad,
relatively exclusive flyway belts in North America. This concept,
based upon analyses of the several thousand records of migratory
waterfowls recoveries then available, was described by Lincoln
(1935a). In this paper (p. 10), Lincoln concluded that:
. . . because of the great attachment of migratory birds for
their ancestral flyways, it would be possible practically to
exterminate the ducks of the West without seriously interfer-
ing with the supply of birds of the same species in the Altantic
and Mississippi flyways, and that the birds of these species
using the eastern flyways would be slow to overflow and re-
populate the devasted areas of the West, even though environ-
mental conditions might be so altered as to be entirely
Since 1948, this concept served as the basis for administrative action
by the Fish and Wildlife Service in setting annual migratory water-
fowl hunting regulations.
The concept of bird populations being confined to four fairly
definite and distinct migration "flyways" is probably most applicable
to those birds that migrate in family groups, namely geese, swans,
and cranes, but does not appear to be very helpful in understanding
the movements of the more widely dispersing ducks. The "pioneering
spirit" in Canada geese, for example, is limited by their social
structure the young travel to and from specific breeding and
wintering areas with their parents. These young later in life usually
breed in the same areas as did their parents. If a goose population is
decimated in one flyway, either by hunting or natural calamities,
other goose populations in other flyways are not seriously
endangered, but also these populations are very slow to repopulate an
area where the previous goose population had been decimated. This is
not the case with ducks because these birds are not always bound by
their intrinsic behavior to return to specific breeding areas. Con-
sequently, vacant breeding areas are more rapidly repopulated by
ducks than by geese.
Although Lincoln's analysis was confined to ducks and geese, some
thought that it applied to other groups of birds as well. Everyone
now realizes that the concept of four flyways, designated as the
Atlantic, Mississippi, Central, and Pacific Flyways, was an
oversimplification of an extremely complex situation involving
crisscrossing of migration routes, varying from species to species. It
can be considered meaningful only in a very general way, even for
waterfowl, and not applicable generally to other groups of birds.
Nevertheless the four "Flyway" areas have been useful in
regionalizing the harvest of waterfowl for areas of different
vulnerability of hunting pressure.
Bellrose (1968) identified corridors of southward migrating
waterfowl east of the Rocky Mountains and determined, through
statistical analyses, the relative abundance of birds in each. He
showed major corridors of dabbling duck movements down the Great
Plains and Missouri-Mississippi river valleys with minor off shoots at
various points from these corridors eastward to the Atlantic coast
where they joined equally minor eastern movements from the North
(Fig. 13). Bellrose's map of migration corridors for the diving ducks
Dabbling Ducks East of Rockies
Figure 13. Migration corridors used by dabbling ducks east of the Rocky Mountains
during their fall migration (After Bellrose 1968).
showed heavy traffic similar to that of dabbling species down the
Great Plains and relatively heavily used corridors from these central
arteries eastward across the Great Lakes area to the Atlantic coast,
terminating particularly in the vicinity of Chesapeake Bay. A fairly
well-used corridor extends along the Atlantic coast.
Lightly Traveled Routes
Heavily Traveled Routes
Figure 14- Distribution and migration of Harris' sparrow. This is an example of a
narrow migration route through the interior of the country.
With our present knowledge of bird migration it is difficult at best
to recognize distinct broad belts of migration down the North
American continent encompassing groups of distinct populations or
species. It seems that so much intermingling of populations occurs
that distinctions between broad "flyway" belts are not discernible.
About all we can say for sure now is that birds travel between certain
breeding areas in the North and certain wintering areas in the South
and that a few heavily traveled corridors used by certain species, and
more generalized routes followed by one or more species, have
Some species exhibit extremely narrow routes of travel. The red
knot and purple sandpiper, for example, are normally found only
along the coasts because they are limited on one side by the broad
waters of the ocean, and on the other by land and fresh water; neither
of these habitats furnish conditions attractive to these species.
The Ipswich race of the savannah sparrow likewise has a very
restricted migration range. It is known to breed only on tiny Sable
Island, Nova Scotia, and it winters from that island south along the
Atlantic coast to Georgia. It is rarely more than a quarter of a mile
from the outer beach and is entirely at home among the sand dunes
with their sparse covering of coarse grass.
The Harris' sparrow supplies an interesting example of a
moderately narrow migration route in the interior of the country
(Fig. 14). This fine, large sparrow is known to breed only in the
narrow belt of stunted timber and brush at or near the limit of trees
from the vicinity of Churchill, Manitoba, on the west shore of Hudson
Bay, to the Mackenzie Delta 1,600 miles to the northwest. When this
sparrow reaches the United States on its southward migration, it is
most numerous in a belt about 500 miles wide, between Montana
and central Minnesota and continues south through a relatively
narrow path in the central part of the continent. Knowledge of
habitat preference by Harris' sparrows suggests the narrow
migration range is restricted to the transition between woodland and
prairie, a type of habitat approaching the woodland-tundra
transition of its breeding area. Development of this migration route,
of course, preceded destruction of the heavy eastern forests by
colonists from Europe. Its winter range lies primarily in similar
country extending from southeastern Nebraska and northwestern
Missouri, across eastern Kansas and Oklahoma and through a
narrow section of eastern Texas, at places hardly more than 150 miles
When birds start their southward migration the movement
necessarily involves the full width of the breeding range. Later, in the
case of landbirds with extensive breeding ranges, there is a
convergence of the lines of flight taken by individual birds owing, in
part, to the conformation of the land mass and in part to the east-west
restriction of habitats suitable to certain species. An example of this
is provided by the eastern kingbird, which breeds in a summer
range 2,800 miles wide from Newfoundland to British Columbia. On
migration, however, the area traversed by the species becomes
constricted until in the southern part of the United States the
occupied area extends from Florida to the mouth of the Rio Grande, a
distance of only 900 miles. Still farther south the migration path
continues to converge, and, at the latitude of Yucatan, it is not more
than 400 miles wide. The great bulk of the species probably moves in
a belt less than half this width.
The scarlet tanager presents another extreme case of a narrowly
converging migration route starting from its 1,900-mile-wide
breeding range in the eastern deciduous forest between New
Brunswick and Saskatchewan (Fig. 15). As the birds move
Figure 15. Distribution and migration of the scarlet tanager. During the breeding sea-
son individual scarlet tanagers may be 1,500 miles apart in an east-and-west line
across the breeding range. In migration, however, the lines gradually converge until in
South America they are about 500 miles apart.
southward in the fall, their path of migration becomes more and
more constricted, until, at the time they leave the United States, all
are included in the 600-mile belt from eastern Texas to the Florida
peninsula. The boundaries continue to converge through Honduras
and Costa Rica where they are not more than 100 miles apart. The
species winters in the heavily forested areas of northwestern South
America including parts of Colombia, Ecuador, and Peru.
The rose-breasted grosbeak also leaves the United States through
the 600-mile stretch from eastern Texas to Apalachicola Bay, but
16. Distribution and migration of the rose-breasted grosbeak. Though the width
of the breeding range is about 2,500 miles, the migratory lines converge until the
boundaries are only about 1,000 miles apart when the birds leave the United States.
thereafter as this grosbeak crosses the Gulf of Mexico and enters the
northern part of its winter quarters in southern Mexico the lines do
not further converge. However, the pathway of those individuals that
continue on to South America is considerably constricted by the
narrowing of the land through Central America to Panama (Fig. 16).
