Chapter III: Introduction: 2 (3)

The incessant crescendo note of the ovenbird is ordinarily associated with the full verdure of May woods, but this bird has been known to reach its breeding grounds in a snowstorm, and the records of its arrival in southern Minnesota show a temperature variation from near freezing to full summer warmth. Temperatures at arrival of several other common birds vary from 14° between highest and lowest temperatures to 37°, the average variation being about 24°. North American species spending the winter months in tropical latitudes experience no marked changes in temperature conditions from November to March or April, yet frequently they will start the northward movement in January or February. This is in obedience to physiological promptings and has no relation to the prevailing weather conditions. For migratory birds the winter season is a period of rest, a time when they have no cares other than those associated with the daily search for food or escape from their natural enemies. Their migrations, however, are a vital part of their life cycles, which have become so well adjusted that the seasons of travel correspond in general with the major seasonal changes on their breeding grounds. With the approach of spring, therefore, the reproductive impulse awakens, and each individual bird is irresistibly impelled to start the journey that ends in its summer home.

In other words, the evidence indicates the urge to migrate is so innate within a species or population that the individuals move north in spring when the average weather is not unendurable. The word "average" must be emphasized since it appears the migrations of birds have evolved in synchrony with average climatic conditions. More northern nesting populations of species such as American robins and savannah sparrows, timed to arrive on their breeding ground when the weather is suitable, pass through areas where their more southern kin are already nesting. The hardy species travel early, fearless of the blasts of retreating winter, while the more delicate kinds come later when there is less danger of encountering prolonged periods of inclement weather. Some of the hardy birds pause in favorable areas and allow the spring season to advance. Then, by rapid travel they again overtake it, or, as sometimes happens, they actually outstrip it. Occasionally this results in some hardship, but rarely in the destruction of large numbers of individuals after arrival. Cases are known where early migrating bluebirds have been overwhelmed by late winter storms. Nevertheless, if such unfavorable conditions are not prolonged, no serious effect on the species is noted. The soundness of the bird's instincts is evidenced by the fact that natural catastrophes, great though they may be, do not permanently diminish the avian populations.

The spring flight of migrants, if interrupted by cold north winds, is resumed when weather conditions again become favorable, and it is probable that all instances of arrival in stormy weather can be explained on the theory that the flight was begun while the weather was auspicious. Even though major movements of migrants in spring generally coincide with periods of warm weather and southerly winds, observations on the beginning of nocturnal spring flights from the coast of Louisiana failed to note any inhibiting factor other than hard rain (Gauthreaux 1971).

Radar studies have indicated that migrant birds possess an amazing understanding of wind patterns (Bellrose 1967). Birds can recognize many characteristics and select for favorable patterns. Head winds are as unfavorable to migration as is rain or snow because they greatly increase the labor of flight and cut down the speed of cross-country travel. If such winds have a particularly high velocity, they may force down the weaker travelers, and when this happens over water, large numbers of birds are lost. Moderate tail winds and cross or quartering breezes appear to offer the best conditions for the flight of migrants. Richardson (1971) found migrants traveling in different directions at different altitudes, but each group of birds was aided by a following wind. Thus we might expect natural selection to operate in favor of those birds that could recognize and respond to favorable wind patterns because it would reduce energy consumption and flight time on long-distance flights (Hassler et al. 1963).

Soaring birds such as hawks, vultures, and storks are very dependent on proper wind conditions for migration. In the fall, often the best day to observe hawk migration in the eastern United States is on the second day after a cold front has passed providing there are steady northwest to west winds and a sunny day for production of thermals (Pettingill 1962). Considerable drifting may be observed in this group of birds because they are literally carried along by the wind or glide from one thermal to the next. Haugh and Cade (1966) found most hawks migrated around Lake Ontario when winds were 10 to 25 miles per hour, but, if the wind exceeded 35 miles per hour, most hawk migration stopped.

In conclusion then, we can say that the weather may be the impetus for migration for many species, but it cannot stimulate a bird to migrate unless it is physiologically prepared. Arrivals on the ground are not necessarily indicative of the number of birds passing overhead. During the fall, peak migrations usually follow the passage of a cold front when the temperature is falling, the barometer is rising, winds are from the west or northwest, and the sky is clearing. In the spring, most migrants proceed north in the warm sector of a low when winds are southerly, warm, and moist, but rain, fog, or snow will often curtail the passage of migrants or prevent the initiation of a migration. Evolution of migratory behavior has probably resulted from the survival of birds capable of selecting those wind conditions, which reduce flight time and energy consumption, during their passage.

=INFLUENCE OF TOPOGRAPHY=

The relation of the world's land masses to each other and the distribution and association of biotypes within these land masses influence the direction birds migrate. Topography may aid, hinder, or prevent the progress of a migrant depending on the bird's particular requirements. Old World migrants must contend with east-west tending mountain ranges and deserts, whereas New World travelers can proceed north and south across a landscape with its major mountain ranges and river systems oriented in the same direction as the birds migrate.

When a distinct feature in the landscape, such as borders between fields and forests, rivers, mountain ridges, desert rims, or peninsulas, appears to influence migratory travel, we call these formations "guiding lines," "diversion-lines," "leading lines," or in German, "Leitlinie." It is an observed fact that some birds in a migratory movement alter their course to travel along a leading line, but whether this feature in the landscape caused the migrants to change their course is only theory (Thomson 1960). Besides topography, many other factors can influence this type of flight behavior including weather, wind speed and direction, time of day, species, age, and experience of the bird (Murray 1964).

