migration route (fig. 10), its breeding range extending in greatest
width from New Brunswick to Saskatchewan, a distance of about 1,900
miles. As the birds move southward in 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. Continuing to converge through
Honduras and Costa Rica, the boundaries there are not more than 100
miles apart. The species winters in northwestern South America, where
it spreads out over most 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
thereafter the lines do not further converge, as this grosbeak enters
the northern part of its winter quarters in Central America and South
America through a door of about the same width (fig. 11).
Although the cases cited represent extremes of convergence, a
narrowing of the migratory path is the rule to a greater or less
degree for the majority of North American birds. The shape of the
continent tends to effect this, and so the width of the migration
route in the latitude of the Gulf of Mexico is usually much less than
in the breeding territory.
The redstart represents a notable case of a wide migration route,
although even in the southern United States this is much narrower
than the breeding range (fig. 12). These birds, however, cross all
parts of the Gulf of Mexico and pass from Florida to Cuba and Haiti
by way of the Bahamas, so that here their route has a width of about
2,500 miles.
[Illustration: _Figure 11._--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 700 miles apart when the birds leave the United
States. For migration paths of other widths see figures 9, 10, and
12.]
=The flyways=
In 1935, as a result of studies of banding data, the author
discovered the existence of the four great flyway systems. This
discovery, based upon analyses of the several thousand records of
the recovery of migratory waterfowl then available, was announced by
the Biological Survey (Lincoln, 1935c) and, beginning in 1948, it
has served as the basis for administrative action by the Fish and
Wildlife Service in the annual hunting regulations.
[Illustration: _Figure 12._--Distribution and migration of the
redstart. An example of a wide migration route, since 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 thus has an
east-and-west width of more than 2,000 miles. For migration paths of
greater or less extent see figures 9, 10, and 11.]
Although this study was confined to this one family of birds there
is a growing mass of evidence in support of the belief that all
populations of migratory birds adhere with more or less fidelity to
their respective flyways. The terms "flyway" and "migration route"
have in the past been used more or less as synonyms but the modern
concept of a flyway is that it is a vast geographic region with
extensive breeding grounds and wintering grounds connected with each
other by a more or less complicated system of migration routes. Each
flyway has its own populations of birds, even of those species that
may have a continental distribution. The breeding grounds of one or
more flyways may (and usually do) overlap broadly, so that during the
nesting season extensive areas may be occupied by birds of the same
species but which belong to different flyways.
The maps (figs. 13, 14, 15, and 16) show the flyways as they are now
understood. It should be pointed out, however, that in the other maps
used in this bulletin, the entire range of a species is shown without
any attempt to distinguish by flyways the different populations. As
banding data accumulate for the nongame species, this distinction
will ultimately be possible, but for the time being, consideration of
their migrations must be chiefly by routes.
The following discussion of the principal routes of North American
birds relates chiefly to the fall migration, for, except as otherwise
noted, the spring flight generally retraces the same course. The
routes indicated on the map (fig. 17) must not be considered as
representing paths with clearly defined borders, but rather as
convenient subdivisions of the four great flyways that, as indicated
above, cover practically the entire width of the North American
Continent and extend from the Arctic coast to South America.
[Illustration: _Figure 13._--The Atlantic flyway.]
=Atlantic oceanic route=
By reference to figure 17 it will be noted that route No. 1 is almost
entirely oceanic, passing directly over the Atlantic Ocean from
Labrador and Nova Scotia to the Lesser Antilles, and then through
this group of small islands to the mainland of South America. This
is not a popular route and its chief claim to fame is that it is the
fall route used by most of the adult eastern golden plovers, and
probably by some other shore birds. Since it lies 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 shore birds that breed on the Arctic tundra of
Mackenzie and Alaska fly southeastward across Canada to the Atlantic
coast and finally follow this oceanic route to the mainland of South
America. The golden plover may accomplish the whole 2,400 miles
without pause or rest, in fair weather the flocks passing 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. As it swims lightly and easily it may make a few short stops
along the way, and it has been seen actually resting on the ocean.
