Chapter II: Introduction: 2 (2)

The intensity of migration depends on circumstances including the need for haste. In fall the flights are more likely to be performed in a leisurely manner, so that after a flight of a few hours the birds often pause to feed and rest for one or several days, particularly if they find themselves in congenial surroundings. Some indication of this is found in the recoveries of banded birds, particularly waterfowl. If we consider only the shortest intervals between banding in the North and subsequent recovery in the South, it is found that usually a month or more is taken to cover straight-line distance of a thousand miles. For example, a black duck banded at Lake Scugog, Ontario, was killed 12 days later at Vicksburg, Mississippi. If the bird was taken shortly after its arrival, the record would indicate an average daily flight of 83 miles, a distance that could have been covered in about 2 hours' flying time. Among the thousands of banding records of ducks and geese, evidences of rapid migrations are decidedly scarce, for with few exceptions, all thousand-mile flights have required 2 to 4 weeks or more. Among sportsmen, the blue-winged teal is well known as a fast-flying duck and quite a few of these banded on Canadian breeding grounds have covered 2,300 to 3,000 miles in a 30-day period. Nevertheless, the majority of those that have traveled to South America were not recovered in that region until 2 or 3 months after they were banded. Probably the fastest flight over a long distance for one of these little ducks was one made by a young male that traveled 3,800 miles from the delta of the Athabaska River, northern Alberta, Canada, to Maracaibo, Venezuela, in exactly 1 month. This flight was at an average speed of 125 miles per day. A very rapid migration speed was maintained by a lesser yellowlegs banded at North Eastham, Cape Cod, Massachusetts, on 28 August 1935 and killed 6 days later, 1,900 miles away, at Lamentin, Martinique, French West Indies. This bird traveled an average daily distance of more than 316 miles.

It seems probable that most migratory journeys are performed at little more than the normal, unforced rate of flight, as this would best conserve the strength of the birds. Migrating birds passing lightships and lighthouses or crossing the face of the moon have been observed to fly without hurry or evidence of straining to attain high speed. The speed or rate of migration would therefore depend chiefly on the duration of flights and tail wind velocity.

The speed of migration is demonstrated by the dates of arrival, particularly during the spring movement. The Canada goose affords a typical example of regular but slow migration. Its advance northward is at the same rate as the advance of the season (Fig. 5). In fact, the isotherm of 35° F appears to be a governing factor in the speed at which these geese move north. (An isotherm is a line that connects points that have the same temperature at the same time.) From an evolutionary viewpoint we might expect this. If the geese continually advanced ahead of the 32° F isotherm, they would always find food and water frozen and unavailable. By migrating north just behind the advance of this isotherm, birds that breed in the far north will find food and open water available and have as long a breeding season as the climate will allow.

Few species perform such leisurely migrations; many wait in their winter homes until spring is well advanced, then move rapidly to their breeding grounds. Sometimes this advance is so rapid, late migrants actually catch up with species that may have been pressing slowly but steadily northward for a month or more. The following several examples of well-known migrants illustrate this.

The grey-cheeked thrush, which winters in the Colombia-Ecuador-Peru-Venezuela-British Guiana area, does not start its northward journey until many other species are well on their way. It does not appear in the United States until the last of April--25 April near the mouth of the Mississippi and 30 April in northern Florida (Fig. 6). A month later, or by the last week in May, the bird is seen in northwestern Alaska. Therefore, the 4,000-mile trip from Louisiana was made at an average distance of about 130 miles per day.

Another example of rapid migration is furnished by the yellow warbler. This species winters in the Tropics and reaches New Orleans about April 5, when the average temperature is 65° F. By traveling north much faster than the spring season progresses, this warbler reaches its breeding grounds in Manitoba the latter part of May, when the average temperature is only 47° F. They encounter progressively colder weather over their entire route and cross a strip of country in the 15 days from May 11 to 25 that spring temperatures normally take 35 days to cross. This "catching up" with spring is habitual in species that winter south of the United States as well as in most northern species that winter in the Gulf States. There appears to be only six exceptions to this rule: the Canada goose, the mallard, the pintail, the common crow, the red-winged blackbird, and the robin.

The snow goose presents a striking example of a late but very rapid spring migration. Most all of these geese winter in the great coastal marshes of Louisiana, where every year over 400,000 spend the winter and congregations of 50,000 or more may be seen grazing in the "pastures" or flying overhead in flocks of various sizes. Their breeding grounds are chiefly on Baffin and Southampton Islands in the northern part of Hudson Bay where conditions of severe cold prevail except for a few weeks each year. The birds are not stimulated to migrate even though the season in their winter quarters is advancing rapidly while their nesting grounds are still covered with a heavy blanket of ice and snow. This suggests the stimulus for spring departure is regulated by an internal mechanism, such as development of the gonads. Accordingly, blue geese remain in the coastal marshes until the last of March or the first of April, when the local birds are already busily engaged in reproduction. The flight northward is rapid, almost nonstop so far as the United States is concerned; although the birds are sometimes recorded in large numbers in the Mississippi Valley, eastern South Dakota, and southeastern Manitoba, there are few records anywhere along the route of the great flocks that winter in Louisiana. When the birds arrive in the James Bay region, they apparently enjoy a prolonged period of rest because they are not seen in the vicinity of their breeding grounds until the first of June. During the first 2 weeks of that month, they pour onto the Arctic tundra by the thousands, and each pair immediately sets about the business of rearing a brood.

