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.

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. 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.

Orientation and Navigation
Factors in a bird's environment select for the expression of migratory behavior, leading to the evolution of a migratory pattern or, on the other hand, to the loss of migratory abilities. Factors in the environment function to provide direct, proximal stimulation for the physiological preparation for migration. Factors in the environment also provide information that allows birds to navigate during migratory passage. Navigation requires knowing three things: current location, destination, and the direction to travel to get from the current location to the destination. Humans eventually learned to use both the sun and the stars to obtain this information. Recently we invented more precise satellite-based technologies that have made these celestial cues for determining geographic positions superfluous and developed electronic aids to navigation that allow orientation without reference to the natural environment. Birds have successfully navigated for eons using environmental information.

Birds are not alone in their ability to navigate long distances. Fish, mammals, and even insects make migratory journeys. But the clarion honking of geese moving in huge skeins across the vault of the heavens, the twittering of migrants filtering down out of the night sky, the flocks of newly arrived birds filling woodlands, fields, and mudflats makes us most aware of the seasonal movements of birds and fills us with awe and wonder as to how such a magnificent event can be accomplished season after season, year after year, with such unerring precision.

Of the three kinds of information necessary for navigation, we know something about the environmental cues that birds use to orient their migratory flight in the proper direction. On the other hand, there also is well-supported experimental evidence that birds use neither the positions of the sun or the stars to know where they are or where they are to go. It has been shown, however, that birds must learn both the location of the wintering area as well as the location of the breeding area in order to navigate properly, but we have no idea what information they are learning. Nor do we know what cues birds use to know the location of their migratory destination when they are in their wintering locale, often thousands of miles away. The recapture of banded birds at the same places along the route of the migratory journey in subsequent years suggests that some species also learn the location of traditional stop-over sites, but how they do that remains a mystery.

Vector Navigation
European Starlings pass through Holland on their migration from Sweden, Finland, and northwestern Russia to their wintering grounds on the channel coast of France and the southern British Isles. Perdeck transported thousands of starlings from The Hague to Switzerland, releasing these banded birds in a geographic location in which the population had never had any previous experience. The subsequent recapture of many of these banded birds demonstrated that the adults, which had previously made the migratory flight, knew they had been displaced and returned to their normal wintering range by flying a direction approximately ninety degrees to their usual southwesterly course. The juveniles, which had never made the trip before, in contrast, continued to fly southwest and were recaptured on the Iberian peninsula. These first-year birds "knew" what direction to fly, but did not recognize they had been displaced, thus ending up in an atypical wintering range. In subsequent years these now adult birds returned to again winter in Spain and Portugal. Coupled with another displacement of starlings to the Barcelona coast in Spain, Perdeck concluded that the proper direction of the migratory flight was innate, that is, inherited in their DNA, since the naive juveniles could fly that direction, and that the birds were also genetically programmed to fly a set distance. This is the same vector or dead-reckoning navigation program Lindberg used to fly from New York to Paris by maintaining a given compass direction (or directions) for a predetermined time (i.e., distance). But this study demonstrated that this navigation system is modified by experience, since adults knew they were not in Holland any longer and knew that in order to get to their normal wintering grounds they needed to fly a direction that they had never flown before! These results are truly amazing. And we don't know how they did it.

Displacement studies in the Western Hemisphere using several species of buntings also demonstrated that birds recognized they had been moved and could fly appropriate, yet unique, routes to return to their normal range. Yet adult Hooded Crows transported latitudinally by over 600 km from wintering grounds in the eastern Baltic to northwestern Germany failed to recognize this displacement. In the spring they oriented properly but migrated to Sweden, west of their normal breeding range. This species used vector navigation, but did not know the location of its traditional destination. Since it is generally accepted that migratory behavior evolved independently again and again in different bird populations, a single explanation to fit all cases perhaps should not be expected.

Orientation Cues
Most of the effort applied to understanding how birds make a migratory flight has been directed toward environmental cues that birds use to maintain a particular flight direction. These cues are landmarks on the Earth's surface, the magnetic lines of flux that longitudinally encircle the Earth, both the sun and the stars in the celestial sphere arching over the Earth, and perhaps prevailing wind direction and odors.

Landmarks are useful as a primary navigation reference only if the bird has been there before. For cranes, swans, and geese that migrate in family groups, young of the year could learn the geographic map for their migratory journey from their parents. But most birds do not migrate in family flocks, and on their initial flight south to the wintering range or back north in the spring must use other cues. Yet birds are aware of the landscape over which they are crossing and appear to use landmarks for orientation purposes. Radar images of migrating birds subject to a strong crosswind were seen to drift off course, except for flocks migrating parallel to a major river. These birds used the river as a reference to shift their orientation and correct for drift in order to maintain the proper ground track. That major geographic features like Point Pelee jutting into Lake Erie or Cape May at the tip of New Jersey are meccas for bird-watchers only reflects the fact that migrating birds recognize these peninsulas during their migration. Migrating hawks seeking updrafts along the north shore of Lake Superior or the ridges of the Appalachians must pay attention to the terrain below them in order to take advantage of the energetic savings afforded by these topographic structures.