Other shore birds have been observed busily feeding in the great area
of ocean known as the Sargasso Sea, where thousands of square miles
of floating seaweed teem with marine life.
[Illustration: _Figure 14._--The Mississippi flyway.]
[Illustration: _Figure 15._--The Central flyway.]
[Illustration: _Figure 16._--The Pacific flyway.]
The annual flight of the adult eastern golden plover is so wonderful
that it may be given in some detail, particularly since it is one of
the exceptions to the general rule that spring and fall movements are
over the same routes (fig. 18). After reaching the South American
coast the birds make a short stop and then continue overland to the
pampas of Argentina, where they remain from September to March.
Leaving their winter quarters, they cross northwestern South America
and the Gulf of Mexico, reaching 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, reaching from the Arctic tundra to the
pampas of Argentina. The older birds are probably accompanied by
some of the young, perhaps those from early nestings, 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 elliptical route is therefore
used chiefly by fully adult birds.
[Illustration: _Figure 17._--Principal migration routes used by birds
in passing from North America to winter quarters in the West Indies,
Central America, and South America. Route No. 4 is the one used most
extensively; only a few species make the 2,400-mile flight from Nova
Scotia to South America.]
[Illustration: _Figure 18._--Distribution and migration of the golden
plover. Adults of the eastern form 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 is
therefore 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 Pacific golden
plover, which breeds in Alaska, apparently makes a nonstop flight
across the ocean to Hawaii, the Marquesas Islands, and the Low
Archipelago, returning in spring over the same route.]
=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 land birds that breed in New England follow the
coast southward to Florida and travel thence by island and mainland
to South America (fig. 17, route 2). As will be seen from the map, a
seemingly natural and convenient highway extends through the Bahamas,
Cuba, Hispaniola, Puerto Rico, and the Lesser Antilles to the South
American coast. Resting places are afforded at convenient intervals,
and at no time need the aerial travelers be out of sight of land. It
is not, however, the favored highway, and only about 25 species of
birds go beyond Cuba to Puerto Rico along this route to their winter
quarters, while only 6 species are known to reach South America by
way of the Lesser Antilles. The obvious draw-back is lack of adequate
food. The total area of all the West Indies east of Puerto Rico is
less than that of Rhode Island, so that if only a small part of
the birds of the eastern United States were to travel this way, it
is doubtful whether even the luxuriant flora and fauna of tropical
habitats would provide food sufficient for their needs. Nevertheless,
many thousands of coots, widgeons, pintails, blue-winged teal, and
other waterfowl and shorebirds regularly spend the winter season
in the coastal marshes and the inland lakes and ponds of Cuba,
Hispaniola, and Puerto Rico.
The map (fig. 17) also will show that route No. 3 presents 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 land birds. 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 some 60
species cross the 150 miles from Florida to Cuba where about half
this number elect to remain for the winter months. The others do not
hesitate to fly the 90 miles between Cuba and Jamaica, but from that
point to the South American coast there is a stretch of islandless
ocean fully 500 miles across. Scarcely a third of the North American
migrants leave the forested mountains of Jamaica to risk the perils
of this ocean trip. Chief among these is the bobolink, which so far
outnumbers all other birds using this route that it may be well
called the "bobolink route" (fig. 19). As traveling companions
along this route, the bobolink may meet a vireo, a kingbird, and a
nighthawk from Florida; the chuck-wills-widow of the Southeastern
States; the black-billed and the yellow-billed cuckoos from New
England; the gray-cheeked thrush from Quebec; bank swallows from
Labrador; and the blackpolled warbler from Alaska. Sometimes this
scattered assemblage will be joined by a tanager or a wood thrush
but the "bobolink route" is not popular with the greater number of
migrants, and although many individuals traverse it, they are only a
small fraction of the multitudes of North American birds that spend
the winter in South America.