The American robin has been mentioned as a slow migrant, and, as a species, it takes 78 days to make the 3,000-mile trip from Iowa to Alaska, a stretch of country that is crossed by advancing spring in 68 days. In this case, however, it does not necessarily mean that individual robins are slow. The northward movement of the species probably depends upon the continual advance of birds from the rear, so that the first individuals arriving in a suitable locality are the ones that nest in that area, while the northward movement of the species is continued by those still to come.

There is great variation in the speed of migration at different latitudes of the broad region between the Gulf of Mexico and the Arctic Ocean. The blackpoll warbler again furnishes an excellent example (Fig. 3). This species winters in northwestern South America and starts to migrate north in April. When the birds reach the southern United States, some individuals fly northwest to the Mississippi Valley, north to Manitoba, northwest to the Mackenzie River, and then almost due west to western Alaska. A fairly uniform average distance of 30 to 35 miles per day is maintained from the Gulf to Minnesota, but a week later this species has reached the central part of the Mackenzie Valley, and by the following week it is observed in northwestern Alaska. During the latter part of the journey, therefore, many individuals must average more than 200 miles per day. Thirty days are spent traveling from Florida to southern Minnesota, a distance of about 1,000 miles, but scarcely half that time is used to cover the remaining 2,500 miles to Alaska. Increased speed across western Canada to Alaska is also shown by many other birds (Figs. 2, 4, 6). A study of all species traveling up the Mississippi Valley indicates an average speed of about 23 miles per day. From southern Minnesota to southern Manitoba 16 species maintain an average speed of about 40 miles per day. From that point to Lake Athabaska, 12 species travel at an average speed of 72 miles per day, while 5 others travel to Great Slave Lake at 116 miles per day, and another 5 species cover 150 miles per day to reach Alaska. This change is in correlation with a corresponding variation in the isothermal lines, which turn northwestward west of the Great Lakes.

As has been previously indicated, the advance of spring in the northern interior is much more rapid than in the Mississippi Valley and on the Gulf coast. In other words, in the North spring comes with a rush, and, during the height of migration season in Saskatchewan, the temperature in the southern part of the Mackenzie Valley just about equals that in the Lake Superior area, 700 miles farther south. Such conditions, coupled with the diagonal course of the birds across this region of fast-moving spring, exert a great influence on migration and are probably factors in the acceleration of travel speed. However, it should be remembered that the birds are getting closer to the breeding season and may be stimulated to travel faster for this reason.

Thus it has been shown that the rate of migration varies greatly under varying circumstances. Radar investigations along the eastern coasts of the United States and England indicate spring migration is several miles per hour faster than in the fall. Also, directions of migrations in spring were much less diverse than in the fall, which suggests less time lost in passage (Tedd and Lack 1958; Nisbet and Drury 1967a). King and Farner (1963) found the same species put on more fat preparatory to migration in the spring. This would give the migrants greater energy reserves for longer flights at that season.

=ALTITUDE OF FLIGHT AND MIGRATION=

The factors regulating the heights of bird migration are not clear. High-altitude flight may be used to locate familiar landmarks, fly over fog or clouds, surmount physical barriers, gain advantage of a following wind, or maintain a better physiological balance. Meteorological conditions probably account for most of the high-altitude records. Wind conditions at ground level are usually quite different in direction and velocity than at points higher up.

In general, human estimates of bird heights are quite unreliable except under special conditions, and these estimates will vary with the eyesight of the observer. Lucanus (1911) found a European sparrow hawk could be distinguished at 800 feet but disappeared from sight at 2,800 feet. A rook (a European member of the crow family) could be recognized at 1,000 feet but disappeared from sight at 3,300 feet. Meinertzhagen (1955) did an interesting experiment with an inflated model of a vulture painted black; it had a wing expanse of 7 feet 10 inches. When released from an airplane at 4,700 feet, it was barely visible and invisible without binoculars at 5,800 feet. At 7,000 feet it was not picked up even when ×12 binoculars were used.

At one time students of bird migration believed normal migratory movements took place at heights above 15,000 feet. They reasoned, somewhat uncertainly, that flying became easier as altitude was gained. It has now been shown, through comprehensive radar studies, that 95 percent of the migratory movements occur at less than 10,000 feet, and the bulk of the movements occur under 3,000 feet. However, birds can and do fly well over 15,000 feet without apparent ill effects. The physiology of long-distance flight at high altitudes is of great interest but can only be touched on briefly in this discussion.

Bird flight at 20,000 feet, where less than half the oxygen is present than at sea level, is impressive if only because the work is achieved by living muscle tissue. A Himalayan mountain climber at 16,000 feet was rather amazed when a flock of geese flew north 2 miles over his head honking as they went (Swan 1970). At 20,000 feet a man has a hard time talking and running or other rapid movements are out of the question; but those geese were probably flying at 27,000 feet and even calling while they traveled at this tremendous height.