Large bodies of water constitute real barriers to soaring birds dependent on thermals and air currents. Good examples of these barriers include the Mediterranean Sea between Europe and Africa and the Great Lakes in North America. Because these water areas do not create good thermals (generally a warm surface, such as a large field on a sunny day, is needed to create the necessary rising air currents for thermals to form) for birds to soar on, migrants are forced to travel around them on up-drafts created where land and water meet. The shoreline, then, may appear to be the guiding line, but more than likely the birds are simply following air currents created by onshore winds replacing the rising air from the surrounding warmer land surface and being deflected upward by the shoreline. These conditions often concentrate our buteos (broad-winged, rough-legged, red-shouldered, and red-tailed hawks) into restricted areas where, on good days, numbers observed can be spectacular. Similar conditions exist over the Bosphorus at the eastern end of the Mediterranean Sea where literally thousands of storks, eagles, and buzzards can be observed on a good day.

While extensive water areas may alter the migratory path of soaring birds, mountain ridges, especially if parallel to the line of flight, are often very conducive to migratory travel. Systematic coverage of the Appalachian ridges indicates all of them aid the migration of soaring birds. Apparently the highest and longest ridges deflect the horizontal winds upward better than the shorter ridges less than 1,000 feet high, and more birds are seen, on the average, along the higher ridges (Robbins 1956).

In general, nocturnal migrants are not influenced by topography as much as diurnal travellers. Radar observations have played an important role in establishing this difference. Bellrose (1967) found that waterfowl migrating at night through the Midwest were not influenced by major river systems, but in the evening or after daybreak ducks and geese tended to alter their course along the rivers. Drury et al. (1961) recorded massive fall and spring movements from the New England area out over the Atlantic Ocean without any apparent regard for the coastline. Until nocturnal migration could be "watched" on a radar screen, many bird observers assumed the guiding effect of the coastline on migratory travel was more restrictive than it really is.

In summary, topography may help or deter a migrant in its passage. It affects different birds in different ways. In North America, migratory movements are continent wide, and no evidence has indicated any particular part of the landscape influences all birds in the same manner. Certain bird populations may use general areas in migration, but they are usually not rigidly restricted to them because of topography.

=PERILS OF MIGRATION=

The migration season is full of peril for birds. Untold thousands of smaller migrants are destroyed each year by storms and attacks by predatory animals. These mortality factors, and others, help keep bird populations in check. Perils of migration are among these causes.

Storms

Of all the hazards confronting birds in migration, particularly the smaller species, storms are the most dangerous. Birds that cross broad stretches of water can be blown off course by a storm, become exhausted, and fall into the waves. Such a catastrophe was once seen from the deck of a vessel in the Gulf of Mexico, 30 miles off the mouth of the Mississippi River. Great numbers of migrating birds, chiefly warblers, were nearing land after having accomplished nearly 95 percent of their long flight when, caught by a "norther" against which they were unable to make headway, hundreds were forced into the waters of the Gulf and drowned. A sudden drop in temperature accompanied by a snowfall can cause a similar affect.

Aerial Obstructions

Lighthouses, tall buildings, monuments, television towers, and other aerial obstructions have been responsible for destruction of migratory birds. Bright beams of lights on buildings and airport ceilometers have a powerful attraction for nocturnal air travelers that may be likened to the fascination for lights exhibited by many insects, particularly night-flying moths. The attraction is most noticeable on foggy nights when the rays have a dazzling effect that not only lures the birds but confuses them and causes their death by collision against high structures. The fixed, white, stationary light located 180 feet above sea level at Ponce de Leon Inlet (formerly Mosquito Inlet), Florida, has caused great destruction of bird life even though the lens is shielded by wire netting. Two other lighthouses at the southern end of Florida, Sombrero Key and Fowey Rocks, have been the cause of a great number of bird tragedies, while heavy mortality has been noted also at some of the lights on the Great Lakes and on the coast of Quebec. Fixed white lights seem to be most attractive to birds; lighthouses equipped with flashing or red lights do not have the same attraction.

For many years in Washington, B.C., the illuminated Washington Monument, towering more than 555 feet into the air, caused destruction of large numbers of small birds. Batteries of brilliant floodlights grouped on all four sides about the base illuminate the Monument so brilliantly, airplane pilots noticed that it could be seen for 40 miles on a clear night. It is certain there is an extensive area of illumination, and on dark nights with gusty, northerly winds, nocturnal migrants seem to fly at lower altitudes and are attracted to the Monument. As they mill about the shaft, they are dashed against it by eddies of wind, and hundreds have been killed in a single night.

In September 1948, bird students were startled by news of the wholesale destruction of common yellowthroats, American redstarts, ovenbirds, and others against the 1,250-foot-high Empire State Building in New York City, the 491-foot-high Philadelphia Saving Fund Society Building in Philadelphia, and the 450-foot-high WBAL radio tower in Baltimore. In New York, the birds continued to crash into the Empire State Building for 6 hours.

