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Migration of Birds

Patterns of Migration


Band recoveries, netting records, and personal observations help to determine migration routes and probe more deeply into the origin and evolution of these pathways. Not surprisingly, certain deviations occur from the expected north and south movements. In the previous section, it was noted that some routes are not poleward at all, but proceed in many directions. After many years of gathering data, a pattern emerges for a particular population, species, or group of species. This section concerns some of the interesting or "eccentric routes," as Cooke referred to them, that birds travel from breeding to wintering grounds and back again.

Loops

Many species do not return north in the spring over the same route they used in the fall; rather, they fly a loop or ellipse. Cooke considered food as the primary factor in determining the course birds took between winter and summer ranges. He speculated that individuals returning by the same route and not finding sufficient food either did not return or did not breed; only the individuals that took a different course with adequate resources survived and left progeny. Assuming that there were genetic components to this variation in orientation and navigation, loop migration routes could evolve. Other investigators considered the prevailing winds to be the major selective factor, since a following wind would require less expenditure of energy. This would give an advantage to individuals who returned north on a different route if the prevailing winds were in more appropriate directions than along the path used during the southward flight.

Whatever the ultimate cause, loop migrations evolved separately in each species to satisfy its particular needs, and this pattern occurs throughout the world among unrelated species. The annual flight of adult American Golden-Plovers illustrates the loop pattern (Figure 24). In the fall, the birds fattened on the rich crop of berries along the coasts of Labrador and Nova Scotia depart south over the Atlantic Ocean to South America. They stop briefly on the coast and then continue overland to the pampas of Argentina where they remain from September to March. When these golden-plovers leave their winter quarters they cross northwestern South America and the Gulf of Mexico to reach the North American mainland on the coasts of Texas and Louisiana. Thence they proceed slowly up the Mississippi Valley and, by the early part of June, are again on their breeding grounds, having performed a round-trip journey in the form of an enormous ellipse with the minor axis about 2,000 miles and the major axis 8,000 miles stretching from the Arctic to the South Temperate Zone. The older birds may be accompanied by some of the young, but most of the immature birds leave their natal grounds later in summer after the adults and move southward through the interior of the country, returning in spring over essentially the same course. The oceanic route is therefore used chiefly by adult birds. A return by the oceanic route in the spring could be fatal. The maritime climate in the Northeast results in foggy conditions along the coast and the frozen soil would offer scanty food resources for the weary travelers. By traveling up the middle of the continent, a much better food supply is assured.

Figure 24: Map showing Breeding Range, Winter Range, and Migration Routes of both Western and Eastern subspecies of the American Golden-Plover
Figure 24.  Distribution and migration of the American Golden Plover. Adults of the eastern subspecies migrate across northeastern Canada and then by a nonstop flight reach South America. In spring they return by way of the Mississippi Valley. Their entire route, therefore, is in the form of a great ellipse with a major axis of 8,000 miles and a minor axis of about 2,000 miles. The western subspecies migrates across the Pacific Ocean to various localities including the Hawaiian and Marquesas islands and the Low, Archipelago.

Other shorebirds follow loop migration routes. White-rumped Sandpipers fly from tundra breeding areas above the Arctic Circle eastward to the Atlantic coast of Labrador, Nova Scotia, and New Brunswick. From there they take the Atlantic Ocean route direct to the Surinam coast of South America, then fly overland to winter in Tierra del Fuego. They return to the arctic by a route through Venezuela and the Great Plains. As for the American Golden-Plovers, the interior route through the continent provides suitable resources while the Atlantic coast is still in the grip of winter. Some Western Sandpipers, instead of following most of their conspecifics southward along the Pacific coast, turn eastward at the Frasier River, moving beyond the Rockies to migrate through the Great Plains, then passing southeastward through Florida and thence to northern South America. In the spring they move westward to the Pacific coast, and the entire species follows the coast north to the tundra in Alaska and eastern Siberia.

Several North American warblers including the Connecticut Warbler (Figure 25) and the western race of the Palm Warbler follow circuitous migration routes. The Connecticut Warbler is not observed on the East coast in spring, but it is recorded farther inland during that season. Thus, this warbler proceeds down the East coast in the fall and up the interior of the continent in the spring. Similarly, the western race of the Palm Warbler moves from its breeding grounds directly east over the Appalachian Mountains before turning south along the Atlantic coast. Television tower kills in northern Florida indicate the population is very concentrated here at this time of year. In the spring this subspecies proceeds north through the interior. The eastern race of the Palm Warbler also proceeds south along the coast in the fall but returns north in the spring along the same path. Scientists had hypothesized that the western population initially migrates eastward to join the rest of the species moving south along the coast because this flight path retraces its past history of range expansion. An alternate hypothesis derived from radar observations, however, suggests that the disparity in seasonal flight directions of many migrants is a positive response to favorable wind directions at that time of year.