Although the cases cited represent extremes of convergence, a
narrowing of the migratory path is the rule to a greater or lesser
degree for the majority of North American birds. Both the shape of
the continent and major habitat belts tend to constrict southward
movement so that the width of the migration route in the latitude of
the Gulf of Mexico is usually much less than in the breeding territory.
The American redstart represents a case of a wide migration route,
but even in the southern United States, this is still much narrower
than the breeding range (Fig. 17). These birds, however, cross all of
the Gulf of Mexico and pass from Florida to Cuba and Haiti by way of
the Bahamas, so here their route is about 2,500 miles wide.
Figure 1 7. Distribution and migration of the redstart. An example of a wide migration
route, birds of this species cross all parts of the Gulf of Mexico, or may travel from
Florida to Cuba and through the Bahamas. Their route has an east-and-west width of
more than 2,000 miles.
Principal Routes From North America
W. W. Cook presented seven of the more important generalized
routes for birds leaving the United States on their way to various
wintering grounds (1915a; Fig. 18). When migrants return
northward in the spring, they may follow these same routes, but it is
not known for certain whether they do. These routes are discussed in
the following sections.
Atlantic Oceanic Route
Route No. 1 (Fig. 18) is almost entirely oceanic and passes directly
over the Atlantic Ocean from Labrador and Nova Scotia to the Lesser
Antilles, then through this group of small islands to the mainland of
South America. Most of the adult eastern golden plovers and some
other shorebirds use this as their fall route. As we mentioned
previously, radar has indicated strong fall movements of warblers
from the New England coast out over the Atlantic to points south
along this route. Since it lies almost entirely over the sea, this route is
definitely known only at its terminals and from occasional
observations made on Bermuda and other islands in its course. Some
of the shorebirds that breed on the Arctic tundra of the District of
Mackenzie (Northwest Territories) and Alaska fly southeastward
across Canada to the Atlantic coast and finally follow this oceanic
Principal Western Routes
Most Extensively Used Routes
Atlantic Coast Routes
Atlantic Oceanic Route
Figure 18. Principal migration routes used by birds in passing from North America to
winter quarters in the West Indies, Central America, and South America. Route 4 is
the one used most extensively while only a few species make the 2,400 mile flight down
Route 1 from Nova Scotia to South America.
route to the mainland of South America. The golden plover may
accomplish the whole 2,400 miles without pause or rest, and in fair
weather the flocks pass Bermuda and sometimes even the islands of
the Antilles without stopping. Although most birds make their
migratory flights either by day or by night, the golden plover in this
remarkable journey flies both day and night. Since this plover swims
lightly and easily, it may make a few short stops along the way.
The Arctic tern follows the Atlantic Ocean route chiefly along the
eastern side of the ocean. Likewise, vast numbers of seabirds such as
auks, murres, guillemots, phalaropes, jaegers, petrels, and
shearwaters follow this over-water route from breeding coasts and
islands in the Northern and Southern Hemispheres.
Atlantic Coast Route and Tributaries
The Atlantic coast is a regular avenue of travel, and along it are
many famous points for observing both land and water birds. About
50 different kinds of landbirds that breed in New England follow the
coast southward to Florida and travel thence by island and mainland
to South America (Fig. 18, route 2). The map indicates a natural and
convenient highway through the Bahamas, Cuba, Hispaniola, Puerto
Rico, and the Lesser Antilles to the South American coast. Resting
places are affored at convenient intervals, and at no time need the
aerial travelers be out of sight of land. It is not, however, the favored
highway; only about 25 species of birds go beyond Cuba to Puerto
Rico along this route to their winter quarters, while only six species
are known to reach South America by way of the Lesser Antilles.
Many thousands of American coots and wigeons, pintails, blue-
winged teal, and other waterfowl as well as shorebirds, regularly
spend the winter season in the coastal marshes, inland lakes, and
ponds of Cuba, Hispaniola, and Puerto Rico.
Route No. 3 (Fig. 18) is a direct line of travel for Atlantic coast
migrants en route to South America, although it involves much
longer flights. It is used almost entirely by landbirds. After taking off
from the coast of Florida there are only two intermediate land masses
where the migrants may pause for rest and food. Nevertheless, tens of
thousands of birds of about 60 species cross the 150 miles from
Florida to Cuba where many elect to remain for the winter months.
The others negotiate the 90 miles between Cuba and Jamaica, but,
from that point to the South American coast, there is a stretch of
islandless ocean 500 miles across. Relatively few North American
migrants on this route go beyond Jamaica. The bobolink so far
outnumbers all other birds using this route that it may be designated
the "bobolink route" (Fig. 19). As traveling companions along this
route, the bobolink may meet vireos, kingbirds, and nighthawks
from Florida, Chuck-will's-widows from the Southeastern States,
black-billed and yellow-billed cuckoos from New England, gray-
cheeked thrushes from Quebec, bank swallows from Labrador, and
blackpoll warblers from Alaska. Sometimes this scattered
assemblage will be joined by a tanager or a wood thrush, but the
Figure 19. Distribution and migration of the bobolink. In crossing to South America,
most of the bobolinks use route 3 (Fig. 18), showing no hesitation in making the
flight from Jamaica across an islandless stretch of ocean. It u-ill be noted that colonies
of these birds have established themselves in western areas, but in migration they
adhere to the ancestral flyways and show no tendency to take the short cut across
Arizona, New Mexico, and Texas.
"bobolink route" is not popular with the greater number of migrants.
Formerly, it was thought most North America landbirds
migrated to Central America via the Florida coast, then crossed to
Cuba, and finally made the short flight from the western tip of Cuba
to Yucatan. A glance at the map would suggest this as a most natural
route, but, as a matter of fact, it is practically deserted except for a
few swallows and shorebirds or an occasional landbird storm-driven
from its normal course. What actually happens in the fall is that
many of the birds breeding east of the Appalachian Mountains travel
parallel to the seacoast in a more or less southwesterly direction and,
apparently maintaining this same general course from northwestern
Florida, cross the Gulf of Mexico to the coastal regions of eastern
Mexico. They thus join migrants from farther inland in using route
No. 4 (Fig. 18).
Routes used by the Atlantic brant merit some detail because their
flight paths were long misunderstood. These birds winter on the
Atlantic coast, chiefly at Barnegat Bay, New Jersey, but depending
upon the severity of the season and the food available, south also to
North Carolina. Their breeding grounds are in the Canadian arctic
archipelago and on the coasts of Greenland. According to the careful
studies of Lewis (1937), the main body travels northward in spring
along the coast to the Bay of Fundy, overland to Northumberland
Strait, which separates Prince Edward Island from mainland New
Brunswick and Nova Scotia. A minor route appears to lead
northward from Long Island Sound by way of the Housatonic and
Connecticut River Valleys to the St. Lawrence River.
After spending the entire month of May feeding and resting in the
Gulf of St. Lawrence, the eastern segment of the brant population
resumes its journey by departing overland from the Bay of Seven
Island area. The eastern and larger segment of the population
appears to fly almost due north to Ungava Bay and from there to
nesting grounds, probably on Baffin Island and Greenland. The
smaller segment travels a route slightly north of west to the
southeastern shores of James Bay, although east of that area some of
the flocks take a more northwesterly course by descending the Fort
George River to reach the eastern shore of James Bay. Upon their
arrival at either of these two points on James Bay, the brants of this
western segment turn northward and proceed along eastern Hudson
Bay to their breeding grounds in the Canadian Arctic.