More recently, the television tower has become the chief hazard. These structures are so tall, sometimes over 1,000 feet, they present more of a menace than buildings or lighthouses. Their blinking lights cause passing migrants to blunder into guy wires or the tower itself while milling around like moths about a flame. Numerous instances (e.g. Stoddard and Norris 1967) throughout the U.S. indicate this peril to migration is widespread. The lethal qualities of airport ceilometers have been effectively modified by conversion to intermittent or rotating beams.

Exhaustion

Both soaring and sailing birds are so proficient in aerial transportation that only recently have the principles been understood and imitated by aircraft pilots. The use of ascending air currents, employed by all soaring birds and easily demonstrated by observing gulls glide hour after hour along the windward side of a ship, are now utilized by man in his operation of gliders. Moreover, the whole structure of a bird makes it the most perfect machine for extensive flight the world has ever known. Hollow, air-filled bones, together with feathers, the lightest and toughest material known for flight, have evolved in combination to produce a perfect flying machine.

Mere consideration of a bird's economy of fuel or energy also is enlightening. The golden plover probably travels over a 2,400-mile oceanic route from Nova Scotia to South America in about 48 hours of continuous flight. This is accomplished with the consumption of less than 2 ounces of body fat (fuel). In contrast, to be just as efficient in operation, a 1,000-pound airplane would consume only a single pint of fuel in a 20-mile flight rather than the gallon usually required. Similarly, the tiny ruby-throated hummingbird weighing approximately 4 grams, crosses the Gulf of Mexico in a single flight of more than 500 miles while consuming less than 1 gram of fat.

One might expect the exertion incident to long migratory flights would result in arrival of migrants at their destination near a state of exhaustion. This is usually not the case. Birds that have recently arrived from a protracted flight over land or sea sometimes show evidences of being tired, but their condition is far from being in a state of emaciation or exhaustion. The popular notion birds find long ocean flights so excessively wearisome that they sink exhausted when terra firma is reached generally does not coincide with the facts.

The truth is, even small landbirds are so little exhausted by ocean voyages, they not only cross the Gulf of Mexico at its widest point but may even proceed without pause many miles inland before stopping. The sora, considered such a weak flyer that at least one writer was led to infer most of its migration was made on foot, has one of the longest migration routes of any member of the rail family and even crosses the wide reaches of the Caribbean Sea. Observations indicate that under favorable conditions birds can fly when and where they please and the distance covered in a single flight is governed chiefly by the amount of stored fat. Exhaustion, except as the result of unusual factors such as strong adverse winds, cannot be said to be an important peril of migration.

=ROUTES OF MIGRATION=

General Considerations

While it is beyond question that certain general directions of flight are consistently followed by migratory birds, it is well to remember the term "migration route" is to some extent a theoretical concept referring to the lines of general advance or retreat of a species, rather than the exact course followed by individual birds or a path followed by a species with specific geographic or ecological boundaries. Even the records of banded birds usually show no more than the place of banding and recovery. One ought to have recourse to intermediate records and reasoning based on probabilities to fill in details of the route actually traversed between the two points. In determining migration routes, one must constantly guard against the false assumption that localities with many grounded migrants are on the main path of migration and localities where no grounded migrants are observed are off the main path.

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 is 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 wintering grounds, and other factors contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide individuals or groups of individuals along more or less definite lines, and it is possible to define such lines of migration for many species.

Except in a few species, individuals probably do not follow precisely the same route twice. This is especially true in the group of soaring birds that utilize thermals. Mueller and Berger (1967b) recaptured only three migrants in subsequent years at Cedar Grove, Wisconsin, after banding over 50,000 birds there. In general, those populations of species with very discernible breeding or wintering grounds have readily discernible migration routes. However, even the whole migration process of certain species may show great yearly fluctuations (Rudebeck 1950).

Aldrich et al. (1949) showed from banding data great variation in migration patterns between species of waterfowl. In some species there was considerable diversity in direction of movement, not only of different breeding populations within a species but also for different individuals of the same breeding population. The impression is inescapable; waterfowl migration is even more complicated than originally supposed, and it is difficult to make generalizations with regard to migration pathways for even a single species let alone waterfowl in general.

Flyways and Corridors

Through plotting accumulated banding data in the 1930's, investigators became impressed by what appeared to be four broad, relatively exclusive flyway belts in North America. This concept, based upon analyses of the several thousand records of migratory waterfowls recoveries then available, was described by Lincoln (1935a). In this paper (p. 10), Lincoln concluded that:

... because of the great attachment of migratory birds for their
ancestral flyways, it would be possible practically to exterminate
the ducks of the West without seriously interfering with the supply
of birds of the same species in the Atlantic and Mississippi
flyways, and that the birds of these species using the eastern
flyways would be slow to overflow and repopulate the devastated
areas of the West, even though environmental conditions might be so
altered as to be entirely favorable.

Since 1948, this concept served as the basis for administrative action by the Fish and Wildlife Service in setting annual migratory waterfowl hunting regulations.