Figure 25: Map showing Breeding Range, Fall Migration Route South, Spring Migration Route North, and the Overlap of Fall & Spring Routes of the Connecticut Warbler
Figure 25.  Breeding range and migration routes of the Connecticut Warbler. From the breeding range in northern United States and southern Canada, it migrates east in the fall to New England, then south along the Atlantic coast to Florida and across the West Indies to winter in South America. In the spring it does not return by the same route but rather completes a loop by migrating northwest across the Allegheny Mountains and the Mississippi Valley (adapted from Cooke 1915a).

In the fall, the Short-tailed Shearwater is observed off the west coast of North America as far south as California. At this time the species is on the eastern leg of a tremendous figure-eight around the Pacific Ocean (Figure 26). The Subalpine Warbler and Red-backed Shrike perform loop migrations between Europe and Africa. Both pass much farther to the east in the spring than in the fall. The Arctic Loon travels south across inland Russia to southern Europe but returns to its arctic breeding grounds via the Gulf Stream in the Atlantic because this water is open while inland waterways are still frozen.

Figure 26: Map showing the Bass Strait Breeding Area, Migration Route, and locations where Museum Specimens have been collected for the Short-tailed Shearwater
Figure 26.  Migration route of the Short-tailed Shearwater. An example of an incredibly large loop migration pattern in a pelagic species. Breeding adults return to two islands in Bass Strait during the last part of October after completing a figure-eight circuit of the northern Pacific Ocean (from Serventy 1953).

Dog-legs

Dog-leg migration patterns are characterized by a prominent bend in the route. Studies have shown some of these indirect pathways connecting wintering and breeding areas are the result of tradition much like the lineage of crooked streets in Boston which can be traced back to old cowpaths. When species have extended their range, many species continue to follow the old route from the original range even if the new areas are not on the same axis as the earlier route. The new extended routes are simply added to the old pathways.

This crooked traditional path can be seen in the routes taken by Old World species extending their ranges into the New World from Europe and Asia. For example, the Northern Wheatear has extended its range into Greenland and Labrador where the local breeding population has become a separate race. When the Labrador individuals depart from their breeding grounds, they proceed north to Greenland, their ancestral home, then east to Europe and south to Africa, the traditional wintering area for all wheatears. Alaskan breeding wheatears migrate to Africa in the opposite direction via Asia where the Alaskan population presumably originated. Alaskan breeding Arctic and Willow warblers and Bluethroats also migrate westward into Siberia and then southward on the Asiatic side of the Pacific Ocean. Some investigators believe the Arctic Tern colonized the New World from Europe because when this bird departs for the south it first crosses the Atlantic to Europe, then moves down the eastern Atlantic coast to Africa and either back across the Atlantic to South America or continues south down past South Africa (Figure 11). To get to South America from the eastern arctic, it would be shorter to follow the American Golden Plover's flight path straight down the Atlantic or along the east coast of the United States but the fact that no Arctic Terns have been observed in the Caribbean indicates that they do no follow that route.

In western United States, California Gulls nest in various colonies around Great Salt Lake and Yellowstone Park. Banding records indicate these populations winter along the California coast (Figure 27). Instead of traveling southwest by the shortest distance to the wintering grounds, they proceed down the Snake and Columbia rivers and reach the coast around Vancouver. From there they proceed south along the coast to Oregon and California. In the spring the adults return over the same course rather than taking the shorter flight northeast in April across the deserts and mountains; this route would be largely made over a cold and inhospitable country.

Figure 27: Map showing Wintering Ground, Migration Route, and Band Recovery locations for California Gulls
Figure 27.  Migration route and wintering grounds of California Gulls banded in northwestern Wyoming. During fall migration, the birds proceed west from the breeding grounds to the Pacific Ocean before turning south to wintering areas in California. A more direct route across Nevada would entail a trip through relatively barren country (After Diem and Condon 1967).

Several dog-leg patterns are apparent in the eastern and western populations of the Tundra Swan (Figure 28). In the eastern population, a sharp change in direction occurs at their major feeding and resting areas in North Dakota. After the birds arrive from the arctic breeding grounds, they proceed east-southeast to their wintering grounds on Chesapeake Bay. In the western population, thousands migrate from the Alaskan breeding grounds to the large marshes along Great Salt Lake. Then after a major stopover, this population heads west over the mountains to California.