In general, the fall migration of the brant follows the routes
utilized in the spring. At this season, the eastern population appears
only on the western and southern shores of Ungava Bay before
continuing their southward journey to the Gulf of St. Lawrence and
beyond. Also, it appears that most of the birds of the western
segment, instead of following the eastern shores of Hudson and
James bays, turn southwestward across the former, by way of the
Belcher Islands, to Cape Henrietta Maria, and from there south
along the western shores of James Bay by way of Akimiski and
Charlton Islands. At the southern end of James Bay, they are joined
by those that have taken the more direct route along the east coasts of
the bays and all then fly overland 570 miles to the estuary of the St.
The Atlantic coast wintering area receives accretions of waterfowl
from three or four interior migration paths, one of which is of first
importance, as it includes great flocks of canvasbacks, redheads,
scaup, Canada geese, and many black ducks that winter in the
waters and marshes of the coastal region south of Delaware Bay. The
canvasbacks, redheads, and scaup coming from breeding grounds on
the great northern plains of central Canada follow the general
southeasterly trend of the Great Lakes, cross Pennsylvania over the
mountains, and reach the Atlantic coast in the vicinity of Delaware
and Chesapeake Bays. Black ducks, mallards, and blue-winged teals
that have gathered in southern Ontario during the fall leave these
feeding grounds and proceed southwest over a course that is
apparently headed for the Mississippi Valley. Many do continue this
route down the Ohio Valley, but others, upon reaching the vicinity of
the St. Clair Flats between Michigan and Ontario, swing abruptly to
the southeast and cross the mountains to reach the Atlantic coast
south of New Jersey. This route, with its Mississippi Valley branch,
has been fully documented by the recovery records of ducks banded
at Lake Scugog, Ontario.
Canvasbacks migrate from the prairie pothole country of the
central United States and Canada to many wintering areas in the
United States. This duck has been the subject of a particular study
(Stewart, Geis, and Evans 1958), and its principle migration routes,
based on recovery of banded birds, are shown to follow an important
trunk route from the major breeding area in the prairie provinces of
Canada and the northern prairies of the United States southeast-
ward through the southern Great Lakes area to Chesapeake Bay, the
chief wintering area (Fig. 20). Relatively few canvasbacks proceed
southward along the Atlantic seaboard. A less important route
branches off from the main trunk in the southern Minnesota region
and extends south along the Mississippi Valley to points along the
river. Other individuals of the prairie breeding population fly
southward on a broad front to the gulf coast of Texas and the interior
of Mexico, while some proceed southwestward on a relatively broad
path to the northern Pacific coast.
Mackenzie Valley-Great Lakes-Mississippi Valley
Route and Tributaries
The route extending from the Mackenzie Valley past the Great
Lakes and down the Mississippi Valley is easily the longest of any in
the Western Hemisphere. Its northern terminus is on the Arctic
coast in the regions of Kotzebue Sound, Alaska, and the mouth of the
Mackenzie River, while its southern end lies in Argentina.
Nighthawks, barn swallows, blackpoll warblers, and individuals of
several other species that breed northward to the Yukon Territory
and Alaska must cover the larger part of the route twice each year.
For more than 3,000 miles from the mouth of the Mackenzie to
the delta of the Mississippi this route is uninterrupted by
mountains. In fact, the greatest elevation above sea level is less than
2,000 feet. Because it is well timbered and watered, the entire region
affords ideal conditions for its great hosts of migrating birds. It is
Figure 20. Principal migratory routes of the canvasback. The major route of travel
extends from breeding areas in central Canada southeast across the Great Lakes and
either south down the Mississippi River or east to Chesapeake Bay (After Stewart
et al 1958).
followed by such vast numbers of ducks, geese, shorebirds,
blackbirds, sparrows, warblers, and thrushes that observers
stationed at favorable points in the Mississippi Valley during the
height of migration can see a greater number of migrants than can
be noted anywhere else in the world.
When many of these species, including ducks, geese, robins, and
yellow-rumped warblers, arrive at the Gulf coast, they spread out
east and west for their winter sojourn. Others, despite the perils of a
trip involving a flight of several hundred miles across the Gulf of
Mexico, fly straight for Central and South America. This part of the
route is a broad "boulevard" extending from northwestern Florida to
eastern Texas and southward across the Gulf of Mexico to Yucatan
and the Isthmus of Tehuantepec (Fig. 18, route 4). This route appears
to have preference over the safer but more circuitous land or island
routes by way of Texas or Florida. During the height of migration
some of the islands off the coast of Louisiana are wonderful
observation points for the student of birds, as the feathered travelers
literally swarm over them.
Present detailed knowledge of the chief tributaries to the
Mackenzie-Great Lakes-Mississippi Valley route relates primarily
to waterfowl. Reference has been made already to the flight of black
ducks that reach the Mississippi Valley from southern Ontario.
Some individuals of this species banded at Lake Scugog, Ontario,
have been recaptured in succeeding seasons in Wisconsin and
Manitoba, but the majority was retaken at points south of the
junction of the Ohio River with the Mississippi indicating their main
route of travel from southern Ontario.
A second route that joins the main artery on its eastern side is the
one used by eastern populations of lesser snow geese, including both
blue and white phases, that breed mainly on Southampton Island
and in the Fox Basin of Baffin Island. In the fall these geese work
southward along the shores of Hudson Bay and, upon reaching the
southern extremity of James Bay, take off on their flight to the great
coastal marshes of Louisiana and Texas west of the Mississippi River
Great Plains Rocky Mountain Routes
This route also has its origin in the Mackenzie River delta and
Alaska. The lesser sandhill cranes, white-fronted geese, and smaller
races of the Canada goose follow this route through the Great Plains
from breeding areas in Alaska and western Canada. It is used chiefly
by the pintails and American wigeons that fly southward through
eastern Alberta to western Montana. Some localities in this area, as
for example, the National Bison Range at Moiese, Montana,
normally furnish food in such abundance that these birds are induced
to pause in their migratory movement. Some flocks of pintails and
wigeons move from this area almost directly west across Idaho to the
valley of the Columbia River, then south to the interior valleys of
California. Others leave Montana by traveling southeastward across
Wyoming and Colorado to join other flocks moving southward
through the Great Plains.
Observations made in the vicinity of Corpus Christi, Texas, have
shown one of the short cuts (Fig. 18, route 5) that is part of the great
artery of migration. Thousands of birds pass along the coast to the
northern part of the State of Veracruz, Mexico. Coastal areas along
the State of Tamaulipas to the north are arid and so entirely unsuited
for frequenters of moist woodlands that it is probable that much, or
all, of this part of the route for these species is a short distance off
shore. It is used by such woodland species as the golden-winged
warbler, the worm-eating warbler, and the Kentucky warbler.
Pacific Coast Route
Although it does present features of unusual interest, the Pacific
coast route is not as important as some of the others described.
Because of the equable conditions that prevail, many species of birds
along the coast from the northwestern states to southeastern Alaska
either do not migrate or else make relatively short journeys. This
route has its origin chiefly in western Alaska, around the Yukon
River delta. Some of the scoters and other sea ducks of the north
Pacific region as well as the diminutive cackling Canada goose of the
Yukon River Delta use the coastal sea route for all or most of their
southward flight. The journey of the cackling goose, as shown by
return records from birds banded at Hooper Bay, Alaska, has been
traced southward across the Alaskan Peninsula and apparently
across the Gulf of Alaska to the Queen Charlotte Islands. The birds
then follow the coast line south to near the mouth of the Columbia
River, where the route swings toward the interior for a short
distance before continuing south by way of the Willamette River
Valley. The winter quarters of the cackling goose are chiefly in the
vicinity of Tule Lake, on the Oregon-California line, and in the
Sacramento Valley of California, although a few push on to the San
A tributary of this "flyway" is followed by Ross' goose, which
breeds in the Perry River district south of Queen Maud Gulf and
other areas farther east on the central Arctic coast of Canada (Fig.