[Illustration: _Figure 19._--Distribution and migration of the
bobolink. In crossing to South America most of the bobolinks use
route 3 (fig. 17), showing no hesitation in making the flight from
Jamaica across an islandless stretch of ocean. It will 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.
(See p. 55.)]
Formerly it was thought that most of the North American land birds
that migrate to Central America made a leisurely trip along the
Florida coast, crossed to Cuba, and thence 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 land bird storm-driven from its accustomed course. What
actually happens is that in the fall many of the birds that breed
east of the Allegheny 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.
The routes used by the Atlantic brant merit some detail for the
reason that these were long misunderstood. These birds winter on
the Atlantic coast, chiefly at Barnegat Bay, N. J., 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, hence overland to Northumberland
Strait, which separates Prince Edward Island from the mainland of New
Brunswick and Nova Scotia. A minor route appears to lead northward
from Long Island Sound by way of such valleys as those of the
Housatonic and Connecticut Rivers, and on across southern Quebec to
the St. Lawrence River.
After spending the entire month of May feeding and resting in various
parts of the Gulf of St. Lawrence, the eastern segment of the brant
population suddenly resumes its journey by crossing to the north
shore of the St. Lawrence estuary. The Bay of Seven Islands, in this
general region, is the point of departure for long overland flights
that are made by the two segments of the population. The eastern
and larger of these appears to fly almost due north to Ungava Bay
and from there to nesting grounds, probably in Baffin Island and
Greenland. The smaller segment travels a route that is but slightly
north of west to the southeastern shores of James Bay, although
somewhere to the east of that area some of the flocks take a more
northwesterly course, descending the Fort George River to reach the
eastern shore of James Bay about two-thirds of the distance north of
its southern extremity. Upon their arrival at either of these two
points on James Bay, the brant of this western segment turn northward
and proceed through the eastern part of Hudson Bay to their breeding
grounds, probably in western Baffin Island, Southampton Island, and
other islands in the Canadian Arctic.
In general, the fall migration of the brant follows the routes
utilized in the spring. At this season, when gathering for the
flight of 570 miles to the St. Lawrence River, they appear only on
the western and southern shores of Ungava Bay. 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. Lawrence River.
The Atlantic flyway 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 ducks,
Canada geese, and many of the black ducks that winter in the waters
and marshes of the coastal region south of Delaware Bay. The
canvasbacks, redheads, and scaups come from their 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, crossing the mountains in a single flight,
reach the Atlantic coast south of New Jersey. This route, with
its Mississippi Valley branch, has been fully demonstrated by the
recovery records of ducks banded at Lake Scugog, Ontario.
The white-winged scoter, which also breeds in the interior of the
continent from northern North Dakota north to the Arctic coast, was
at one time credited with an elliptical migration route, at least
insofar as those wintering on the Atlantic coast are concerned.
This sea duck nests only near fresh water but spends the winters on
the ocean along both the Atlantic and Pacific coasts of the United
States. It migrates over land surfaces mainly at night and it is now
believed that after working northward to the waters of Long Island
Sound it starts up the valleys of the Hudson and Connecticut Rivers
and flies overland to the Great Lakes, from which region it continues
west and northwest to the breeding grounds, returning to its winter
quarters over the same route. Early ideas alleging an elliptical
route probably arose from the fact that great numbers of first-year
nonbreeding birds regularly pass up the New England coast, cross the
Gulf of St. Lawrence and spend the summer loafing off the coast of
Labrador. In the fall these birds form into large flocks and retrace
their flight, chiefly during daylight hours, to winter quarters from
southeastern Maine south at least to Chesapeake Bay. As it was not
known that the white-winged scoter does not usually breed until it is
2 years old, and since the southward movement of yearling birds was
conspicuous while the travels of those from the nesting grounds were
chiefly at night, the theory was advanced that the latter flew 1,500
miles due east from the region west of Hudson Bay to the coast of
Labrador, thence southward to the known winter quarters.
