When birds start their southward migration, the movement necessarily
involves the full width of the breeding range. Later, there is a
convergence of the lines of flight taken by individual birds, owing
to the conformation of the land mass, and as the species proceeds
southward the width of the occupied region becomes less and less.
An example of this is provided by the common kingbird (_Tyrannus
tyrannus_), which breeds from Newfoundland to British Columbia, a
summer range 2,800 miles wide. On migration, however, its paths
converge, 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, and still farther south the migration
path is further restricted. In the latitude of Yucatan it is not more
than 400 miles wide, and it is probable that the great bulk of the
species moves in a belt that is less than half that width.
A migration route, therefore, may be anything from a narrow path
that adheres closely to some definite geographical feature, such
as a river valley or a coast line, to a broad boulevard that leads
in the desired direction and follows only the general trend of the
land mass. Also it is to be remembered that whatever main routes are
described, there remain 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 routes, therefore, must
be considered merely as indicating paths of migration on which the
tendency to concentrate is particularly noticeable.
In considering the width of migration lanes it will be obvious that
certain species, as the knot (_Calidris canutus_) and the purple
sandpiper (_Arquatella maritima_), which are normally found only
along the coasts, must have extremely narrow routes of travel. They
are limited on one side by the broad waters of the ocean and on the
other by land and fresh water, both of which are unsuited to furnish
the food that is desired and necessary to the well being of these
species.
Among land birds that have a definite migration, the Ipswich sparrow
(_Passerculus princeps_) has what is probably the most rstricted
migration range of any species. It is known to breed only on Sable
Island, Nova Scotia, and it winters along the Atlantic coast south
to Georgia. Living constantly within sound of the surf, it is rarely
more than a quarter of a mile from the outer beach, and is entirely
at home among the sand dunes and their sparse covering of coarse
grass.
[Illustration: B4630M
Figure 16.--Breeding and wintering ranges and migration of Harris's
sparrow, an example of a narrow migration route through the interior
of the country. The heavy broken lines enclose the region traversed
by the majority of these finches; the light broken line encloses the
country where they occur with more or less regularity; while the
spots indicate records of accidental or sporadic occurrence.]
Harris's sparrow (_Zonotrichia querula_) supplies an interesting
example of a narrow migration route in the interior of the country
(fig. 16). This fine, large finch is known to breed only in the
region from Fort Churchill, on the west shore of Hudson Bay,
northwest to the shores of Great Bear Lake. Very few actual breeding
records of the species are available, but these are sufficient
to indicate that the breeding range is in the strip of country
characterized by more or less stunted timber just south of the limit
of trees. When it begins its fall migration, this bird necessarily
covers the full width of its breeding area. Then it proceeds almost
directly south, or slightly southeasterly, the area covered by the
majority of the species becoming gradually constricted, so that
by the time it reaches the United States it is most numerous in a
belt about 500 miles wide, extending across North Dakota to central
Minnesota. Harris's sparrows are noted on migration with fair
regularity east to the western shore of Lake Michigan, and west to
the foothills of the Rocky Mountains, but the great bulk of the
species moves north and south through a relatively narrow path in
the central part of the continent. Present knowledge suggests that
the reason for this narrow migration range is the close association
that Harris's sparrow maintains with a certain type of habitat,
including brushy places, thickets, edges of groves, and weed patches.
While these environmental conditions are found in other parts of
the country, the region crossed by this sparrow presents almost a
continuous succession of habitat of this type. Its winter range
extends from southeastern Nebraska and northwestern Missouri, across
eastern Kansas and Oklahoma and through a narrow section of central
Texas, at places hardly more than 150 miles wide.
[Illustration: B4502M
Figure 17.--Distribution and migration of the scarlet tanager.
During the breeding season individual scarlet tanagers may be 1,900
miles apart in an east-and-west line across the breeding range. In
migration, however, the lines converge until in southern Central
America they are not more than 100 miles apart. For migration paths
of other widths see figures 16, 18, and 19.]
The scarlet tanager presents another extreme case of narrowness of
migration route (fig. 17), 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 700-mile stretch from eastern Texas to Appalachicola Bay, but
thereafter the lines do dot 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. 18).
[Illustration: B4501M
Figure 18.--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 16, 17, and 19.]
While 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 (_Setophaga ruticilla_) 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. 19). 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: B4500M
Figure 19.--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 fly way thus has an east and west width of more
than 2,000 miles. For migration paths of greater or less extent see
figures 16, 17, and 18.]
In the following, the 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 maps (figs. 20 and 21) must not
be considered as representing paths with clearly defined borders, but
rather as convenient subdivisions of the one great flyway that covers
practically the entire width of the North American Continent and
extends from the Arctic coast to South America.
[Illustration: B1032M
Figure 20.--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.]