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Migration of Birds

Geographic Patterns of Migration


Populations Within Species

Both length and duration of migratory journeys vary greatly between families, species, or populations within a species. Northern Bobwhite and other North American quails, Northern Cardinals, Canyon, Cactus and Carolina wrens, Wrentits, some of the titmouses and most woodpeckers are largely nonmigratory. These species may live out their entire existence without going more than 10 miles from the nest where they were hatched.

Song Sparrows, Eastern and Western meadowlarks, and Blue Jays 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 movements of these partial migrant species can be gained by observing birds that are banded or color-marked. The American Robin, for example, occurs in the southern United States throughout the year, but only during the summer in Canada and Alaska. Its movements are readily ascertained from museum specimens. The breeding robin of the southeastern states is the southern race. In autumn most of the 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 in 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 Song Sparrows, males remain all year on their northern breeding grounds while the females and young migrate south. In Dark-eyed Juncos, adult females migrate the farthest south, while young males winter the farthest north. Adult male and young female juncos winter at intermediate distances.

Several species containing more than one distinguishable population exhibit "leap-frog" migration patterns. The eastern population of the 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 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 breeds from the Puget Sound-Vancouver Island area northward along 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 migratory populations 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 (Figure 10). Among the differentially sized subspecies of Canada Geese, the populations of lowest body mass breed the farthest north but winter the farthest south, while the heaviest subspecies is a relatively permanent resident in the northern United States. This pattern is clearly related to the increased survival under cold stress afforded by large body size.

Figure 10: Map showing Breeding, Winter, and Winter/Breeding Ranges of the Fox Sparrow
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).

The Palm Warbler breeds from Nova Scotia and Maine west and northwest to the southern Mackenzie River valley. 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 the West Indies and Central America, moving 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.

Short Distance Migration

Some species have extensive summer ranges (e.g., the Pine Warbler, Rock Wren, Field Sparrow, Loggerhead Shrike, and Black-headed 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.

Many species, including American Tree Sparrows, Snow Buntings, and Lapland Longspurs, nest in the far north and winter in the eastern United States, while others, including 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 regions 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.

Long Distance Migration

More than 300 breeding species leave the United States and Canada 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.

Some of the common summer residents of North America migrate even farther, pushing across the Equator and finally coming to rest for the winter on the Argentine pampas or in Patagonia. Common Nighthawks, Barn Swallows, Cliff Swallows, and 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, Ruddy 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.

The Arctic Tern is the champion "globe trotter" and long-distance flier. 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 degrees (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 south to Massachusetts. After the young are grown, 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 flyers was a mystery. Although a few scattered individuals had 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, a migrant on the west coast of Europe and Africa. As a result of band recoveries, its migratory pattern was disclosed (Figure 11). Few other animals in the world enjoy as many hours of daylight as the Arctic Tern. For these birds, the sun shines most of the day during the nesting season in the northern part of the range, and during their winter sojourn to the south, daylight is almost continuous as well.

Figure 11: Map showing the Breeding Areas, Winter Areas, Recovery Points, and Migration Points of Artic Terns
Figure 11.  Distribution and migration of Arctic Terns. The route indicated 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.


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