The concept of bird populations being confined to four fairly definite and distinct migration "flyways" is probably most applicable to those birds that migrate in family groups, namely geese, swans, and cranes, but does not appear to be very helpful in understanding the movements of the more widely dispersing ducks. The "pioneering spirit" in Canada geese, for example, is limited by their social structure the young travel to and from specific breeding and wintering areas with their parents. These young later in life usually breed in the same areas as did their parents. If a goose population is decimated in one flyway, either by hunting or natural calamities, other goose populations in other flyways are not seriously endangered, but also these populations are very slow to repopulate an area where the previous goose population had been decimated. This is not the case with ducks because these birds are not always bound by their intrinsic behavior to return to specific breeding areas. Consequently, vacant breeding areas are more rapidly repopulated by ducks than by geese.

Although Lincoln's analysis was confined to ducks and geese, some thought that it applied to other groups of birds as well. Everyone now realizes that the concept of four flyways, designated as the Atlantic, Mississippi, Central, and Pacific Flyways, was an oversimplification of an extremely complex situation involving crisscrossing of migration routes, varying from species to species. It can be considered meaningful only in a very general way, even for waterfowl, and not applicable generally to other groups of birds. Nevertheless the four "Flyway" areas have been useful in regionalizing the harvest of waterfowl for areas of different vulnerability of hunting pressure.

Bellrose (1968) identified corridors of southward migrating waterfowl east of the Rocky Mountains and determined, through statistical analyses, the relative abundance of birds in each. He showed major corridors of dabbling duck movements down the Great Plains and Missouri-Mississippi river valleys with minor off shoots at various points from these corridors eastward to the Atlantic coast where they joined equally minor eastern movements from the North (Fig. 13). Bellrose's map of migration corridors for the diving ducks showed heavy traffic similar to that of dabbling species down the Great Plains and relatively heavily used corridors from these central arteries eastward across the Great Lakes area to the Atlantic coast, terminating particularly in the vicinity of Chesapeake Bay. A fairly well-used corridor extends along the Atlantic coast.

With our present knowledge of bird migration it is difficult at best to recognize distinct broad belts of migration down the North American continent encompassing groups of distinct populations or species. It seems that so much intermingling of populations occurs that distinctions between broad "flyway" belts are not discernible. About all we can say for sure now is that birds travel between certain breeding areas in the North and certain wintering areas in the South and that a few heavily traveled corridors used by certain species, and more generalized routes followed by one or more species, have become obvious.

Narrow Routes

Some species exhibit extremely narrow routes of travel. The red knot and purple sandpiper, for example, are normally found only along the coasts because they are limited on one side by the broad waters of the ocean, and on the other by land and fresh water; neither of these habitats furnish conditions attractive to these species.

The Ipswich race of the savannah sparrow likewise has a very restricted migration range. It is known to breed only on tiny Sable Island, Nova Scotia, and it winters from that island south along the Atlantic coast to Georgia. It is rarely more than a quarter of a mile from the outer beach and is entirely at home among the sand dunes with their sparse covering of coarse grass.

The Harris' sparrow supplies an interesting example of a moderately narrow migration route in the interior of the country (Fig. 14). This fine, large sparrow is known to breed only in the narrow belt of stunted timber and brush at or near the limit of trees from the vicinity of Churchill, Manitoba, on the west shore of Hudson Bay, to the Mackenzie Delta 1,600 miles to the northwest. When this sparrow reaches the United States on its southward migration, it is most numerous in a belt about 500 miles wide, between Montana and central Minnesota and continues south through a relatively narrow path in the central part of the continent. Knowledge of habitat preference by Harris' sparrows suggests the narrow migration range is restricted to the transition between woodland and prairie, a type of habitat approaching the woodland-tundra transition of its breeding area. Development of this migration route, of course, preceded destruction of the heavy eastern forests by colonists from Europe. Its winter range lies primarily in similar country extending from southeastern Nebraska and northwestern Missouri, across eastern Kansas and Oklahoma and through a narrow section of eastern Texas, at places hardly more than 150 miles wide.

Converging Routes

When birds start their southward migration the movement necessarily involves the full width of the breeding range. Later, in the case of landbirds with extensive breeding ranges, there is a convergence of the lines of flight taken by individual birds owing, in part, to the conformation of the land mass and in part to the east-west restriction of habitats suitable to certain species. An example of this is provided by the eastern kingbird, which breeds in a summer range 2,800 miles wide from Newfoundland to British Columbia. On migration, however, the area traversed by the species becomes constricted until in the southern part of the United States the occupied area extends from Florida to the mouth of the Rio Grande, a distance of only 900 miles. Still farther south the migration path continues to converge, and, at the latitude of Yucatan, it is not more than 400 miles wide. The great bulk of the species probably moves in a belt less than half this width.

The scarlet tanager presents another extreme case of a narrowly converging migration route starting from its 1,900-mile-wide breeding range in the eastern deciduous forest between New Brunswick and Saskatchewan (Fig. 15). As the birds move southward in the fall, their path of migration becomes more and more constricted, until, at the time they leave the United States, all are included in the 600-mile belt from eastern Texas to the Florida peninsula. The boundaries continue to converge through Honduras and Costa Rica where they are not more than 100 miles apart. The species winters in the heavily forested areas of northwestern South America including parts of Colombia, Ecuador, and Peru.

The rose-breasted grosbeak also leaves the United States through the 600-mile stretch from eastern Texas to Apalachicola Bay, but thereafter as this grosbeak crosses the Gulf of Mexico and enters the northern part of its winter quarters in southern Mexico the lines do not further converge. However, the pathway of those individuals that continue on to South America is considerably constricted by the narrowing of the land through Central America to Panama (Fig. 16).