Accurate observations on the altitude of migratory flights is scanty, although altimeter observations from airplanes and radar are becoming more frequent in the literature. An example is the report of a mallard struck by a commercial airliner at 21,000 feet over the Nevada desert (Manville 1963). It is, of course, obvious that some birds must cross mountain ranges during migration and attain great altitudes. Numerous observations have come from the Himalayas (Geroudet 1954; Swan 1970). Observers at 14,000 feet recorded storks and cranes flying so high that they could be seen only through field glasses. In the same area large vultures were seen soaring at 25,000 feet and an eagle carcass was found at 26,000 feet. The expedition to Mt. Everest in 1952 found skeletons of a pintail and a black-tailed godwit at 16,400 feet on Khumbu Glacier (Geroudet 1954). Bar-headed geese have been observed flying over the highest peaks (29,000+ feet) even though a 10,000-foot pass was nearby. Probably 30 or more species regularly cross these high passes (Swan 1970).

Except to fly over high mountain ranges, birds rarely fly as high as those traveling down the western Atlantic (Richardson 1972). Many of these birds are making long-distance flights to eastern South America and beyond. Therefore, flight at high altitudes in this region is probably advantageous for them. Richardson postulated stronger advantageous tail winds were found higher up and the cooler air minimized evaporative water losses. This investigator found air temperatures averaged 35° F at 10,000 feet over Nova Scotia in September. The lower the ambient temperature, the more heat can be lost by convection and the less water is required for cooling. Also, a bird flying high can achieve the same range as one flying at sea level but must cruise at a higher speed with a corresponding increase in power output and oxygen consumption. But the increased cruising speed results in shorter flight time and less interference from wind (Pennycuick 1969).

Another postulate favoring the high-altitude flying theory was that the wonderful vision of birds was their sole guidance during migratory flights. To keep landmarks in view, birds were obliged to fly high, particularly when crossing wide areas of water. This will be considered in greater detail in the section, "Orientation and Navigation," so here it will be sufficient to say that birds rely only in part upon landmarks to guide them on migration. Also, it must be remembered that definite physical limitations to the range of visibility exist even under perfect atmospheric conditions. Chief of these is the curvature of the earth's surface. Thus, if birds crossing the Gulf of Mexico to Louisiana and Florida flew at a height of 5 miles, they would still be unable to see a third of the way across (during daylight hours). And yet this trip is made twice each year, much of the distance probably at night, by thousands of thrushes, warblers, and others.

The altitude of migration depends upon the species of bird, weather, time of day or year, and geographical features. Nocturnal migrants, studied by radar, appear to fly at different altitudes at different times during the night. Birds generally take off shortly after sundown and rapidly gain maximum altitude. This peak is maintained until around midnight, then the travelers gradually descend until daylight. For most small birds the favored altitude appears to be between 500 and 1,000 feet (Bellrose 1971), but radar studies have found some nocturnal migrants (probably shorebirds) over the ocean were at 15,000 or even 20,000 feet (Lack 1960b; Nisbet 1963b; Richardson 1972). Observations made from lighthouses and other vantage points indicate that certain migrants commonly travel at altitudes of a very few feet to a few hundred feet above sea or land. Sandpipers, northern phalaropes, and various sea ducks have been seen flying so low they were visible only as they topped a wave. Observers stationed at lighthouses and lightships off the English coast have similarly recorded the passage of landbirds flying just above the surface of the water and rarely above 200 feet. During the World Wars, broad areas in the air were under constant surveillance, and many airplane pilots and observers took more than a casual interest in birds. Of the several hundred records resulting from their observations, only 36 were of birds flying above 5,000 feet and only 7 above 8,500 feet. Cranes were once recorded at an altitude of 15,000 feet, while the lapwing was the bird most frequently seen at high levels, 8,500 feet being its greatest recorded altitude. Records of the U.S. Civil Aeronautics Administration show that over two-thirds of all the bird-aircraft collisions occur below 2,000 feet and practically none occur above 6,000 feet (Williams 1950).

Recently, radar has aided greatly in determining differences in the altitude of bird flight. Nocturnal migrants appear to fly slightly higher, on the average, than diurnal migrants, but daytime flights may be influenced more by cloud cover (Lack 1960a; Eastwood and Rider 1965). Bellrose (1971) found little difference in the altitudinal distribution of small nocturnal migrants under clear or overcast skies. Many night migrating birds are killed each year by striking lighthouses, television towers or other man-made illuminated obstructions, but this does not furnish proof that low altitudes are generally used during nocturnal flight because these accidents occur chiefly in foggy weather. Under such conditions, migrating birds seem to be attracted to and confused by lights. Seabirds, such as loons, eiders, and scoters, generally fly very low over the water but gain altitude when land is crossed. The reverse is true for landbirds (Dorst 1963; Bergman and Donner 1964; Eastwood and Rider 1965). There may be a seasonal difference in the altitude of migration, but the evidence is conflicting. Radar echoes studied by Bellrose and Graber in Illinois (1963) showed fall migrants flew higher than spring migrants. They speculated this difference was related to the winds during the fall being more favorable for southerly migration at higher altitudes, while winds at these altitudes in the spring would be less favorable for northerly migration. Eastwood and Rider (1965) studied seasonal migration patterns in England and found the reverse to be true. They suggested one reason for this seasonal difference was that flocks of fall migrants included many young birds whose flight capabilities are inferior to adults and consequently are unable to achieve the higher altitudes in the fall.

=SEGREGATION DURING MIGRATION=

By Individuals or Groups of Species

During the height of northward movement in spring, the woods and thickets may suddenly be filled with several species of wood warblers, thrushes, sparrows, flycatchers, and other birds. It is natural to conclude they traveled together and arrived simultaneously. Probably they did, but such combined migration is by no means the rule for all species.

