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 race.
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.
|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: 1. Shumagin fox sparrow; 2. Kodiak fox sparrow; 3. Valdez fox sparrow; 4. Yakutat fox sparrow; 5. Townsend fox sparrow; 6. sooty fox sparrow (After Swarth 1920).|
Many species, including the tree sparrow, snow bunting, and Lapland
longspur, nest in the far north and winter in the eastern 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 Hispaniola.
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 sanderlings. 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.
|Figure 11. Distribution and migration of arctic terns. The route indicatedf or 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.|
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, 11,000 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, 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.