21). Its fall migration is southwest and south across the barren
grounds to Great Slave and Athabaska Lakes, where it joins
thousands of other waterfowl bound for winter homes along the
eastern coast of the United States and the Gulf of Mexico. But when
Ross' geese have traveled south approximately to the northern
boundary of Montana, most of them separate from their companions
and turn southwest across the Rocky Mountains to winter in
California. In recent years a few Ross' geese have been found
wintering east of the Rocky Mountains along with flocks of lesser
snow geese and may be correlated with an eastward extension of
their breeding range.
The southward route of those migratory landbirds of the Pacific
area that leave the United States in winter extends chiefly through
the interior of California to the mouth of the Colorado River and on to
winter quarters in western Mexico (Fig. 18, routes 6 and 7).
The movements of the western tanager show a migration route that
is in some ways remarkable. The species breeds in the mountains
from the northern part of Baja California and western Texas north to
northern British Columbia and southwestern Mackenzie. Its winter
range is in two discontinuous areas southern Baja California and
r-L r ' ^r 'WM>r
i T S v--< T \ r
i i } \ IH^
' -f- -rl -V-'-^ 7 "
Figure 21. The breeding range, wintering range, and main migration route of Ross'
geese. This is the only species of which practically all members breed in the Arctic,
migrate south through the Canadian prairie, and upon reaching the United States,
turn to the southwest rather than the southeast. The southern part of this route, how-
ever, is followed by some mallards, pintails, wigeons, and other ducks.
eastern and southwestern Mexico south to Guatemala (Fig. 22).
During spring migration the birds appear first in western Texas and
the southern parts of New Mexico and Arizona about April 20 (Fig.
23). By April 30 the vanguard has advanced evenly to an
approximate east-west line across central New Mexico, Arizona, and
Figure 22. Breeding and wintering ranges of the western tanager. See Fig. 23 for the
spring route taken by the birds breeding in the northern part of the range.
southern California. By May 10 the easternmost birds have advanced
only to southern Colorado, while those in the far west have reached
northern Washington. Ten days later the northward advance of the
species is shown as a great curve, extending northeastward from
Vancouver Island to central Alberta and thence southeastward to
Isochronal Migration Lines
Figure 23. Migration of the western tanager. The birds that arrive in eastern A Iberta by
May 20 do not travel northward along the eastern base of the Rocky Mountains,
because the vanguard has then only reached northern Colorado. Instead the isochronal
lines indicate that they migrate north through California, Oregon, and Washington
and then cross the Rockies in British Columbia.
northern Colorado. Since these tanagers do not reach northern
Colorado until May 20, it is evident those present in Alberta on that
date, instead of traveling northward through the Rocky Mountains,
their summer home, actually reached there by a route that carried
them west of the Rockies to southern British Columbia and thence
eastward across the still snowy northern Rocky Mountains.
Pacific Oceanic Route
The Pacific oceanic route is used by the Pacific golden plover,
bristle-thighed curlew, ruddy turnstone, wandering tattler and
other shorebirds. The ruddy turnstone, and probably other
shorebirds, migrating from the islands of the Bering Sea, have an
elliptical route that takes them southward via the islands of the
central Pacific and northward along the Asiatic coast. In addition,
many seabirds that breed on far northern and southern coasts or
islands migrate up and down the Pacific well away from land except
when the breeding season approaches.
The Pacific golden plover breeds chiefly in the Arctic coast region
of Siberia and in a more limited area on the Alaskan coast. Some of
the birds probably migrate south via the continent of Asia to winter
quarters in Japan, China, India, Australia, New Zealand, and
Oceania. Others evidently go south by way of the Pacific Ocean to the
Hawaiian Islands and other islands of the central and southern
Pacific. Migrating golden plovers have been observed at sea on a line
that apparently extends from Hawaii to the Aleutian Islands; it
appears certain some of the Alaskan birds make a nonstop flight
across the sea from Alaska to Hawaii. While it would seem incredible
that any birds could lay a course so accurately as to land on these
small isolated oceanic islands, 2,000 miles south of the Aleutians,
2,000 miles west of Baja California, and nearly 4,000 miles east of
Japan, the evidence admits only the conclusion that year after year
this transoceanic round-trip journey between Alaska and Hawaii is
made by considerable numbers of golden plovers.
Some Arctic nesting birds retreat only a short distance south in the
winter. These species include the red-legged kittiwake, Ross' gull,
emperor goose, and various eiders. The latter group of ducks winter
well south of their nesting areas but nevertheless remain farther
north than do the majority of other species of ducks. The routes
followed by these birds are chiefly parallel to the coast and may be
considered as being tributary either to the Atlantic or Pacific coast
routes. The heavy passage of gulls, ducks, black brants, and other
water birds at Point Barrow, Alaska, and other points on the Arctic
coast, has been noted by many observers. The best defined Arctic
route in North America is the one following the coast of Alaska.
A migration route, therefore, may be anything from a narrow path
closely adhering to some definite geographical feature, such as a
river valley or a coastline, to a broad boulevard that leads in the
desired direction and follows only the general trend of the land mass.
Oceanic routes appear to be special cases that are not fully
understood at the present time. Also it must be remembered that all
the main routes contain a multitude of tributary and separate minor
routes. In fact, with the entire continent of North America crossed by
migratory birds, the different groups or species frequently follow
lines that may repeatedly intersect those taken by others of their own
kind or by other species. The arterial or trunk routes, therefore, must
be considered merely as indicating paths of migration on which the
tendency to concentrate is particularly noticeable.
PATTERNS OF MIGRATION
Band recoveries, netting records, and personal observations help
us to critically examine migration routes and probe deeper into the
origin and evolution of these pathways. We are beginning to realize
certain deviations occur from the "normal" north and south
movements expected in most species. In the previous section on
routes, we touched briefly on the fact that some routes are not
poleward at all, but in some other direction. We know that many
migrants do not stop at the exact localities year after year but they
probably do follow the same general course each season. After many
years of observations a pattern emerges for that population, species,
or group of species. In this section we would like to take a closer look
at some of the interesting patterns (or "eccentric routes" as Cook
(1915a) referred to them) in migration that birds are annually to
travel from breeding to wintering grounds and back again. In many
cases, the causative agents are unknown or pure conjecture, but in
others, sound biological principles can be put forth that may indicate
why a particular species could have evolved the specific pattern it
Many species do not return north in the spring over the same route
they used in the fall; rather, they fly around an enormous loop or
ellipse. Cook (1915a) considered food as the primary factor in
determining the course birds took between winter and summer
ranges. Individuals that returned by the same route and did not find
sufficient food for their needs at that time were eliminated from the
population, and only progeny from individuals that took a
different course with sufficient food lived to build the tradition of a
loop migration. Other investigators consider prevailing winds a
major factor in the evolution of loop migration. Whatever the reason
may be, it has most likely evolved separately in each species to satisfy
its particular needs, and the fact that this pattern occurs all over the
world in completely unrelated species is a good illustration of
The annual flight of adult golden plovers is so unusual, it will be
given in some detail. The species is observed by hundreds of bird
watchers every year and it well illustrates loop migration (Fig. 24).