As a group, the wood warblers probably travel more in mixed companies than do any other single family of North American birds. In spring and fall, the flocks are likely to be made up of the adults and young of several species. Sometimes swallows, sparrows, blackbirds, and some of the shorebirds also migrate in mixed flocks. In the fall, great flocks of blackbirds frequently sweep south across the Plains States, with common grackles, red-winged blackbirds, yellow-headed blackbirds, and Brewer's blackbirds included in the same flock.

On the other hand many species keep strictly to themselves. It would be difficult for any other kind of bird to keep company with the rapid movements of the chimney swift. Besides flight speed, feeding habits or roosting preferences can be so individual as to make traveling with other species incompatible. Nighthawks also fly in separate companies, as do crows, waxwings, crossbills, bobolinks, and kingbirds. Occasionally, a flock of ducks will be observed to contain several species, but generally when they are actually migrating, individuals of each species separate and travel with others of their own kind.

Although different species generally do not migrate together, we often find many species passing through an area at the same time. If the different kinds of birds observed in a specific area are counted every day throughout the entire migration season, this count often rises and falls much like the bell-shaped curve exhibited when the number of individuals of a given species are counted through the same time period. Figure 7 shows two peaks in the number of species passing through the desert at the north end of the Gulf of Eilat (=Akaba) in the Red Sea. These two peaks happen to coincide with peaks in the numbers of individuals (mostly from the order of perching birds) traveling through the area. Therefore, in the latter part of March and again in April, one notices not only more birds in the area but also more different kinds.

Closely related species or species that eat the same food organisms are not often found migrating through the same area at the same time. Ornithologists call this species replacement. In North America, peaks in the migration of the five kinds of spotted thrushes generally do not coincide. Dates of departure in these species have evolved so all the individuals of these closely related birds do not converge on one area at the same time and subsequently exhaust the food supply. By selection of staggered peak migration dates, evolution has distributed the members of this family more or less evenly throughout the entire season. Likewise, in the eastern Mediterranean area, we find a similar situation in spring migration for three closely related buntings; Cretzschmar's bunting comes through first, followed a few weeks later by the Ortolan bunting and, at the tail end of the migration period, the black-headed bunting appears (Fig. 8).

By Age

The adults of most birds leave the young when they are grown. This gives the parents an opportunity to rest and renew their plumage before starting for winter quarters. The young are likely to move south together ahead of their parents. This has been documented in a number of species including our mourning dove, the common swift of Europe, and storks. Mueller and Berger (1967) found an age-specific migration pattern in sharp-shinned hawks passing through Wisconsin. The immatures were much in evidence during mid-September while the adults came through a month later. Far to the south in the Antarctic, young Adelie penguins depart for northern wintering grounds much earlier than adults.

In a few species, adults depart south before the young. Adult golden plovers, Hudsonian godwits, and probably most of the Arctic breeding shorebirds leave the young as soon as they are capable of caring for themselves and set out for South America ahead of the juveniles. Likewise, data for the least flycatcher indicate adults migrate before the young, but Johnson (1963) did not find this segregation in the Hammond's flycatcher. In Europe, adult red-backed shrikes are known to migrate ahead of their young.

In contrast to this loss of parental concern, geese, swans, and cranes remain in family groups throughout migration. The parent birds undergo a wing molt that renders them flightless during the period of growth of their young so that both the adults and immatures acquire their flight capabilities at the same time and are able to start south together. Large flocks of Canada geese, for example, are composed of many families banded together. When these flocks separate into small V-shaped units it is probably correct to assume an old goose or gander is leading the family. After female ducks start to incubate their eggs, the males of most species of ducks flock by themselves and remain together until fall. When segregation of the sexes such as this occurs the young birds often accompany their mothers south. Murray and Jehl (1964) concluded from mist-netting many thousands of migrant passerines at Island Beach, New Jersey, that adults and juveniles travel at approximately the same time.

By Sex

Males and females of some species may migrate either simultaneously or separately. In the latter case it is usually the males, rarely the females, that arrive first. Sometimes great flocks of male red-winged blackbirds reach a locality several days before any females; this is particularly the rule in spring. The first robins are usually found to be males, as are also the first song sparrows, rose-breasted grosbeaks, and scarlet tanagers. In Europe, the three buntings mentioned previously are also segregated as to sex during migration. Figure 8 shows two prominent peaks for both the Cretzschmar's and Ortolan buntings; during passage the first peak was primarily males while the second peak consisted mostly of females. This early arrival of males on the breeding grounds is associated with territorial possession whereby the male selects the area where it intends to breed and each individual attempts to protect a definite territory from trespass by other males of his own kind, while announcing his presence to rival males and later arriving females by song or other display. The female then selects the site where she wishes to nest. The long-billed marsh wren is a noteworthy example; the males may enthusiastically build several nests before the females arrive. In the fall, common and king eiders are sexually segregated during migration. During July, flocks crossing Point Barrow are composed almost entirely of males, while after the middle of August the flocks are almost all females (Thompson and Person 1963). In the Chicago area, Annan (1962) reported that some males, such as the hermit thrush, Swainson's thrush, gray-cheeked thrush, and veery, arrive before any females and predominate during the first week of occurrence.

