Part 6

There are many species, including the tree sparrow, slate-colored junco, and Lapland longspur (Calcarius lapponicus), that nest in Canada and winter in the United States; while others, including the vesper sparrow (Pooecetes gramineus), chipping sparrow (Spizella passerina), grackles, red-winged blackbird, bluebird, the woodcock (Philohela minor), and several species of ducks, nest in the northern United States and move south for the winter to areas along the Gulf of Mexico. This list includes the more hardy species, some individuals of which may linger in protected places well within the reach of severe cold, as, for example, Wilson's snipe, or jacksnipe (Capella delicata), which frequently is found during subzero weather in parts of the Rocky Mountain region where warm springs assure a food supply. More than 100 of our summer birds leave the United States entirely and spend the winter id the West Indies or in Central America or South America. For example, the Cape May warbler (Dendroica tigrina), which breeds from northern New England, northern Michigan, and northern Minnesota, north to New Brunswick, Nova Scotia, and nearly to Great Slave Lake, is concentrated in winter chiefly in the West Indies, its metropolis at this season being the island of Hispaniola.

Figure 13.--Barn Swallow, a bird that has so long a migration route that some individuals breed north to Yukon and Alaska, while the winter range extends south to Argentina, 7,000 miles away.]

LONG-DISTANCE MIGRATIONS

Some of the common summer residents are not content with a trip to northern South America, but push on across the Equator and finally come to rest for the winter in the pampas of Argentina, or even in Patagonia. Thus some species that are more or less associated with, each other in summer, as nighthawks, barn swallows, cliff swallows, and some of the thrushes may also occupy the same general winter quarters in Brazil. Some individual nighthawks and barn swallows (fig. 13) travel still farther, and of all North American land birds these species probably have the longest migration route, as they occur north to Yukon and Alaska, and south to Argentina, 7,000 miles away. Such seasonal flights are exceeded in length, however, by the journeys of several species of water birds, chiefly members of the suborder of shore birds. In this group are 19 species that breed north of the Arctic Circle and winter in South America, 6 of them going as far south as Patagonia, and thus having a migration route more than 8,000 miles in length.

Figure 14.--Arctic tern. The longest flight known for an individual bird was accomplished by an arctic tern that in 3 months flew from the coast of Labrador to the coast of southeastern Africa.]

The arctic tern (Sterna paradisaea) is the champion "globe trotter" and long-distance flier (figs. 14 and 15). Its name "arctic" is well earned, as its breeding range is circumpolar and it nests as far north as it can find a suitable place. The first nest to be found in this region was only 7½° from the North Pole, and it contained a downy chick surrounded by a wall of newly-fallen snow that had been scooped out by the parent. In North America it 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 a few months later they may be found in the Antarctic region, 11,000 miles away. Until very recently the route followed by these hardy fliers was a complete mystery, for although a few scattered individuals have been noted south as far as Long Island, the species is otherwise practically unknown along the Atantic coasts of North America and South America. It is, however, known as a migrant on the west coast of Europe and Africa. By means of numbered bands the picture is now developing of what is apparently not only the longest but also one of the most remarkable of all migratory journeys.

Judging by the evidence at present available, it seems likely that the arctic terns of eastern North America originally found their way here from the Old World, probably by way of Iceland and Greenland. Consequently when the time comes for them to migrate to winter quarters they do not go directly south as do the common (Sterna hirundo) and Forster's terns (S. forsteri), but instead, they fly back eastward along their ancestral route across the Atlantic to the shores of Europe and then go south along the African coast to their winter home; those that breed in the northwestern part of the continent, as in Alaska, probably migrate chiefly down the western coast, as the species is not infrequently reported on the coast of California and also on the western coast of South America.

Figure 15.— Distribution and the migration of the arctic terns of eastern North America. The route indicated for this bird is unique, as 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, and as the route taken is circuitous, these terns probably fly at least 25,000 miles each year.]

The evidence yielded by banding consists of only three definite cases, but their interpretation seems to permit but one conclusion: All three birds were banded as downy chicks, one on July 3, 1913, at Eastern Egg Rock, Maine, and the other two at the Red Islands, Turnevik Bay, Labrador, on July 22, 1927, and July 23, 1928. The first was found dead in the Niger River delta, West Africa, in August 1917, while the Labrador birds were recovered near La Rochelle, France, on October 1, 1927, and at Margate, near Port Shepstone, Natal, South Africa, on November 14, 1928. The flight shown by this last record is the longest known, the trip, between 8,000 and 9,000 miles, being accomplished in less than 3 months.

Probably no other animal in the world enjoys as many hours of daylight as does the arctic tern, since for these birds the sun never sets during their nesting season in the northern part of the range, while during their sojourn in the south, daylight is continuous. During several months of the year they have 24 hours of daylight and during the other months considerably more daylight than darkness.

=ROUTES OF MIGRATION=

While it is beyond question that certain general directions of flight are constantly followed by migratory birds, it is well to remember that the term "migration route" is to some extent a theoretical concept, concerned entirely with the lines of general advance or retreat of a species, rather than the exact course followed by individual birds. Even the records of banded birds usually show no more than the places of banding and recovery, and one must have recourse to intermediate records and to reasoning based on probabilities to fill in details of the flyway actually traversed between the two points.

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 it seems as if the routes of no two species coincide. Differences in distance traveled, in time of starting, in speed of flight, in geographical position, in latitudes of breeding and of wintering grounds, and in other factors, all contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide the avian travelers along more or less definite lines, and it is possible to define general lines of migration for the majority of species.

It has frequently been observed that migrating birds have a tendency to follow major topographic lines on the earth's surface when their trend is in the general direction of the birds' journey. Bird migration is generally thought of as a north-and-south movement, with the lanes of heavier concentration following the coasts, mountain ranges, and principal river valleys. To a considerable extent this is the case, particularly in North America, where the coast lines, mountain chains, and the larger rivers in general run north and south. Students of American birds thus have exceptionally good opportunities to study migratory movements. In cases where the migration is a long one, however, the notion must be abandoned that the birds' flight is restricted to particular narrow routes that follow river valleys and the like, as many species seem to disregard utterly such apparently good natural flyways as river valleys. For example, the Arkansas River has a general east and west course for a great part of its length, and while it does constitute a highway for many perching birds en route from the Mississippi Valley to the Rocky Mountain region, some of the hawks and many ducks and shore birds pay the valley scant attention. They may arrest their fall journey to feed among cottonwoods or along sand bars, but when ready to resume their flight they leave the river and fly directly south over the more or less arid region that lies between the Arkansas and the Rio Grande.

WIDE AND NARROW MIGRATION LANES

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.

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.

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).

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.

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.]