In the fall, the birds fatten on the multitude of berries along the
coasts of Labrador and Nova Scotia, then depart south over the
Atlantic Ocean to South America. After reaching the South
American coast the birds make a short stop, then continue overland
to the pampas of Argentina, where they remain from September to
Eastern Subspecies American Golden Plover
+ Migratory Routes
Figure 24. Distribution and migration of the American golden plover. Adults of the
eastern subspecies migrate across northeastern Canada and then by a nonstop flight
reach South America. In spring they return by way of the Mississippi Valley. Their
entire route, therefore, is in the form of a great ellipse with a major axis of 8,000 miles
and a minor axis of about 2,000 miles. The western subspecies migrates across the
Pacific Ocean to various localities including the Hawaiian and Marquesas islands
and the Low Archipelago.
March. When these golden plovers leave their winter quarters they
cross northwestern South America and the Gulf of Mexico to reach
the North American mainland on the coasts of Texas and Louisiana.
Thence they proceed slowly up the Mississippi Valley and, by the
early part of June, are again on their breeding grounds, having
performed a round-trip journey in the form of an enormous ellipse
with the minor axis about 2,000 miles and the major axis 8,000 miles
stretching from the Arctic tundra to the pampas of Argentina. The
older birds may be accompanied by some of the young, but most of the
immature birds leave their natal grounds late in summer and move
southward through the interior of the country, returning in spring
over essentially the same course. The oceanic route is therefore used
chiefly by adult birds.
A return by the oceanic route in the spring could be fatal. The
maritime climate in the Northeast results in foggy conditions along
the coast and the frozen soil would offer few rewards for the weary
travelers. By traveling up the middle of the continent, a much better
food supply is assured (Welty 1962).
Several North American warblers including the Connecticut
warbler (Fig. 25) and the western race of the palm warbler have been
found to follow circuitous migration routes. The Connecticut warbler
is not observed or banded on the East coast in spring, but it is
recorded farther inland during the season. Thus this warbler
proceeds down the East coast in the fall and up the interior of the
continent in the spring. Similarly, the western race of the palm
warbler moves from its breeding grounds directly east to the
Appalachian Mountains before turning south along the coast.
Television tower kills in northern Florida indicate the population is
very concentrated here at this time of year. In the spring this race
also proceeds north through the interior. Graber (1968) points out
that the eastern race of the palm warbler also proceeds south along
the coast in the fall and poses this question: "does the western popula-
tion of this species intentionally move toward the ancestral range, or
is the fall flight direction merely a consequence of the temperate zone
Graber concluded from radar observations that the disparity in
seasonal flight directions of many migrants was a positive response of
migrants to favorable wind directions at that time of year. The east-
oriented transgulf migrants followed an elliptical migration because
postfrontal air flow in the fall at latitude 40 N is northwesterly, and,
in the spring southerly; whereas winds over the Gulf of Mexico are
consistently easterly or southeasterly. Therefore, transgulf migrants
returning north in the spring would be moved westward across the
Gulf unless they compensated for wind drift. Observers were not
aware of high-altitude drift before radar (Bellrose and Graber 1963).
Numerous other loop migrations have been documented through-
out the world. In the fall, the short-tailed shearwater, is observed off
the west coast of North America as far south as California. At this
time the species is on the eastern leg of a tremendous figure-eight
/ i j
/ V- _r i
Fall Migration Route South
Overlapping Spring and Fall
Spring Migration Route North
Figure 25. Breeding range and migration routes of the Connecticut warbler. From the
breeding range in northern United States and southern Canada, it migrates east in
the fall to New England, then south along the Atlantic coast to Florida and across the
West Indies to winter in South America. In the spring it does not return by the same
route but rather completes a loop by migrating northwest across the Allegheny Moun-
tains and the Mississippi Valley (Adapted from Cooke 1915a).
circuit around the Pacific Ocean (Fig. 26). The subalpine warbler
and red-backed shrikes perform loop migrations between Europe
and Africa. Both pass much farther to the east in the spring than in
the fall (Moreau 1961). The Arctic loon travels south across inland
Russia to southern Europe but returns to its Arctic breeding grounds
via the Gulf Stream on the sea because this water is open much
earlier in the spring than the inland waterways (Welty 1962).
Bass Strait Breeding Area
Short -Tailed Shearwater
Figure 26. Migration route of the short-tailed shearwater. An example of an incredi-
bly large loop migration pattern in a pelagic species. Breeding adults return to two
islands in Bass Strait during the last part of October after completing a figure-eight
circuit of the northern Pacific Ocean (From Serventy 1953).
Dog-leg migration patterns are characterized by a prominent bend
or twist in the route. Studies have shown some of these illogical, out-
of-the-way means for connecting wintering and breeding areas have
no biological function, but instead, are the result of tradition much
like the lineage of crooked streets in Boston can be traced back to old
cowpaths (Welty 1962). Many species have extended their range in
recent years, but the pioneers continue to retrace the old route from
the point of origin even if the new areas are not on the same axis as the
earlier route. The old pathways have apparently become implanted
as part of the migratory instinct in all members of particular
populations even after extending their ranges considerable distances
from the original.
Good examples of this crooked traditional path can be seen in the
routes taken by Old World species extending their ranges into the
New World from Europe and Asia. The European wheatear has
extended its range into Greenland and Labrador where the local
breeding population has become a separate race. When the Labrador
individuals depart from their breeding grounds, they proceed north
to Greenland, their ancestral home, then west to Europe and south to
Africa, the traditional wintering area for all wheatears. Alaskan
breeding wheatears migrate to Africa in the opposite direction via
Asia where the Alaskan population presumably originated. Alaskan
breeding Arctic and willow warblers and bluethroats also migrate
westward into Siberia and then southward on the Asiatic side. Some
investigators believe the Arctic tern colonized the New World from
Europe because when this bird departs for the south it first crosses
the Atlantic to Europe, then moves down the eastern Atlantic coast to
Africa and either back across the Atlantic to South America or
continues south down past South Africa (Fig. 11). To get to South
America from the eastern Arctic, it would be shorter to follow the
golden plover's flight path straight down the Atlantic or along the
east coast of the United States but the fact that no Arctic terns have
been observed in the Caribbean indicates that they do no follow that
In western United States, California gulls nest in various colonies
around Great Salt Lake and Yellowstone Park. Banding records
indicate these populations winter along the California coast (Fig. 27).
Instead of traveling southwest by the shortest distance to the
wintering grounds, they proceed longitudinally down the Snake and
Columbia Rivers and reach the coast around Vancouver ( Woodbury
et al. 1946). Thence they proceed south along the coast to Oregon and
California. In the spring the adults return over the same course
rather than taking the shorter flight northeast in April across the
deserts and mountains; this route would be largely made over a cold
and inhospitable country (Oldaker 1961).
Sladen (1973) has mapped the migration routes of whistling swans,
and several dog-leg patterns are apparent in the eastern and western
Figure 27. Migration route and wintering grounds of California gulls banded in north-
western Wyoming. During fall migration, the birds proceed west from the breeding
grounds to the Pacific Ocean before turning south to wintering areas in California. A
more direct route across Nevada would entail a trip through relatively barren country
(After Diem and Condon 1967).
populations (Fig. 28). In the eastern population, a sharp change in
direction occurs at their major feeding and resting areas in North
Dakota. After the birds arrive from the Arctic breeding grounds,
they proceed east-southeast to their wintering grounds on
Chesapeake Bay. In the western population, thousands of birds
migrate from the Alaskan breeding grounds to the large marshes
along Great Salt Lake. Then after a major stopover, this population
heads west over the mountains to California.