The general trend of migration in most northern populations of North American birds is along a northwest to southeast axis. This again is a reflection of eastern species having extended their ranges by pushing westward. For example, in the Stikine River Valley of northern British Columbia and southwestern Alaska the Common Nighthawk, Chipping Sparrow, Rusty Blackbird, Yellow Warbler, and American Redstart have established breeding populations in areas that are just 20 to 100 miles from the Pacific Ocean. The American Robin, Northern Flicker, Dark-eyed Junco, Blackpoll Warbler, Yellow-rumped Warbler, and Ovenbird, all common eastern species, also are established as breeding birds in western Alaska. The Ovenbird has even been detected on the lower Yukon River, and the Sandhill Crane, Pectoral Sandpiper, and Gray-cheeked Thrush have moved across the Bering Strait into Siberia. Yet these birds continue to migrate through the eastern part of the continent. Instead of taking the shortest route south, they retrace the direction of their westward expansion and move southward along the same avenues as their more eastern relatives.

Figure 28: Map showing Fall Migration of both Western and Eastern Populations of Tundra Swans
Figure 28.  Distribution and migration routes of Tundra Swans in North America. Birds from the central arctic head south to North Dakota before proceeding east to Chesapeake Bay, while many Alaskan breeders migrate to Great Salt Lake before turning west to winter in California (after Sladen, 1973).

The Red-eyed Vireo is primarily an inhabitant east of the Great Plains, but an arm of its breeding range extends northwest to the Pacific coast in British Columbia (Figure 29). It has been suggested that this range extension has taken place comparatively recently via deciduous woodland corridors, and the invaders retrace in spring and fall the general route by which they originally entered the region.

Figure 29: Map showing the Breeding Range, Winter Range, and Migration Limits of the Red-eyed Vireo
Figure 29.  Distribution and migration of the Red-eyed Vireo. It is evident that the Red-eyed Vireo has only recently invaded Washington by an extension of its breeding range almost due west from the upper Missouri Valley. Like the Bobolink (Fig. 19), however, the western breeders do not take the short cut south or southeast from their nesting grounds but migrate spring and fall along the route traveled in making the extension.

In the case of the Bobolink, a new extension of the breeding range and a subsequent change in the migration of the species has taken place since settlement by Euroamericans (Figure 19). Because the Bobolink is a bird of damp meadows, it was originally cut off from the western states by the intervening arid Great Plains, but with the advent of irrigation and the bringing of large areas under cultivation, small colonies of nesting Bobolinks appeared at various western points. Now the species is established as a regular breeder in the great mountain parks and irrigated valleys of Colorado and elsewhere almost to the Pacific coast. These western pioneers fly long distances east and west to join the western edge of the route followed by the bulk of the Bobolinks that breed in the northern United States and southern Canada.

Pelagic Wandering

Many of the pelagic birds observed off our coasts appear to be nomadic when they are not breeding. These movements are not necessarily random, because there is usually a seasonal shift in the population, often for great distances and in specific directions, away from the breeding area after completion of the nesting cycle. Also the return from the sea to nesting areas is at a definite time of year.

Observations on the movements of pelagic birds are difficult at best and accurate records are few. We do know some of these species have regular routes (e.g., Arctic Terns) and specific patterns of migration (e.g., the loop in the Short-tailed Shearwater). As more knowledge is accumulated on the "nomadic" species, we may actually find they, too, have regular migration routes.

Movements of some of the tubenoses (albatrosses, fulmars, shearwaters, and storm-petrels) have been correlated with ocean currents, prevailing winds, temperature, and water fertility. Commercial fishermen have long known ocean currents are very important factors in the supply of nutrients, plankton, and forage fish for larger fish. These same foodstuffs often attract pelagic birds as evidenced by the tremendous concentrations of birds off the Peruvian coasts where there is an upwelling of cold, nutrient-bearing water. The migration routes of many albatrosses are over temperate marine waters of high biological productivity; that of the Laysan Albatross is correlated with cold currents, while the Black-footed Albatross occurs over warm currents. Many Southern Hemisphere pelagic species have been extremely successful by migrating across the equator to exploit rich northern waters during the North Temperate summer.

Leap-frogging

When two or more races of the same species occupy different breeding ranges on the same migratory axis, the races breeding the farthest north often winter the farthest south. Thus, a northern race "leap-frogs" over the breeding and wintering range of the southern populations. This has been well documented in the Fox Sparrow discussed previously (Figure 10) and has been demonstrated for populations of the Eastern Bluebird.

Vertical Migration

In order to find winter quarters furnishing suitable conditions, many North American birds fly hundreds of miles across land and sea. Others, however, are able to reach satisfactory areas merely by moving down the sides of a mountain. In such cases, a few hundred feet of altitude corresponds to hundreds of miles of latitude. These vertical or altitudinal migrations occur worldwide wherever there are large mountain ranges. Aristotle (cited by Dorst) first mentions vertical migration, ". . . birds in winter and in frosty weather come down to the plains for warmth, and in summer migrate to the hills for coolness . . . "

In the Rocky Mountain region vertical migrations are particularly notable. Chickadees, rosy-finches, juncos, Pine Grosbeaks, Williamson's Sapsuckers and others nest at high altitudes and move down to the lower levels to spend the winter.

