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 travel still farther, and of all North American land birds these species probably have the longest migration route, as they occur north in summer to Yukon and Alaska, and south in winter 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 there 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.
The arctic tern is the champion "globe trotter" and long-distance flier ([fig. 8]). 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 degrees 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 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 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 Atlantic 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.
Figure 8.—Distribution and 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.
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 and Forster's terns, 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.
The evidence yielded by banding consists of only six definite cases, but their interpretation permits but one conclusion. All were banded either as downy chicks or as nonflying immature birds. The details of their banding and recovery are of sufficient interest to justify citing in detail. The first was banded on July 3, 1913, at Eastern Egg Rock, Maine,[2] and in August 1917 was found dead in the Niger River delta, West Africa. The second was banded at the Red Islands, Turnevik Bay, Labrador, on July 22, 1927, and was recovered near La Rochelle, France, on October 1, 1927. The third, also banded on the Red Islands, on July 23, 1928, was retaken at Margate, near Port Shepstone, Natal, South Africa, on November 14, 1928. The fourth, banded at Machias Seal Island, New Brunswick, on July 20, 1935, was captured near St. Nazaire, France, October 8, 1935. The fifth and sixth were banded at Machias Seal Island also; the fifth, banded July 5, 1947, was found on November 10, 1948, at Kingfisher Creek, Sedgefield, near Wilderness, Eastern Cape Province, South Africa, while the sixth, banded on July 18, 1948, was picked up dead during the latter part of September 1948 on the hills near Kyle Strome, Sutherland, Scotland. All that remained of the bird that provides the last-named case was a mutilated foot and it appeared that it had been the victim of some predator. It should be pointed out that the flights indicated in the third and fifth cases detailed above, are the longest known for any birds. Both are between 8,000 and 9,000 miles, which in the case of No. 3 was accomplished in less than 3 months.
[2] Recorded at the time of banding as a common tern, a natural error, as the downy young of common and arctic terns look almost exactly alike.
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 the 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 place of banding and recovery, and one must have recourse to intermediate records and to reasoning based on probabilities to fill in details of the route 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, time of starting, speed of flight, geographical position, 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 been frequently 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 larger rivers in general run north and south. In cases where the migration is a long one, however, the notion must be abandoned that the birds' flight is always restricted to narrow routes that follow river valleys and the like, as many species seem to disregard utterly such apparently good natural highways. 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 route 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 the autumn journey to feed among the 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.