Fall Migration of
Fall Migration of
Figure 28. Distribution and migration routes of whistling swans in North America.
Birds from the central arctic head south to North Dakota before proceeding east to
Chesapeake Bay, while many Alaskan breeders migrate to Great Salt Lake before
turning west to winter in California (After Sladen, 1973).
Many of the pelagic birds observed off our coasts or at sea appear to
be nomadic when they are not breeding. These movements are not
necessarily at random because there is usually a seasonal shift in the
population, often for great distances and in specific directions, away
from the breeding area after completion of the nesting cycle. Also the
return from the sea to nesting areas is at a definite time of year. This
may not be true migration in the classical sense (Thomson 1964),
although it is similar in most respects.
Because of the extensive and often inhospitable habitat of pelagic
birds (to human observers at least), observations on their movements
are difficult at best and accurate records are few. We do know some of
these species have regular routes (e.g., Arctic terns) and specific
patterns of migration (e.g., the loop in the short-tailed shearwater).
As more knowledge is accumulated on the "nomadic" species, we may
actually find they too have regular migration routes based on
Movements of some of the tubenoses (Order Procellariiformes, that
includes albatrosses, fulmars, shearwaters, and petrels) have been
correlated with ocean currents, prevailing winds, temperatures, and
general water fertility (Kuroda 1957; Shuntov 1968; Fisher and
Fisher 1972). Commercial fishermen have long known ocean
currents are very important factors in the supply of nutrients,
plankton, and forage fish for larger fish. These same foodstuffs often
attract pelagic birds as evidenced by the tremendous concentrations
that occur off the Peruvian coasts where the upwelling of cold
nutrient-bearing water is evident. Kuroda (1957) found some fine
correlations between the route of the short-tailed shearwater and
ocean currents. Likewise Shuntov (1968) found the migratory routes
of albatrosses were over temperate marine waters of high biological
productivity. The Laysan albatross was correlated with cold
currents, while the black-footed albatross occurred over warm
currents. Many Southern Hemisphere pelagic species have been
extremely successful in exploiting rich northern waters during
the summer; the group is probably the most abundant and
widespread in the world (Bourne 1956).
When two or more races of the same species occupy different
breeding ranges on the same axis as migratory flight, the races
breeding the farthest north often winter the farthest south. Thus, a
northern race "leap-frogs" over the breeding and wintering range of
the southern populations. This has been well documented in the fox
sparrow discussed previously (Fig. 10) and is exhibited by races of
Canada geese breeding in central Canada as well. One of the smaller
races of this goose breeds along the Arctic coast of the Northwest
Territories and winters on the Gulf coast of Texas and northeastern
Mexico, while a much larger race breeds in the central United States
and Canada but winters in the central part of the United States. This
leaping over occurs in other species as well, including the bluebird
In the effort to find winter quarters furnishing satisfactory living
conditions, many North American birds fly hundreds of miles across
land and sea. Others, however, are able to attain their objectives
merely by moving down the sides of a mountain. In such cases a few
hundred feet of altitude corresponds to hundreds of miles of latitude.
Movements of this kind, known as "vertical migrations," are found
worldwide wherever there are large mountain ranges. Aristotle first
mentions vertical migration: "Weakly birds in winter and in frosty
weather come down to the plains for warmth, and in summer migrate
to the hills for coolness . . ."(Dorst 1962). The number of species that
can perform this type of migration pattern is obviously limited to
those species adapted to breeding in alpine areas.
In the Rocky Mountain region vertical migrations are particularly
notable. Chickadees, rosy finches, juncos, pine grosbeaks, William-
son's sapsuckers, and western wood pewees nest at high altitudes and
move down to the lower levels to spend the winter. The dark-eyed
juncos breeding in the Great Smoky Mountains make a vertical
migration, but other members of the species, breeding in flatter
areas, make an annual north-south migration of hundreds of miles
(Van Tyne and Berger 1959). There is a distinct tendency among the
young of mountain-breeding birds to work down to the lower levels as
soon as the nesting season is over. The sudden increases among birds
in the edges of the foothills are particularly noticeable when cold
spells with snow or frost occur at the higher altitudes. In the Dead
Sea area of the Middle East, some birds that breed in this extremely
hot desert move up into the surrounding cooler hill during the winter
The vertical migrations of some mountain dwelling gallinaceous
birds (mountain quail and blue grouse) are quite interesting because
the annual journey from breeding to wintering grounds is made on
foot. Mountain quail make this downward trek quite early in the fall
well before any snows can prevent them from reaching their goal.
Blue grouse perform essentially the same journey in reverse. During
midwinter, these birds can be found near timberline eating spruce
buds protruding above the snow.
These illustrations show that the length and direction of a
migration route are adapted to the needs for survival and are met in
some cases by a short vertical movement or great latitudinal travels
Recent banding studies have demonstrated many migrants,
especially young of the year, have a tendency to disperse after
fledging. These premigatory movements have also been called "post-
fledging dispersal," "reverse migration," and "postbreeding north-
ward migration." Demonstration of this phenomenon is especially
important as it relates to locality-faithfulness (Ortstreue), range
extension, and gene mixture between populations. These movements
cannot be considered as true migrations even though they are
repeated annually by the species between breeding grounds and
some other area. These movements are generally repeated by the
same age class in the population but not the same individuals.
Nevertheless, these regular northward movements are quite
striking, especially in herons. The young of some species commonly
wander late in the summer and fall for several hundred miles north
of the district in which they were hatched. Young little blue herons as
well as great and snowy egrets are conspicuous in the East as far
north as New England and in the Mississippi Valley to southeastern
Kansas and Illinois. Black-crowned night herons banded in a large
colony at Barnstable, Massachusetts, have been recaptured the same
season northward to Maine and Quebec and westward to New York.
In September most of them return to the south.
These movements have been noted in several other species as well.
Broley (1947) nicely illustrated this northward movement of bald
eagles along the Atlantic coast (Fig. 29). Birds banded as nestlings in
Florida have been retaken that summer 1,500 miles away in Canada.
Van Tyne and Berger (1959) surmised the summer heat of Florida
was too great for this eagle, a northern species that has only recently
spread into Florida to take advantage of abundant food and nesting
sites, which it exploits during the cooler season. Postbreeding
northward movements are also shared by wood ducks, yellow-
breasted chats, eastern bluebirds, and white pelicans.
A somewhat different type of postbreeding migration is the so-
called "molt migration" exhibited by many species of waterfowl
(Salomonsen 1968). These birds may travel considerable distances
away from their nesting area to traditional molting sites where they
spend the flightless period of the eclipse plumage. At such times they
may move well into the breeding ranges of other geographic races of
their species. These movements may be governed by the availability
of food and are counteracted in fall by a directive migratory impulse
that carries those birds that attained more northern latitudes after
the nesting period, back to their normal wintering homes in the
The occasional great invasions beyond the limits of their normal
range of certain birds associated with the far North are quite
different from migration patterns discussed previously. Classic
examples of such invasions in the eastern part of the country are the
periodic flights of crossbills. Sometimes these migrations will extend
well south into the southern States.