Although the cases cited represent extremes of convergence, a narrowing of the migratory path is the rule to a greater or lesser degree for the majority of North American birds. Both the shape of the continent and major habitat belts tend to constrict southward movement so that the width of the migration route in the latitude of the Gulf of Mexico is usually much less than in the breeding territory.

The American redstart represents a case of a wide migration route, but even in the southern United States, this is still much narrower than the breeding range (Fig. 17). These birds, however, cross all of the Gulf of Mexico and pass from Florida to Cuba and Haiti by way of the Bahamas, so here their route is about 2,500 miles wide.

Principal Routes From North America

W. W. Cook presented seven of the more important generalized routes for birds leaving the United States on their way to various wintering grounds (1915a; Fig. 18). When migrants return northward in the spring, they may follow these same routes, but it is not known for certain whether they do. These routes are discussed in the following sections.

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Atlantic Oceanic Route
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Route No. 1 (Fig. 18) is almost entirely oceanic and passes directly over the Atlantic Ocean from Labrador and Nova Scotia to the Lesser Antilles, then through this group of small islands to the mainland of South America. Most of the adult eastern golden plovers and some other shorebirds use this as their fall route. As we mentioned previously, radar has indicated strong fall movements of warblers from the New England coast out over the Atlantic to points south along this route. Since it lies almost entirely over the sea, this route is definitely known only at its terminals and from occasional observations made on Bermuda and other islands in its course. Some of the shorebirds that breed on the Arctic tundra of the District of Mackenzie (Northwest Territories) and Alaska fly southeastward across Canada to the Atlantic coast and finally follow this oceanic route to the mainland of South America. The golden plover may accomplish the whole 2,400 miles without pause or rest, and in fair weather the flocks pass Bermuda and sometimes even the islands of the Antilles without stopping. Although most birds make their migratory flights either by day or by night, the golden plover in this remarkable journey flies both day and night. Since this plover swims lightly and easily, it may make a few short stops along the way.

The Arctic tern follows the Atlantic Ocean route chiefly along the eastern side of the ocean. Likewise, vast numbers of seabirds such as auks, murres, guillemots, phalaropes, jaegers, petrels, and shearwaters follow this over-water route from breeding coasts and islands in the Northern and Southern Hemispheres.

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Atlantic Coast Route and Tributaries
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The Atlantic coast is a regular avenue of travel, and along it are many famous points for observing both land and water birds. About 50 different kinds of landbirds that breed in New England follow the coast southward to Florida and travel thence by island and mainland to South America (Fig. 18, route 2). The map indicates a natural and convenient highway through the Bahamas, Cuba, Hispaniola, Puerto Rico, and the Lesser Antilles to the South American coast. Resting places are afforded at convenient intervals, and at no time need the aerial travelers be out of sight of land. It is not, however, the favored highway; only about 25 species of birds go beyond Cuba to Puerto Rico along this route to their winter quarters, while only six species are known to reach South America by way of the Lesser Antilles. Many thousands of American coots and wigeons, pintails, blue-winged teal, and other waterfowl as well as shorebirds, regularly spend the winter season in the coastal marshes, inland lakes, and ponds of Cuba, Hispaniola, and Puerto Rico.

Route No. 3 (Fig. 18) is a direct line of travel for Atlantic coast migrants en route to South America, although it involves much longer flights. It is used almost entirely by landbirds. After taking off from the coast of Florida there are only two intermediate land masses where the migrants may pause for rest and food. Nevertheless, tens of thousands of birds of about 60 species cross the 150 miles from Florida to Cuba where many elect to remain for the winter months. The others negotiate the 90 miles between Cuba and Jamaica, but, from that point to the South American coast, there is a stretch of islandless ocean 500 miles across. Relatively few North American migrants on this route go beyond Jamaica. The bobolink so far outnumbers all other birds using this route that it may be designated the "bobolink route" (Fig. 19). As traveling companions along this route, the bobolink may meet vireos, kingbirds, and nighthawks from Florida, Chuck-will's-widows from the Southeastern States, black-billed and yellow-billed cuckoos from New England, gray-cheeked thrushes from Quebec, bank swallows from Labrador, and blackpoll warblers from Alaska. Sometimes this scattered assemblage will be joined by a tanager or a wood thrush, but the "bobolink route" is not popular with the greater number of migrants.

Formerly, it was thought most North America landbirds migrated to Central America via the Florida coast, then crossed to Cuba, and finally made the short flight from the western tip of Cuba to Yucatan. A glance at the map would suggest this as a most natural route, but, as a matter of fact, it is practically deserted except for a few swallows and shorebirds or an occasional landbird storm-driven from its normal course. What actually happens in the fall is that many of the birds breeding east of the Appalachian Mountains travel parallel to the seacoast in a more or less southwesterly direction and, apparently maintaining this same general course from northwestern Florida, cross the Gulf of Mexico to the coastal regions of eastern Mexico. They thus join migrants from farther inland in using route No. 4 (Fig. 18).