In a few species the males and females apparently arrive at the breeding grounds together and proceed at once to nest building. In fact, among shorebirds, ducks, and geese, courtship and mating often takes place in whole or in part while the birds are in the South or on their way north, so that when they arrive at the northern nesting grounds they are paired and ready to proceed at once with raising their families. Mallards and black ducks may be observed in pairs as early as December, the female leading and the male following when they take flight. Naturally, these mated pairs migrate north in company, and it was largely to protect such pairings that duck shooting in spring was abolished by Federal law.

In the coastal subspecies of the western flycatcher, the sexes appear to migrate in synchrony during the spring in contrast to migration of Hammond's flycatcher in which the adult males usually precede the females (Johnson 1973). Both sexes of the common blackcap of Europe appear to migrate together at least across the eastern end of the Mediterranean during the spring (Fig. 9).

By Kinds of Flocks

Migratory flights are frequently accomplished in close flock formation, as with shorebirds, blackbirds, waxwings, and especially some of the buntings, longspurs, juncos, and tree sparrows. Other species maintain a very loose flock formation; examples are turkey vultures, hawks, swifts, blue jays, swallows, warblers, and bluebirds. Still others, the grebes, snowy owls, winter wrens, shrikes, and belted kingfishers, ordinarily travel alone, and when several are found in close proximity it is an indication they have been drawn together by unusual conditions, such as abundant food.

Just as flocking among resident birds provides group protection against predation, flocking in migration greatly facilitates the attainment of destination (Pettingill 1970). The V-shaped flocks often associated with Canada geese have a definite energy conserving function by creating favorable air currents for every member of the flock but the leader; when the leader becomes tired, it will often change places with a member behind. Night migrating flocks generally fly in looser formations than do day migrating flocks.

=WHERE BIRDS MIGRATE=

Migration by Populations Within Species

Both length and duration of migratory journeys vary greatly between families, species, or populations within a species. Bobwhite, western quails, cardinals, Carolina wrens, and probably some of the titmice and woodpeckers are apparently almost or entirely nonmigratory. These species may live out their entire existence without going more than 10 miles from the nest where they were hatched.

Many song sparrows, meadowlarks, blue jays, and other species make such short migrations that the movement is difficult to detect because individuals, possibly not the same ones, may be found in one area throughout the year while other individuals that move south may be replaced by individuals from the north. Information on different movements of this type, within a species, can be gained by observing birds marked with numbered bands, colored materials, or identification of racially distinct museum specimens.

The American robin is a good example of this type of movement. This species occurs in the southern United States throughout the year, but in Canada and Alaska only during the summer. Its movements are readily ascertained from study specimens. The breeding robin of the southeastern states is the southern race. In autumn most of its more northern nesters, such as those from Maryland and Virginia move into the southern part of the breeding range or slightly farther south. At about the same time the northern American robin moves south and winters throughout the breeding and wintering range of its smaller and paler southern relative. Thus there is complete overlap of wintering ranges of northern and southern American robin populations, although some individuals of the northern race winter in areas vacated earlier by the southern race.

Among many migratory species there is considerable variation among individuals and populations with respect to distances moved. Certain populations may be quite sedentary while others are strongly migratory, and certain individuals of the same population can be more migratory than others. For example, red-winged blackbirds nesting on the Gulf Coast are practically sedentary, but in winter they are joined by other subspecies that nest as far north as the Mackenzie Valley. In certain populations of the song sparrow and other species, males remain all year on their northern breeding grounds while the females and young migrate south.

Several species containing more than one distinguishable population exhibit "leap-frog" migration patterns. The familiar eastern fox sparrow breeds from northeastern Manitoba to Labrador, but during the winter it is found concentrated in the southeastern part of the United States. On the west coast of the continent, however, a study of museum specimens by Swarth (1920), indicated six subspecies of this bird breeding in rather sharply delimited ranges extending from Puget Sound and Vancouver Island to Unimak Island, at the end of the Alaskan Peninsula. One of these subspecies, known as the sooty fox sparrow, breeds from the Puget Sound-Vancouver Island area northward along part of the coast of British Columbia. It hardly migrates at all, while the other races, nesting on the coast of Alaska, are found in winter far to the south in Oregon and California. Although much overlap exists, the races breeding farthest north generally tend to winter farthest south. This illustrates a tendency for those populations forced to migrate to pass over those subspecies so favorably located as to be almost sedentary. If the northern birds settled for the winter along with the sedentary population, winter requirements may not be as sufficient as in the unoccupied areas farther south (Fig. 10). Therefore, natural selection has insured the different populations will survive the winter by separating the subspecies into different wintering areas.

Another example of this "leap-frog" migration is illustrated by the common yellowthroat of the Atlantic coast. Birds occupying the most southern part of the general range are almost nonmigratory and reside throughout the year in Florida, whereas the population that breeds as far north as Newfoundland goes to the West Indies for the winter. Thus the northern population literally "jumps" over the home of the southern relatives during migratory journeys.

The palm warbler breeds from Nova Scotia and Maine west and northwest to southern Mackenzie. The species has been separated into two subspecies: those breeding in the interior of Canada and those breeding in northeastern United States and Canada. The northwestern subspecies makes a 3,000-mile journey from Great Slave Lake to Cuba and passes through the Gulf States early in October. After the bulk of these birds have passed, the eastern subspecies, whose migratory journey is about half as long, drifts slowly into the Gulf Coast region and remains for the winter.