A study of the Canada geese that winter abundantly in the waters of
Back Bay, Va., and Currituck Sound, N. C., reveals another important
tributary to the Atlantic coast route. Banding has shown that the
principal breeding grounds of these birds are among the islands
and on the eastern shore of Hudson Bay. From this region they move
south in autumn to the point of Lower Ontario between Lakes Erie
and Huron. Occasionally one of these geese will be recovered in the
Mississippi Valley but the great majority are retaken either on their
breeding grounds or on the Atlantic coast south of Delaware Bay,
showing another instance of a long cross-country flight by waterfowl.
Although the Canada goose is abundant in migration on the coast of
New England, the birds taken there rarely include any of those banded
in southern Ontario. The northeastern population of these geese comes
from breeding areas in New England, the Maritime Provinces of Canada,
Newfoundland, and the desolate coast of Labrador, their migration
being entirely coastwise. Still another cross-country route between
the Mississippi Valley and the Atlantic coast remains to be briefly
described. It is not yet well understood, but the banding of ducks
such as the blue-winged teal on the coastal saw-grass marshes of
South Carolina has revealed that there is a migration route across
the Appalachians to the Mississippi Valley. Birds marked in these
marshes have been retaken in Tennessee and Kentucky, as well as in
other States farther north in the Mississippi Flyway.
=Mackenzie Valley--Great Lakes--Mississippi Valley route and
tributaries=
Easily the longest route of any in the Western Hemisphere is that
extending from the Mackenzie Valley past the Great Lakes and down the
Mississippi River, including its tributaries. 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 Patagonia. During the spring migration some of the shore birds
traverse the full extent of this great path, and it seems likely
that the nighthawk, the barn swallow, the blackpolled warbler, and
individuals of several other species that breed north to Yukon and
Alaska must twice each year cover the larger part of it.
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, there is not even a ridge of hills high enough to interfere
with the movements of the feathered travelers, and the greatest
elevation above sea level is less than 2,000 feet. Well timbered and
watered, the entire region affords ideal conditions for the support
of its great hosts of migrating birds. It is followed by such vast
numbers of ducks, geese, shore birds, blackbirds, sparrows, warblers,
and thrushes, that observers stationed at favorable points in the
Mississippi Valley daring the height of migration can see a greater
number of species and individuals than can be noted anywhere else in
the world.
Starting in the region of Kotzebue Sound, Alaska, the route extends
eastward across northern Alaska and joins another that has its origin
at the mouth of the Mackenzie River. The line of flight then trends a
little east of south through the great lake system of central Canada,
where it is joined by two or three other routes from the northeast
that have their origin on the central Arctic coast. Continuing
southward the migrating flocks are constantly augmented by additions
to their numbers as they pass over the great breeding grounds of
central and southern Canada. Upon reaching the headwaters of the
Missouri and Mississippi Rivers the route follows these streams to
the Gulf coast, Arriving in this latitude many species, including
the ducks and geese, the robin, the myrtle warbler, and some others
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, strike out boldly for Central America
and South America. This part of the route is a broad "boulevard"
extending from northwestern Florida to eastern Texas and reaching
southward across the Gulf of Mexico to Yucatan and the Isthmus of
Tehuantepec (fig. 17, route 4). In other words, when most of the
birds (chiefly shore birds and land birds) that have come south
directly through the Mississippi Valley have reached the coastal
plains of Mississippi and Louisiana, they continue directly across
the Gulf of Mexico. This route appears to have preference over the
safer but more circuitous land or island routes by way of Texas and
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 highway relates chiefly
to waterfowl. Reference already has been made to the flight of the
black ducks (p. 58) 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 have been retaken at points south
of the junction of the Ohio River with the Mississippi, definitely
indicating their route of travel from southern Ontario.