Routes used by the Atlantic brant merit some detail because their flight paths were long misunderstood. These birds winter on the Atlantic coast, chiefly at Barnegat Bay, New Jersey, but depending upon the severity of the season and the food available, south also to North Carolina. Their breeding grounds are in the Canadian arctic archipelago and on the coasts of Greenland. According to the careful studies of Lewis (1937), the main body travels northward in spring along the coast to the Bay of Fundy, overland to Northumberland Strait, which separates Prince Edward Island from mainland New Brunswick and Nova Scotia. A minor route appears to lead northward from Long Island Sound by way of the Housatonic and Connecticut River Valleys to the St. Lawrence River.

After spending the entire month of May feeding and resting in the Gulf of St. Lawrence, the eastern segment of the brant population resumes its journey by departing overland from the Bay of Seven Island area. The eastern and larger segment of the population appears to fly almost due north to Ungava Bay and from there to nesting grounds, probably on Baffin Island and Greenland. The smaller segment travels a route slightly north of west to the southeastern shores of James Bay, although east of that area some of the flocks take a more northwesterly course by descending the Fort George River to reach the eastern shore of James Bay. Upon their arrival at either of these two points on James Bay, the brants of this western segment turn northward and proceed along eastern Hudson Bay to their breeding grounds in the Canadian Arctic.

In general, the fall migration of the brant follows the routes utilized in the spring. At this season, the eastern population appears only on the western and southern shores of Ungava Bay before continuing their southward journey to the Gulf of St. Lawrence and beyond. Also, it appears that most of the birds of the western segment, instead of following the eastern shores of Hudson and James bays, turn southwestward across the former, by way of the Belcher Islands, to Cape Henrietta Maria, and from there south along the western shores of James Bay by way of Akimiski and Charlton Islands. At the southern end of James Bay, they are joined by those that have taken the more direct route along the east coasts of the bays and all then fly overland 570 miles to the estuary of the St. Lawrence River.

The Atlantic coast wintering area receives accretions of waterfowl from three or four interior migration paths, one of which is of first importance, as it includes great flocks of canvasbacks, redheads, scaup, Canada geese, and many black ducks that winter in the waters and marshes of the coastal region south of Delaware Bay. The canvasbacks, redheads, and scaup coming from breeding grounds on the great northern plains of central Canada follow the general southeasterly trend of the Great Lakes, cross Pennsylvania over the mountains, and reach the Atlantic coast in the vicinity of Delaware and Chesapeake Bays. Black ducks, mallards, and blue-winged teals that have gathered in southern Ontario during the fall leave these feeding grounds and proceed southwest over a course that is apparently headed for the Mississippi Valley. Many do continue this route down the Ohio Valley, but others, upon reaching the vicinity of the St. Clair Flats between Michigan and Ontario, swing abruptly to the southeast and cross the mountains to reach the Atlantic coast south of New Jersey. This route, with its Mississippi Valley branch, has been fully documented by the recovery records of ducks banded at Lake Scugog, Ontario.

Canvasbacks migrate from the prairie pothole country of the central United States and Canada to many wintering areas in the United States. This duck has been the subject of a particular study (Stewart, Geis, and Evans 1958), and its principle migration routes, based on recovery of banded birds, are shown to follow an important trunk route from the major breeding area in the prairie provinces of Canada and the northern prairies of the United States southeastward through the southern Great Lakes area to Chesapeake Bay, the chief wintering area (Fig. 20). Relatively few canvasbacks proceed southward along the Atlantic seaboard. A less important route branches off from the main trunk in the southern Minnesota region and extends south along the Mississippi Valley to points along the river. Other individuals of the prairie breeding population fly southward on a broad front to the gulf coast of Texas and the interior of Mexico, while some proceed southwestward on a relatively broad path to the northern Pacific coast.

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Mackenzie Valley-Great Lakes-Mississippi Valley
Route and Tributaries
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The route extending from the Mackenzie Valley past the Great Lakes and down the Mississippi Valley is easily the longest of any in the Western Hemisphere. Its northern terminus is on the Arctic coast in the regions of Kotzebue Sound, Alaska, and the mouth of the Mackenzie River, while its southern end lies in Argentina. Nighthawks, barn swallows, blackpoll warblers, and individuals of several other species that breed northward to the Yukon Territory and Alaska must cover the larger part of the route twice each year.

For more than 3,000 miles--from the mouth of the Mackenzie to the delta of the Mississippi--this route is uninterrupted by mountains. In fact, the greatest elevation above sea level is less than 2,000 feet. Because it is well timbered and watered, the entire region affords ideal conditions for its great hosts of migrating birds. It is followed by such vast numbers of ducks, geese, shorebirds, blackbirds, sparrows, warblers, and thrushes that observers stationed at favorable points in the Mississippi Valley during the height of migration can see a greater number of migrants than can be noted anywhere else in the world.

When many of these species, including ducks, geese, robins, and yellow-rumped warblers, arrive at the Gulf coast, they spread out east and west for their winter sojourn. Others, despite the perils of a trip involving a flight of several hundred miles across the Gulf of Mexico, fly straight for Central and South America. This part of the route is a broad "boulevard" extending from northwestern Florida to eastern Texas and southward across the Gulf of Mexico to Yucatan and the Isthmus of Tehuantepec (Fig. 18, route 4). This route appears to have preference over the safer but more circuitous land or island routes by way of Texas or Florida. During the height of migration some of the islands off the coast of Louisiana are wonderful observation points for the student of birds, as the feathered travelers literally swarm over them.