Fall Flights Not Far South of Breeding Range

Some species have extensive summer ranges (e.g., the pine warbler, rock wren, field sparrow, loggerhead shrike, and 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 the tree sparrow, snow bunting, and Lapland longspur, nest in the far north and winter in the eastern United States, while others, including the vesper and chipping sparrows, common grackle, red-winged blackbird, eastern bluebird, American woodcock, and several species of ducks, nest much farther south in the United States and Canada and move south a relatively short distance for the winter to areas along the Gulf of Mexico. In a few of the more hardy species, individuals may linger in protected places well within reach of severe cold. The common snipe, for example, is frequently found during subzero weather in parts of the Rocky Mountain region where warm springs assure a food supply. More than 100 summer birds leave the United States entirely and spend the winter in the West Indies, Central America, or South America. For example, the Cape May warbler breeds from northern New England, northern Michigan, and northern Minnesota, north to New Brunswick, Nova Scotia, and nearly to Great Slave Lake. In winter it is concentrated chiefly in the West Indies on the island of Hispaniola.

Long Distance Migration

Some of the common summer residents of North America are not content with a trip to northern tropical areas of the West Indies and Central America, but push on across the Equator and finally come to rest for the winter in Patagonia or the pampas of Argentina. Species such as nighthawks, some barn swallows, cliff swallows, and a few thrushes may occupy the same general winter quarters in Brazil, but other nighthawks and barn swallows go farther south. Of all North American landbirds these species probably travel the farthest; they are found north in summer to the Yukon Territory and Alaska, and south in winter to Argentina, 7,000 miles away. Such seasonal flights are exceeded in length, however, by the remarkable journeys of several species of shorebirds including white-rumped and Baird's sandpipers, greater yellowlegs, turnstones, red knots, and sanderlings. In this group, 19 species breed north of the Arctic Circle and winter in South America; six of these go as far south as Patagonia, a distance of over 8,000 miles.

The Arctic tern is the champion "globe trotter" and long-distance flier (Fig. 11). Its name "Arctic" is well earned, as its breeding range is circumpolar and it nests as far north as the land extends in North America. The first nest found in this region was only 7½° (518 miles) from the North Pole and contained a downy chick surrounded by a wall of newly fallen snow scooped out by the parent. In North America the Arctic tern breeds south in the interior to Great Slave Lake, and on the Atlantic coast to Massachusetts. After the young are grown, the Arctic terns disappear from their North American breeding grounds and turn up a few months later in the Antarctic region, 11,000 miles away. For a long time the route followed by these hardy fliers was a complete mystery; although a few scattered individuals have been noted south as far as Long Island in the United States, the species is otherwise practically unknown along the Atlantic coasts of North America and northern South America. It is, however, known as a migrant on the west coast of Europe and Africa. By means of numbered bands, a picture disclosed what is apparently not only the longest, but also one of the most remarkable migratory journeys (Austin 1928).

Few other animals in the world enjoy as many hours of daylight as the Arctic tern. For these birds, the sun never sets during the nesting season in the northern part of the range, and during their winter sojourn to the south, daylight is continuous as well. In other months of the year considerably more daylight than darkness is encountered.

=ORIENTATION AND NAVIGATION=

There probably is no single aspect of the entire subject of bird migration that increases our admiration so much as the unerring certainty with which birds cover thousands of miles of land and water to come to rest in exactly the same spot where they spent the previous summer or winter. Records from birds marked with numbered bands offer abundant proof that the same individuals of many species will return again and again to identical nesting or winter feeding sites.

This ability to travel with precision over seemingly featureless stretches of land or water is not limited to birds but is likewise possessed by certain mammals, reptiles, fishes, and insects; the well-known migrations of salmon and eels are notable examples.

For an animal to return to a specific spot after a lengthy migration, it must use true navigation to get there. That is, it needs to not only travel in a given compass heading and know where it is at any given time so the course may be altered when necessary but also be able to recognize its goal when it has arrived. It is dangerous to generalize on the means of orientation and navigation in migration; different groups of birds with different modes of existence have evolved different means of finding their way from one place to another (Pettingill 1970). We are only beginning to realize the complexities involved in the many modes of bird orientation and navigation. All we can do in this section is present a brief summary of some of the more important principles involved and the studies that have enhanced our knowledge in the area.

Ability to follow a more or less definite course to a definite goal is evidently part of an inherited faculty. Both the direction and the goal must have been implanted in the bird's genetic code when the particular population became established at its present location. The theory is sometimes advanced that older and more experienced birds lead the way and thereby show the route to their younger companions. This explanation may be acceptable for some species such as geese, swans, and cranes because they stay in family groups, but not for species in which adults and young are known to migrate at different times, especially when young migrate ahead of the adults. As indicated in a previous section on segregation, many North American shorebirds as well as the cuckoos of New Zealand do this. An inherited response to its surroundings, with a definite sense of the goal to be reached and the direction to be followed, must be attributed to these latter birds.

It is well known that birds possess wonderful vision. If they also have retentive memories subsequent trips over the route may well be steered in part by recognizable landmarks. Arguments against the theory of landmark memory are chiefly that unescorted young birds, without previous experience, can find their way to the winter quarters of their species, even if the wintering area has a radically different landscape and vegetation than the breeding grounds. Experimental findings and field observations indicate landmarks are used in navigation by certain birds, but the degree of use varies considerably among the species (Bellrose 1972a).