Snowy owls are noted for occasional invasions that have been
correlated with periodic declines in lemmings, a primary food
resource of northern predators. According to Gross (1947), 24 major
invasions occurred between 1833 and 1945. The interval between
these varied from 2 to 14 years, but nearly half (11) were at intervals
of 4 years. A great flight occurred in the winter of 1926-27 when more
than 1,000 records were received from New England alone, but the
Fi(ju re 29. Northern recoveries of young bald eagles banded as nestlings in Florida. The
birds sometimes "migrate" over 1,500 miles up the coast during their first summer
before returning south (From Broleij 1947).
largest on record was in 1945-46 when the "Snowy Owl Committee" of
the American Ornithologists' Union received reports of 13,502 birds,
of which 4,443 were reported killed. It extended over the entire width
of the continent from Washington and British Columbia to the
Atlantic coast and south to Nebraska, Illinois, Indiana, Pennsyl-
vania, and Maryland. One was taken as far south as South Carolina.
In the Rocky Mountain region, great flights of the beautiful
Bohemian waxwing are occasionally recorded. The greatest invasion
in the history of Colorado ornithology occurred in February 1917,
when it was estimated that at least 10,000 were within the corporate
limits of the city of Denver. The last previous occurrence of the
species in large numbers in that section was in 1908.
Evening grosbeaks likewise are given to more or less wandering
journeys, and, curiously enough, in addition to occasional trips south
of their regular range, they travel east and west for considerable
distances. For example, grosbeaks banded at Sault Ste. Marie,
Michigan, have been recaptured on Cape Cod, Massachusetts, and in
the following season were back at the banding station. Banding
records and museum specimen identifications demonstrate that this
east-and-west trip across the northeastern part of the country is
sometimes made also by purple finches, red crossbills, and mourning
ORIGIN AND EVOLUTION OF MIGRATION
The origin and evolution of bird migration has been discussed in
ornithological literature for centuries. As we have seen from the
foregoing discussion, migration exists in many forms throughout the
world and probably arose to satisfy many different needs in different
orders of birds at the same time. New pattens, traditions, and
routes are arising today as well as disappearing. Currently, the
migration patterns we see are a composite result of historic
influences mixed with present day influences. Even though the
migration of several different species may be very similar, the
patterns exhibited today can be the result of quite different
evolutionary processes. Because it cannot be substantiated by
experimental facts, any explanation of how a particular pattern or
route originates is pure conjecture.
The general anatomical and physiological attributes of birds
enable them to develop more diverse and spectacular migratory
behavior than any other group of animals. Their potential for long
sustained flights is of primary importance in pre-adapting birds to
successful migrations. Migration has long since become a definite
hereditary habit of many species of birds that recurs in annual cycles,
evidently because of physiological changes which prompt a search
for an environment suitable for reproduction and survival. Like the
bird's other habits its migratory behavior is just as characteristic as
the color of its plumage and, like it, evolved through natural selection
because it was advantageous for the survival of the population. Its
origin has been thought by some to be a mystery locked in past ages,
but by study of the history of how birds came to occupy their present
ranges, information becomes available which suggests theories that
may be developed and explored. Two that are commonly mentioned
are termed the "Northern Ancestral Home Theory" and the
"Southern Ancestral Home Theory."
According to the former of these hypotheses, in earlier ages when
conditions of climate, food, and habitat were favorable for existence
of birds throughout the year much further north than is the case
today, many species remained in these nothern latitudes as
permanent residents. Today, such conditions are found only in more
southern regions where migrations are much shorter or nonexistent.
Gradually, however, in the Northern Hemisphere the glacial ice
fields advanced southward, causing a southward movement of
conditions favorable to northern birds, until finally all bird life was
confined to southern latitudes. As the ages passed, the ice cap
gradually retreated, and each spring the birds whose ancestral home
had been in the North moved in again to fill newly opened breeding
habitat only to be driven south again at the approach of winter. As the
size of the ice-covered area diminished, the journeys to the summer
breeding areas became even longer until eventually the climatic
conditions of the present age became established, and with them,
present patterns of the annual advance and retreat we call migration.
The opposing theory is simpler in some respects and supposes the
ancenstral home of Northern Hemisphere migratory birds was in the
Tropics. As all bird life tends to overpopulation, there was a constant
effort of young individuals to pioneer and seek breeding grounds
where competition was less severe. Species better adapted to more
northern latitudes moved in that direction for nesting but were kept
in check by the glacial ice and forced to return southward with the
recurrence of winter conditions. Gradually, as the ice retreated, vast
areas of virgin country became successively suitable for summer
occupancy, but the winter habitat in the South remained the home to
which the birds returned after the nesting season.
The above two theories presume that the Quaternary glaciations,
which occurred 10,000 to 1 million years ago, have been the
predominate influence on bird migration in North America and
Europe as we observe it today. There is no doubt these extreme
climatic and ecologic barriers played a part in shaping or modifying
some patterns, but as Moreau (1951) has pointed out, well-developed
migrations occur in parts of the world, including the Southern
Hemisphere or even within the tropics, where continental glaciation
was not a factor. Furthermore, migrations to fit various needs have
probably been going on ever since birds could fly. The tremendous
Pleistocene glaciations actually occupied less than a hundreth of the
time birds have existed on the earth and probably only determined
the details of migrations as we see them today (Moreau 1951).
The northern and southern ancestral home theories appear
diametrically opposed to each other but Dorst (1963) concludes they
are perfectly compatible. Since the phenomena probably occurred
simultaneously, northern migrants then originated from two stocks:
the North Temperate Zone birds sought refuge to the south during
the glacial periods and the tropical avifauna expanded their range
during the interglacial periods. Dorst also stated this double origin is
more prevalent in North America where the tropical element is most
abundant. Birds representing this element include hummingbirds,
tyrant flycatchers, orioles, tanagers, and blackbirds. At some
latitudes, they nest in the same area as the shorebirds which are of
arctic parental stock.
These theories assume migration is a genetic, inherited character,
but we now know in some species it can be modified in the lifetime of
one individual and the place some individuals return to nest or winter
is not the ancestral home but a place to which they had been
transported at an early stage in their development. Traditions that
have lived for countless generations may die overnight if experienced
individuals are lost or no longer active (Hochbaum 1955); migration
patterns remain constant only as long as the factors influencing these
patterns remain constant. But the landscape and the interacting
ecological stresses are forever changing, and we would expect the
adaptive behavior of birds to respond with them. One of these
responses to an expanding habitat is colonization of new territory and
expansion of a species' range with accompanying development of a
migratory habit. The search for favorable conditions under which to
breed in summer and to feed in winter, as influenced by competition
for space, has been the principal factor underlying the extension of
ranges, usually by young, nonconditioned individuals. This is
exemplified by the northward extension in historic times of a number
of species. Many of these range extensions have closely followed
man's settlement of the area and the subsequent changes in habitat
that man has made.
From the previous descriptions of migration patterns and routes, it
will be observed that the general trend of migration in most northern
populations of North American birds is northwest and southeast.