Present detailed knowledge of the chief tributaries to the Mackenzie-Great Lakes-Mississippi Valley route relates primarily to waterfowl. Reference has been made already to the flight of black ducks that reach the Mississippi Valley from southern Ontario. Some individuals of this species banded at Lake Scugog, Ontario, have been recaptured in succeeding seasons in Wisconsin and Manitoba, but the majority was retaken at points south of the junction of the Ohio River with the Mississippi indicating their main route of travel from southern Ontario.

A second route that joins the main artery on its eastern side is the one used by eastern populations of lesser snow geese, including both blue and white phases, that breed mainly on Southampton Island and in the Fox Basin of Baffin Island. In the fall these geese work southward along the shores of Hudson Bay and, upon reaching the southern extremity of James Bay, take off on their flight to the great coastal marshes of Louisiana and Texas west of the Mississippi River delta.

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Great Plains-Rocky Mountain Routes
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This route also has its origin in the Mackenzie River delta and Alaska. The lesser sandhill cranes, white-fronted geese, and smaller races of the Canada goose follow this route through the Great Plains from breeding areas in Alaska and western Canada. It is used chiefly by the pintails and American wigeons that fly southward through eastern Alberta to western Montana. Some localities in this area, as for example, the National Bison Range at Moiese, Montana, normally furnish food in such abundance that these birds are induced to pause in their migratory movement. Some flocks of pintails and wigeons move from this area almost directly west across Idaho to the valley of the Columbia River, then south to the interior valleys of California. Others leave Montana by traveling southeastward across Wyoming and Colorado to join other flocks moving southward through the Great Plains.

Observations made in the vicinity of Corpus Christi, Texas, have shown one of the short cuts (Fig. 18, route 5) that is part of the great artery of migration. Thousands of birds pass along the coast to the northern part of the State of Veracruz, Mexico. Coastal areas along the State of Tamaulipas to the north are arid and so entirely unsuited for frequenters of moist woodlands that it is probable that much, or all, of this part of the route for these species is a short distance off shore. It is used by such woodland species as the golden-winged warbler, the worm-eating warbler, and the Kentucky warbler.

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Pacific Coast Route
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Although it does present features of unusual interest, the Pacific coast route is not as important as some of the others described. Because of the equable conditions that prevail, many species of birds along the coast from the northwestern states to southeastern Alaska either do not migrate or else make relatively short journeys. This route has its origin chiefly in western Alaska, around the Yukon River delta. Some of the scoters and other sea ducks of the north Pacific region as well as the diminutive cackling Canada goose of the Yukon River Delta use the coastal sea route for all or most of their southward flight. The journey of the cackling goose, as shown by return records from birds banded at Hooper Bay, Alaska, has been traced southward across the Alaskan Peninsula and apparently across the Gulf of Alaska to the Queen Charlotte Islands. The birds then follow the coast line south to near the mouth of the Columbia River, where the route swings toward the interior for a short distance before continuing south by way of the Willamette River Valley. The winter quarters of the cackling goose are chiefly in the vicinity of Tule Lake, on the Oregon-California line, and in the Sacramento Valley of California, although a few push on to the San Joaquin Valley.

A tributary of this "flyway" is followed by Ross' goose, which breeds in the Perry River district south of Queen Maud Gulf and other areas farther east on the central Arctic coast of Canada (Fig. 21). Its fall migration is southwest and south across the barren grounds to Great Slave and Athabaska Lakes, where it joins thousands of other waterfowl bound for winter homes along the eastern coast of the United States and the Gulf of Mexico. But when Ross' geese have traveled south approximately to the northern boundary of Montana, most of them separate from their companions and turn southwest across the Rocky Mountains to winter in California. In recent years a few Ross' geese have been found wintering east of the Rocky Mountains along with flocks of lesser snow geese and may be correlated with an eastward extension of their breeding range.

The southward route of those migratory landbirds of the Pacific area that leave the United States in winter extends chiefly through the interior of California to the mouth of the Colorado River and on to winter quarters in western Mexico (Fig. 18, routes 6 and 7).

The movements of the western tanager show a migration route that is in some ways remarkable. The species breeds in the mountains from the northern part of Baja California and western Texas north to northern British Columbia and southwestern Mackenzie. Its winter range is in two discontinuous areas--southern Baja California and eastern and southwestern Mexico south to Guatemala (Fig. 22). During spring migration the birds appear first in western Texas and the southern parts of New Mexico and Arizona about April 20 (Fig. 23). By April 30 the vanguard has advanced evenly to an approximate east-west line across central New Mexico, Arizona, and southern California. By May 10 the easternmost birds have advanced only to southern Colorado, while those in the far west have reached northern Washington. Ten days later the northward advance of the species is shown as a great curve, extending northeastward from Vancouver Island to central Alberta and thence southeastward to northern Colorado. Since these tanagers do not reach northern Colorado until May 20, it is evident those present in Alberta on that date, instead of traveling northward through the Rocky Mountains, their summer home, actually reached there by a route that carried them west of the Rockies to southern British Columbia and thence eastward across the still snowy northern Rocky Mountains.