The Dark-eyed Juncos breeding in the Great Smoky Mountains and northward to the Blue Ridge make a vertical migration, but other members of the species, breeding in the eastern north woods make an annual north-south migration of hundreds of miles. The young of mountain-breeding juncos work down to the lower levels as soon as the nesting season is over, while the adults come later. The sudden increases in numbers of birds in the foothills are particularly noticeable when cold spells with snow or frost occur at the higher altitudes. In the Dead Sea area of the Middle East, some birds that breed in this extremely hot desert move up into the surrounding cooler hills during the winter.

The vertical migrations of some mountain-dwelling gallinaceous birds (for example, Mountain Quail and Blue Grouse) are quite interesting because the annual journey from breeding to wintering grounds is made on foot. Mountain Quail make this downward trek quite early in the fall well before any snows can prevent them from reaching their goal. Blue Grouse perform essentially the same journey in reverse. During midwinter, these birds can be found near timberline eating spruce buds protruding above the snow.

Premigratory Movements

Recent banding studies have demonstrated many migrants, especially young of the year, have a tendency to disperse after fledging. These premigratory movements have also been called post-fledging dispersal, reverse migration, and postbreeding northward migration. Demonstration of this phenomenon is especially important as it relates to locality-faithfulness (philopatry), range extension, and gene flow between populations. These movements cannot be considered as true migrations even though they are repeated annually by the species between breeding grounds and some other area, since these movements are generally repeated by the same age class in the population but not the same individuals.

Nevertheless, these regular northward movements are quite striking, especially in herons. The young of some species commonly wander late in the summer and fall for several hundred miles north of the area in which they were hatched. Young Little Blue Herons as well as Great and Snowy egrets are conspicuous in the East as far north as New England and in the Mississippi Valley to Minnesota and Michigan. Black-crowned Night-Herons banded in a large colony at Barnstable, Massachusetts have been recaptured the same season northward to Maine and Quebec and westward to New York. In September most of them return to the south.

These movements have been noted in several other species as well, for example, the northward movement of Bald Eagles along the Atlantic coast (Figure 30). Birds banded as nestlings in Florida have been retaken that summer 1,500 miles away in Canada. Postbreeding northward movements are also shared by Wood Ducks, Yellow-breasted Chats, Eastern Bluebirds, and American White Pelicans.

Figure 30: Map showing Nesting Area, distances traveled, and Northern Recovery locations of banded Bald Eagles
Figure 30.  Northern recoveries of young Bald Eagles banded as nestlings in Florida. The birds sometimes "migrate" over 1,500 miles up the coast during their first summer before returning south (from Broley 1947).

A somewhat different type of postbreeding migration is the molt migration exhibited by many species of waterfowl. These birds may travel considerable distances away from their nesting area to traditional molting sites where they spend a flightless period in eclipse plumage. At such times they may move well into the breeding ranges of other geographic races of their species. These movements may be governed by the availability of food or a reduction in the chances for predation while they are flightless, This is counteracted in the fall by migration that carries those birds from more northern latitudes after the nesting period back to their normal wintering homes in the south.

Vagrant Migration

The occasional great invasions beyond the limits of their normal range of certain birds, especially species breeding in the far North, are quite different from migration patterns discussed previously. Classic examples of such invasions in the eastern part of the country are the periodic flights of crossbills. Sometimes these migrations will extend well south into the southern States.

Snowy Owls are noted for periodic invasions that have been correlated with declines in lemmings, a primary food resource of northern predators. The interval between these incursions varied from 2 to 14 years, but nearly half were at intervals of 4 years. A great flight occurred in the winter of 1926-27 when more than 1,000 records were received from New England alone, but the largest on record was in 1945-46 when the "Snowy Owl Committee" of the American Ornithologists' Union received reports of 13,502 birds, of which 4,443 were reported killed. It extended over the entire width of the continent from Washington and British Columbia to the Atlantic coast and south to Nebraska, Illinois, Indiana, Pennsylvania, and Maryland. One was taken as far south as South Carolina.

In the Rocky Mountain region, flights of the beautiful Bohemian Waxwing are occasionally recorded. The greatest invasion in the history of Colorado ornithology occurred in February 1917, when it was estimated that at least 10,000 were observed in Denver. The previous large occurrence of waxwings in Denver was in 1908.

Likewise, Evening Grosbeaks illustrate similar wanderings. In addition to occasional trips south of their regular range, they will also travel east and west for considerable distances. Evening Grosbeaks banded at Sault Ste. Marie, Michigan have been recaptured in winter on Cape Cod, Massachusetts and in the following breeding season were back at the original banding location. Banding records and museum specimen identifications demonstrate that this east-and-west trip across the northeastern part of the country is sometimes made also by Purple Finches, Red Crossbills, and Mourning Doves.


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