Since humans learned to use celestial cues, it was only natural that studies were undertaken to demonstrate that birds could use them as well. Soon after the end of the Second World War, Gustav Kramer showed that migratory European Starlings oriented to the azimuth of the sun when he used mirrors to shift the sun's image by ninety degrees in the laboratory and obtained a corresponding shift in the birds' orientation. Furthermore, since the birds would maintain a constant direction even though the sun traversed from east to west during the day, the compensation for this movement demonstrated that the birds were keeping time. They knew what orientation to the sun was appropriate at 9 a.m. They knew what different angle was appropriate at noon, and again at 4 p.m. It has been recently shown that melatonin secretions from the light-sensitive pineal gland on the top of the bird's brain are involved in this response. Not only starlings but homing pigeons, penguins, waterfowl, and many species of perching birds have been shown to use solar orientation. Even nocturnal migrants take directional information from the sun. European Robins and Savannah Sparrows that were prevented from seeing the setting sun did not orient under the stars as well as birds that were allowed to see the sun set. Birds can detect polarized light from sunlight's penetration through the atmosphere, and it has been hypothesized that the pattern of polarized light in the evening sky is the primary cue that provides a reference for their orientation.

Using the artificial night sky provided by planetariums demonstrated that nocturnal migrants respond to star patterns. (quite analogous to Kramer's work on solar orientation, Franz Sauer demonstrated that if the planetarium sky is shifted, the birds make a corresponding shift in their orientation azimuth. Steve Emlen was able to show that the orientation was not dependent upon a single star, like Polaris, but to the general sky pattern. As he would turn off more and more stars so that they were no longer being projected in the planetarium, the bird's orientation became poorer and poorer. While the proper direction for orientation at a given time is probably innate, Emlen was able to show that knowing the location of "north" must be learned. When young birds were raised under a planetarium sky in which Betelgeuse, a star in Orion of the southern sky, was projected to the celestial north pole, the birds oriented as if Betelgeuse was "north" when they were later placed under the normally orientated night sky, even though in reality it was south!

Radar studies have shown that birds do migrate above cloud decks where landmarks are not visible, under overcast skies where celestial cues are not visible, and even within cloud layers where neither set of cues is available. The nomadic horsemen of the steppes of Asia used the response of lodestones to the Earth's magnetic field to find their way, and the hypothesis that migrating birds might do the same was suggested as early as the middle of the nineteenth century. Yet it was not until the mid-twentieth century that Merkel and Wiltschko demonstrated in a laboratory environment devoid of any other cues that European Robins would change their orientation in response to shifts in an artificial magnetic field that was as weak as the Earth's natural field. Although iron-containing magnetite crystals are associated with the nervous system in homing pigeons, Northern Bobwhite, and several species of perching birds, it is unknown whether they are associated with the sensory receptor for the geomagnetic cue. An alternate hypothesis for the sensory receptor suggests that response of visual pigments in the eye to electromagnetic energy is the basis for geomagnetic orientation. It has been shown, however, that previous exposure to celestial orientation cues enhances the ability of a bird to respond more appropriately when only geomagnetic cues are available.

Radar observations indicate that birds will decrease their air speed when their ground speed is augmented by a strong tail wind. We also know that birds can sense wind direction as gusts ruffling the feathers stimulate sensory receptors located in the skin around the base of the feather. Since there are characteristic patterns of wind circulation around high and low pressure centers at the altitude most birds migrate, it has been hypothesized that birds could use these prevailing wind directions as an orientation cue. However, there presently is no experimental support for this hypothesis.

The sense of smell in birds was considered for a long time to be poorly developed, but more recent evidence suggests that some species can discriminate odors quite well. If the olfactory nerves of homing pigeons are cut, the birds do not return to their home loft as well as birds whose olfactory nerves were left intact. A similar experiment has demonstrated that European Starlings with severed olfactory nerves returned less often than unaffected control birds even at distances as great as 240 km from their home roosts. And even more interesting, when these starlings returned to the nesting area the following spring, the starlings with nonfunctioning olfactory nerves returned at a significantly lower frequency than the other starlings.

Considering the array of demonstrated and suggested cues that birds might use in their orientation, it is clear that they rely upon a suite of cues rather than a single cue. For a migrating bird this redundancy is critical, since not all sources of orientation information are equally available at a given time, nor are all sources of information equally useful in a given situation.