To a land-dweller traveling the ocean, the vast expanse may seem featureless but the reverse may be true for a seabird blown over land by a storm. In the latter situation the differences in vegetation and topography "obvious" to land-dwellers are completely foreign to a seabird as it has had little previous experience to help interpret these "strange objects." Griffin and Hock (1949) observed the flight behavior of gannets displaced far inland away from their nests. The bird appeared to search randomly until the coastline was met, then the fliers pursued a much more direct course home. Herring gulls, displaced about 250 miles from their nest in 2 consecutive years, returned the second year in one-sixth the time required the first year (Griffin 1943). To birds such as gannets, albatrosses, and shearwaters, which spend almost their entire lives traveling thousands of miles at sea and return to very specific nesting areas, the "featureless ocean expanses" are probably very rich in visual cues. It is difficult to believe a bird dependent on the sea for its livelihood cannot help but be aware of wave direction, islands, reefs, atolls, concentrations of floating flotsam, organisms, currents, clouds over islands, fog belts, etc.

Much migration takes place at night and great stretches of the open sea are crossed to reach destinations. Nights are rarely so dark that all terrestrial objects are totally obscured, and features such as coastlines and rivers are just those that are most likely to be seen in the faintest light, particularly by the acute vision of birds from their aerial points of observation. Even if terrestrial objects are completely obscured on a very dark night, the migrants are still able to assess their surroundings during the day before starting out again.

Some birds, especially colonial seabirds, seem to be able to fly unerringly through the densest fog, particularly in the vicinity of their nest site. Members of the Biological Survey, proceeding by steamer through a dense fog from the island of Unalaska to Bogoslof Island in the Bering Sea, recorded flocks of murres, returning to Bogoslof after quests for food. The birds broke through the wall of fog astern, flew by the vessel, and disappeared into the mists ahead on the same course as the ship. On the other hand, radar observations of migrating birds have indicated strong directional movements on clear nights but often completely random movements in heavily overcast or stormy weather. Possibly some birds can perceive the position of the sun through an overcast as honey bees are known to do. It is less likely the stars could be detected through overcast at night.

Careful studies have been made on the homing instinct in various seabirds such as Laysan albatrosses, Manx shearwaters, and several tropical species of terns. Sooty and noddy terns reach their most northern breeding point on the Dry Tortugas, off the southwest coast of Florida. They are not known to wander any appreciable distance farther north. Displaced breeding birds returned to their nests on the Dry Tortugas after they had been taken on board ship, confined in cages below decks, and carried northward 400 to 800 miles before being released in a region where they had had no previous experience. Likewise, Laysan albatrosses and Manx shearwaters have returned over 3,000 miles in similar homing experiments.

Possibly the "homing instinct," as shown by pigeons, terns, shearwaters, albatrosses, and by the frigatebirds trained as message carriers in the South Pacific, may not be identical with the sense of perceptive orientation that figures in the flights of migratory birds. Nevertheless, it seems closely akin and is probably governed by the same mechanisms. There are good reasons to assume that once we know the processes governing displaced homing we will know, in general, how birds navigate; this question is still far from being answered (Wallraff 1967).

Some students have leaned toward the possible existence of a "magnetic sense" as being the important factor in the power of geographical orientation. The theory was conceived as early as 1855 and reported in 1882 by Viguier. Investigations of this have been conducted by Yeagley (1947) and Gordon (1948) with contradictory results. In 1951, Yeagley incorporated the idea that sensitivity to the effect of the earth's rotary motion through the vertical component of the magnetic field is the means of orientation. The basic question asked of the theory is: "Can birds detect such minute differences in the earth's magnetic field and can these forces affect bird behavior?"

Attempts to demonstrate the effect of radio waves on the navigational ability of birds have produced contradictory results. In some of these tests, homing pigeons released near broadcasting stations have appeared to be hopelessly confused, whereas in others, apparently conducted in the same manner, no effects could be discerned. Before sensitivity of birds to electromagnetic stimuli of any kind can be accepted or rejected, much additional experimental work is necessary.

Human navigators have used the heavenly bodies in determining their course and position for centuries. It would not be surprising then to find other long-distance travelers using the same method. One of the most constant visual cues a migrating bird could use would be the sun's or moon's path and the location of the stars.

Some of the more recent experimental work on bird navigation has been with astronomical (sun) and celestial (star) directional clues. Studies by Kramer, Sauer, and others have indicated a phenomenal inherited ability in birds to use the position of the sun by day and the stars by night to chart their courses. This involves an intricate compensation for daily, seasonal, and geographical changes in the positions of these heavenly bodies. Kramer (1957, 1961) placed diurnal migrants in circular cages and "changed" the position of the sun with mirrors. The birds shifted their position to compensate for these changes. Sauer (1957, 1958), in a fascinating study with nocturnal migrant warblers, placed birds in a round cage open to the sky. These birds oriented in the normal direction for that locality and time of year. He next placed the cage and birds in a planetarium and projected overhead the night sky star patterns for different seasons and localities. The familiar star pattern produced a normal orientation but an unfamiliar sky caused confusion and complete disorientation. These experiments, begun in Germany, are still continuing in other countries with other species. Emlen (1969) used photoperiod manipulation to change the physiological states of spring and fall migratory readiness in indigo buntings. Half the sample of birds were in breeding condition whereas the other half were already past the reproductive stage even though it was spring "outside." When these birds were subjected to a spring star pattern in a planetarium, the birds in spring condition oriented northward but those in autumnal condition oriented southward. Although some results have been negative, by and large the evidence supports the original findings that the sun and stars are visual "landmarks" used by at least some birds as well as bees and probably many other creatures in finding their way home as well as to their winter and summer quarters.