Eastern species tend to extend their ranges by pushing westward,
particularly in the North. For example, in the Stikine River Valley of
northern British Columbia and southwestern Alaska the common
nighthawk, chipping sparrow, rusty blackbird, yellow warbler,
American redstart, and others have established breeding stations at
points 20 to 100 miles from the Pacific Ocean. The northern race of
the American robin, common flickers, dark-eyed juncos, blackpoll
warblers, yellow-rumped warblers, and ovenbirds, all common
eastern species, also are established as breeding birds in western
Alaska. The ovenbird has even been detected on the lower Yukon
River, and the sandhill crane and gray-cheeked thrush have moved
across Bering Strait into Siberia. These birds continue to migrate
through the eastern part of the continent. Instead of taking the
shortest route south, they retrace the direction of their westward
expansion and move southward along the same avenues as their more
The red-eyed vireo is essentially an inhabitant of states east of the
Great Plains, but an arm of its breeding range extends northwest to
the Pacific coast in British Columbia (Fig. 30). It seems evident this is
a range extension that has taken place comparatively recently by a
westward movement via deciduous woodland corridors, and the
invaders retrace in spring and fall the general route by which they
originally entered the country.
In the case of the bobolink, a new extension of the breeding range
and a subsequent change in the migration of the species has taken
place since settlement by European man (Fig. 19). Because the
bobolink is a bird of damp meadows, it was originally cut off from the
Western States by the intervening arid regions, but with the advent
of irrigation and the bringing of large areas under cultivation, small
colonies of nesting bobolinks appeared at various western points.
Now the species is established as a regular breeder in the great
mountain parks and irrigated valleys of Colorado and elsewhere
almost to the Pacific coast. These western pioneers must fly long
Figure 30. Distribution and migration of the red-eyed vireo. It is evident that the red-
eyed vireo has only recently invaded Washington by an extension of its breeding range
almost due west from the upper Missouri Valley. Like the bobolink (Fig. 19), however,
the western breeders do not take the short cut south or southeast from their nesting
grounds but migrate spring and fall along the route traveled in making the extension.
distances east and west to reach the western edge of the route
followed by the bulk of the bobolinks that breed in the northern
United States and southern Canada.
During the past few decades, various populations of Canada geese
have altered their migration patterns as a result of transplanting
brood stock, development of refuges or changing agricultural
practices. These routes will continue tachange in the coming years as
long as these factors are in a state of flux. It has been shown
that man can establish breeding colonies of Canada geese with young
birds almost anywhere.
Europe also has several good examples of changes in migration
routes through range extension. One of the best examples is the serin.
During the past century, this European finch has spread its breeding
range from around the Mediterranean Sea to include the entire
continent. While the Mediterranean populations remain sedentary,
the more northern breeding birds are migratory. Most likely, those
birds that did not migrate from the North were eliminated by severe
weather. Similarly, the wheatear, yellow wagtail, and Arctic
warbler have extended their breeding ranges eastward across the
Bering Sea into Alaska, but the wheatear, for instance, migrates all
the way back across Asia to Africa where it winters with other
wheatears coming from Europe, Iceland, and Greenland.
As bird populations become more and more migratory, we might
expect their flight capabilities to be enhanced accordingly. These
changes in morphology are readily seen in wing shape. Several
groups of birds have closely related species or populations some of
which are migratory and others sedentary. The sedentary species or
populations have more rounded wings because of the relative length
of the wing quills. On the other hand, populations that migrate great
distances, such as albatrosses, falcons, swifts, various shorebirds,
and terns, have more pointed wings. Kipp(1942, 1958) demonstrated
this using orioles. The sedentary black-headed oriole of India has a
well-rounded wing whereas the closely related black-naped oriole is
migratory between India and Siberia and has primaries that are
much more pointed and well developed.
Thus it seems the origin and evolution of migration have roots in
the present that are deep in the past. The important thing to consider
in the evolution of a migratory trait is whether a population can adapt
to new conditions by genetic modification of its physiology and
habits. The migratory habit has evolved in those populations in
which, on the average, more individuals survive by moving to a
different area part of the year than if they remained in the same area
WHERE WE STAND
The migration of birds had its beginning in times so remote its
origins have been largely obscured and can be interpreted now only
in terms of present conditions. The causes underlying migration are
exceedingly complex. The mystery that formerly cloaked the
periodic travels of birds, however, has been largely dispelled through
the fairly complete information now available concerning the extent
and times of seasonal journeys of most species. Many gaps still
remain in our knowledge of the subject, but present knowledge is
being placed on record, and the answers to many uncertainties that
continue to make bird migration one of the most fascinating subjects
in the science of ornithology must be left for future studies. In some
areas we are on the threshold of discovery. More and more
sophisticated approaches including radar, radio telemetry, com-
puter processing of banding data, and physiological and behavior
studies are being developed.
With the widespread use of these new techniques, we are beginning
to realize the benefits, aside from aesthetic reasons, for studying
migration. Radar alone has aided tremendously in documenting
flock size, heights, and speeds of migration as well as the descriptions
and locations of patterns and routes of specific migrants in relation to
aircraft flight lanes. Recent studies have indicated local, nonmigra-
tory populations of various blackbirds cause nearly all of the rice
damage in southern States and the "hordes from the North"
contribute very little to the losses. In addition, the transport of
arborviruses from one continent to another via these long distance
migrants is being investigated. People have started to uncover the
secrets of migration and utilize this knowledge for the betterment of
Each kind of bird seems to have its own reaction to the
environment, so that the character of movement differs widely in the
various species, and seldom do any two present the same picture. In
fact, bird migration has been described as a phase of geographic
distribution wherein there is a more or less regular seasonal shifting
of the avian population caused by the same factors that determine the
ranges of the sedentary species. If this view is correct, then it must be
recognized that the far-reaching works of man in altering the natural
condition of the Earth's surface can so change the environment
necessary for the well-being of the birds as to bring about changes in
their yearly travels. The nature and extent of the changes wrought by
man on the North American Continent are readily apparent.
Extensive forests have been burned or cut away, rolling prairies
turned over with the plow, and wetlands drained or filled. Their
places have been taken by a variety of human activites. These great
changes are exerting pressure on native bird populations, and
various species may be either benefited or adversely affected.
The Federal Government has recognized its responsibility to
migratory birds under these changing conditions. Enabling acts
allow for carrying out migratory bird treaty obligations in
cooperation with other countries, and now most species have legal
protection under regulations administered by the U.S. Fish and
Wildlife Service. The effectiveness of conservation laws, however, is
increased in the same measure that the people of the country become
acquainted with the migratory bird resource and interest themselves
personally in the well-being of the various species. Long before
European man came to America, the birds had established their
seasonal patterns of migration throughout the Western Hemisphere.
The economic, scientific, and esthetic values of these migratory
species dictate they be permitted to continue their long-accustomed
and to some extent still-mysterious habits of migration.
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LIST OF BIRD SPIECIES MENTIONED IN TEXT
Branta bemicla hreta
Branta bemicla nigricans
Ay thy a valisineria
* For all North American species the authors have followed nomenclature in the 1957 edition of the A.O.U.
Check-list. Also, we have incorporated the new names presented in the April 1973 issue of ThfAuk( volume 90.
number 2, pages 411-419), the quarterly journal of the A.O.U. For other parts of the world we have used the
most authoritative sources available.
Goose, Snow [Blue]
Hawk, Sparrow (European)
Heron, Black-crowned Night
Heron, Little Blue
Oriole, Black-headed (Indian)
Petrel, Wilson's Storm
Pewee, Western Wood
Pigeon (Rock Dove)
Ay thy a americana
Sparrow, Andean (Rufous-collared)
Sparrow, White- throated
Warbler, Black-throated Blue
irbler, Cape May
Wren, Long-billed Marsh
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fish, wildlife, mineral, forest, and park and recreational
resources. It also has major responsibilities for Indian and
As the Nation's principal conservation agency, the Depart-
ment works to assure that nonrenewable resources are developed
and used wisely, that park and recreational resources are con-
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