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Pacific Oceanic Route
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The Pacific oceanic route is used by the Pacific golden plover, bristle-thighed curlew, ruddy turnstone, wandering tattler and other shorebirds. The ruddy turnstone, and probably other shorebirds, migrating from the islands of the Bering Sea, have an elliptical route that takes them southward via the islands of the central Pacific and northward along the Asiatic coast. In addition, many seabirds that breed on far northern and southern coasts or islands migrate up and down the Pacific well away from land except when the breeding season approaches.

The Pacific golden plover breeds chiefly in the Arctic coast region of Siberia and in a more limited area on the Alaskan coast. Some of the birds probably migrate south via the continent of Asia to winter quarters in Japan, China, India, Australia, New Zealand, and Oceania. Others evidently go south by way of the Pacific Ocean to the Hawaiian Islands and other islands of the central and southern Pacific. Migrating golden plovers have been observed at sea on a line that apparently extends from Hawaii to the Aleutian Islands; it appears certain some of the Alaskan birds make a nonstop flight across the sea from Alaska to Hawaii. While it would seem incredible that any birds could lay a course so accurately as to land on these small isolated oceanic islands, 2,000 miles south of the Aleutians, 2,000 miles west of Baja California, and nearly 4,000 miles east of Japan, the evidence admits only the conclusion that year after year this transoceanic round-trip journey between Alaska and Hawaii is made by considerable numbers of golden plovers.

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Arctic Routes
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Some Arctic nesting birds retreat only a short distance south in the winter. These species include the red-legged kittiwake, Ross' gull, emperor goose, and various eiders. The latter group of ducks winter well south of their nesting areas but nevertheless remain farther north than do the majority of other species of ducks. The routes followed by these birds are chiefly parallel to the coast and may be considered as being tributary either to the Atlantic or Pacific coast routes. The heavy passage of gulls, ducks, black brants, and other water birds at Point Barrow, Alaska, and other points on the Arctic coast, has been noted by many observers. The best defined Arctic route in North America is the one following the coast of Alaska.

A migration route, therefore, may be anything from a narrow path closely adhering to some definite geographical feature, such as a river valley or a coastline, to a broad boulevard that leads in the desired direction and follows only the general trend of the land mass. Oceanic routes appear to be special cases that are not fully understood at the present time. Also it must be remembered that all the main routes contain a multitude of tributary and separate minor routes. In fact, with the entire continent of North America crossed by migratory birds, the different groups or species frequently follow lines that may repeatedly intersect those taken by others of their own kind or by other species. The arterial or trunk routes, therefore, must be considered merely as indicating paths of migration on which the tendency to concentrate is particularly noticeable.

=PATTERNS OF MIGRATION=

Band recoveries, netting records, and personal observations help us to critically examine migration routes and probe deeper into the origin and evolution of these pathways. We are beginning to realize certain deviations occur from the "normal" north and south movements expected in most species. In the previous section on routes, we touched briefly on the fact that some routes are not poleward at all, but in some other direction. We know that many migrants do not stop at the exact localities year after year but they probably do follow the same general course each season. After many years of observations a pattern emerges for that population, species, or group of species. In this section we would like to take a closer look at some of the interesting patterns (or "eccentric routes" as Cook (1915a) referred to them) in migration that birds are annually to travel from breeding to wintering grounds and back again. In many cases, the causative agents are unknown or pure conjecture, but in others, sound biological principles can be put forth that may indicate why a particular species could have evolved the specific pattern it exhibits.

Loops

Many species do not return north in the spring over the same route they used in the fall; rather, they fly around an enormous loop or ellipse. Cook (1915a) considered food as the primary factor in determining the course birds took between winter and summer ranges. Individuals that returned by the same route and did not find sufficient food for their needs at that time were eliminated from the population, and only progeny from individuals that took a different course with sufficient food lived to build the tradition of a loop migration. Other investigators consider prevailing winds a major factor in the evolution of loop migration. Whatever the reason may be, it has most likely evolved separately in each species to satisfy its particular needs, and the fact that this pattern occurs all over the world in completely unrelated species is a good illustration of convergent evolution.

The annual flight of adult golden plovers is so unusual, it will be given in some detail. The species is observed by hundreds of bird watchers every year and it well illustrates loop migration (Fig. 24).

In the fall, the birds fatten on the multitude of berries along the coasts of Labrador and Nova Scotia, then depart south over the Atlantic Ocean to South America. After reaching the South American coast the birds make a short stop, 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 tundra to the pampas of Argentina. The older birds may be accompanied by some of the young, but most of the immature birds leave their natal grounds late in summer 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 few rewards for the weary travelers. By traveling up the middle of the continent, a much better food supply is assured (Welty 1962).

Several North American warblers including the Connecticut warbler (Fig. 25) and the western race of the palm warbler have been found to follow circuitous migration routes. The Connecticut warbler is not observed or banded on the East coast in spring, but it is recorded farther inland during the 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 to the Appalachian Mountains before turning south along the coast. Television tower kills in northern Florida indicate the population is very concentrated here at this time of year. In the spring this race also proceeds north through the interior. Graber (1968) points out that the eastern race of the palm warbler also proceeds south along the coast in the fall and poses this question: "does the western population of this species intentionally move toward the ancestral range, or is the fall flight direction merely a consequence of the temperate zone westerly circulation?"