Influence of Weather
Weather, especially temperature, affects that rate of premigratory preparation. A warmer, earlier spring accelerates the process, while a cooler, later spring inhibits the process. For example, the maintenance of body temperature under cold stress competes for energy that might be stored as fat in preparation for the migratory journey if temperatures were a little more salubrious. Additionally, there might be a more direct response from temperature receptors in the skin that direct impulses to the areas of the brain that regulate hormonal factors affecting the development of the migratory state. Thus in warm, early springs a species arrives earlier than average, while in cool, late springs they tend to arrive later.

During both spring and fall migrations, radar studies have demonstrated that weather has a defining role in determining when a bird will actually begin a migratory flight. The primary stimulus for departure is a following wind; in the spring this is a wind from the south, in the fall it is a wind from the north. Clear skies, presumably providing for celestial orientation cues, are of secondary importance, since major flights will occur under an overcast if adequate tail winds are blowing.

In the North Temperate Zone, migrations are concurrent with periods of rapid seasonal change. In the summer, warm, moist air masses dominate, but as fall approaches colder, drier air pushes southward to eventually bring the grip of winter to the land. The battle for domination of air masses is then reversed in the spring as the longer daylengths increase the heat load in the atmosphere, again giving the advantage to the northward expansion of warmer air. It is along this frontal boundary between these air masses that low pressure centers develop and move eastward, steered by the high velocity jet stream aloft. Winds flow in toward these low pressure centers in a counter-clockwise circulation, fed by air spiralling outward in a clockwise direction from intervening high pressure centers within the air masses. Thus, in the southeastern quadrant of a low pressure center, warm moist winds drive a warm front northward into the colder air, the warmer air being pushed gradually above the colder air forming large areas of cloud cover and widespread rainfall. In the northwestern quadrant of a low pressure center, cold dry air pushes a cold front southeastward into the warmer air mass, abruptly forcing the warm, moist air aloft, sometimes with violent and severe consequences.

Since prevailing wind direction determines whether a migratory flight will occur, the patterns of wind circulation around highs and lows affects migratory movement (Figure 12). During fall migration, the best passage of migrants usually occurs the day after the day of cold front passage with brisk north winds, dropping temperatures, a rising barometer, and clearing skies. The intensity of this flight only wanes as migrating flocks become less and less influenced by the prevailing winds following the cold front as it moves eastward. Since wind direction becomes more variable and wind velocity decreases as high pressure begins to dominate, mass migratory flights are curtailed. This is the time birds stop and feed.

Figure 12. A hypothetical weather system that could be ideal for mass migrations of waterfowl in the fall. The strong southerly flow of air created by counter-clockwise winds about the lows and the clockwise rotation of air about the highs, aids the rapid movement of waterfowl from their breeding grounds in the Canadian prairies to wintering areas in southern United States.

During spring, weather conditions in the southeastern or warm sector of a low pressure are conducive to movements of birds since the prevailing wind flows strongly from the south. But when these migrating flocks are overtaken by the cold front sweeping in from the west with its abrupt reversal of wind direction, towering clouds, turbulent air, and often torrential rain, migration stops, and the birds are grounded. If northward migrating flocks overtake the warm front, they are also faced with a shift in wind direction now blowing out of the east, increased cloud cover, and precipitation, but since the air is less turbulent, the wind shift less inappropriate, and the rains gentler, they will often continue northward awhile before they land and begin to forage.

The passage of low pressure system and the associated winds, often results in "waves" of migrants grounded by the storm being seen by observers. This is especially the case in the spring. This phenomenon reaches its superlative expression along the Gulf of Mexico if a cold front is positioned along or just off the coast. Then trans-Gulf migrants nearing the end of their flight from Central America and enjoying the advantage of a following wind must struggle against the adverse headwinds until landfall is reached, and the exhausted birds settle immediately to rest and forage in whatever habitat the coastal strand provides. It is a day to be remembered by any bird watcher. Orioles and tanagers by the dozens crowd the scrubby seaside bushes; Blackburnian and Cerulean warblers forage with Indigo and Painted buntings in the lawns of bayside homes. But if there is no front, there are no birds, the migrants having sufficient fat stores to continue flying northward on the following wind until they must stop to eat and drink.

Soaring birds such as hawks, Ospreys, eagles, and vultures are very dependent on proper wind conditions for migration. In the fall, often the best day to observe hawk migration along mountains in the eastern United States is on the second day after a cold front has passed, providing there are steady northwest to west winds to produce updrafts as the strong air currents are forced over the north-south oriented ridges. Migrants also soar on convective thermals that are generated by the differential heating of the Earth's surface. It has been estimated that the normal premigratory fat load of 100 grams in a Broad-winged Hawk would be exhausted in only five days of flapping flight. But by spiralling in the updrafts of one thermal and gliding down to the next to again to take advantage of the rising air currents, its stored fat would last 20 days, more than enough to provide energy for its 3,000 mile journey from the Neotropics.

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