In conclusion, then, we can say this about bird orientation and navigation: 1) many cues are available to birds for migratory guidance and one or several of these may be used by any migrant; 2) different species and groups of birds use different cues, depending on their migration traits; 3) visual cues probably play a predominant role in migration (radar studies have indicated that some birds can maintain their orientation even under completely overcast nights, although they usually become disoriented under such conditions); and 4) long-distance migrants and pelagic species have a much higher developed sense of orientation than those species that migrate only short distances or not at all.

=INFLUENCE OF WEATHER=

It is thought by some that the weather has little to do with the time of arrival of migratory birds. It is assumed that if the bird is physiologically prepared for migration it departs, irrespective of the weather. Even if this were the case, weather can influence the progress of migration by not only controlling the advance of the seasons but also by helping, hindering, or even stopping bird flight (Welty 1962).

Some scientists believe that birds not only avoid bad weather at the start of a journey but usually finish the journey in good weather (Nesbit and Drury 1967b). Contrary to what many observers believe, the arrival of birds in an area, whether they stop or continue on, is more often controlled by the weather at the point of departure than at the point of arrival. During the peak of migration, suitable weather may occur at an observation site, but strong migratory movements may be arrested before the birds arrive there because the weather was not suitable at the point of departure or somewhere in between. In addition, if there is good weather at the point of departure as well as farther down the migration route, the migrants, once air-borne in a favorable weather pattern, may continue on right over an expectant observer and the whole flight will be missed. Nesbit and Drury's (1967b) radar study on air-ground comparisons found, with few exceptions, ground observers missed the largest movements observed on radar. Observation of a large wave of arrivals indicated migrants had been stopped by a meteorological barrier, and people were actually not reporting maximum migration but an interruption to migration. Therefore, when migration is proceeding normally under safe conditions, very little movement is visible to the ground observer but a large arrival of birds on the ground often indicates something is not in order and the migrants have been forced to stop for one reason or another.

The question is frequently asked: "How can I identify weather conditions suitable or unsuitable for migration?" It is almost impossible to discuss separately the effects of different weather factors on migration because barometric pressure, temperature, wind, and other meteorological phenomena are very closely related.

On the North American continent, air masses generally proceed about 600 miles per day from the west to the east. These air masses vary in pressure, temperature, humidity, and wind. The wind within these masses travels in either a clockwise (anticyclonic) or counter-clockwise (cyclonic) direction. Cyclonic air masses contain relatively moist warm air with low barometric pressure centers and are designated "lows"; anticyclonic air masses are characterized by dry cool air with high barometric pressure areas and are called "highs." Where these air masses meet, a "front" is formed, and the rapidity with which this front moves through an area depends on the temperature and pressure gradient on either side of the front.

An understanding of frontal systems, with their associated wind, temperature and humidity, is one of the keys to understanding when birds migrate. You must not only watch the fronts in your area but the progress of nearby air masses as well because the birds migrating through your area have started their journey to the north or south of you depending on the season. The weather conditions at point of departure will dictate if and when birds will be passing through your area in the near future.

During fall migration, the best passage of migrants usually occurs 2 days after a cold front has gone through. That is, the low has passed and it is being followed by a high characterized by dropping temperatures, a rising barometer, and clearing skies. The 24 hours just after a low has passed are not always conducive to a good passage of birds because winds are often too strong and turbulent in the trough between the two air masses. Hochbaum (1955) correlated mass movements of ducks through the prairies with weather systems and noted the combination of weather conditions described above was ideal for mass migrations of ducks during November. During this period, observers at Delta, Manitoba, south to Louisiana recorded a tremendous flight of ducks as the proper conditions of barometric pressure, temperature, wind, and cloud cover passed across the central United States and Canada. An example of the type of weather system that is often associated with mass movements is illustrated in Fig. 12.

Records of lapwings on Newfoundland and the Gulf of St. Lawrence appear to be the result of a particular series of meteorological events (Bagg 1967). The lapwing is a European species rarely found in the New World. If cold air moves into western Europe from the east, lapwings move westward into England, Wales, and Ireland. Occasionally, the development of an anomalous weather pattern over the North Atlantic including an elongated low from Europe to eastern Canada causes some birds to be literally "blown" in the counter-clockwise airstream across the Atlantic to the Gulf of St. Lawrence.

During spring migration, weather conditions conducive to strong movements of birds are somewhat the opposite from those in the fall. Migrants will move north on the warm sector of an incoming low. When a high pressure area has just passed, the influx of warm moist tropical air is extended and intensified (Bagg et al. 1950). However, during this time, cloudiness and rain associated with the low may curtail migration or squeeze it into a narrow period proceeding along the warm front. If a fast moving cold front approaches from the northwest, the rapid movement of migrants will be sharply curtailed or even grounded until more favorable conditions occur.