Figure 5. Migration of the Canada Goose. The northward movement keeps pace with the progress of spring, because the advance of the isotherm of 35° F agrees with that of the birds.

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

The Gray-cheeked Thrush, which winters in northern South America, 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 end of April: 25 April near the mouth of the Mississippi and 30 April in northern Florida (Figure 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 rate of about 130 miles per day.

Figure 6. Isochronal migration lines of the Gray-cheeked Thrush, an example of rapid migration. The distance from Louisiana to Alaska is about 4,000 miles and is covered at an average speed of about 130 miles per day. The last part of the journey is covered at a speed several times what it is in the Mississippi Valley.

Another example or 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 (31°C). By traveling north much faster than the spring progresses, this warbler reaches its breeding grounds in Manitoba in the latter part of May, when the average temperature is only 47°F (22°C). They encounter progressively colder weather over their entire route and cross a strip of country in the 15 days from May 11 to May 25 that spring temperatures normally take 35 days to cross. This "catching up: with spring is typical in many species that winter south of the United States as well as in most northern species that winter in the Gulf States.

The Snow Goose presents a striking example of a late but very rapid spring migration. Most of these geese winter in the great coastal marshes of Louisiana, where every year over 400,000 spend the winter. Congregations of 50,000 or more may be seen grazing in 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. Even though the season in their winter quarters is advancing rapidly, their nesting grounds are still covered with a heavy blanket of ice and snow. Thus, Snow Geese remain in the coastal marshes until the last of March or the first of April, when local birds are already busily engaged in reproduction. These data support the general hypothesis that a species' premigratory development in response to stimuli such as daylength and temperature has evolved so that the timing of its physiological preparation will lead to its arrival on the breeding range at the optimum conditions for 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, along the Platte in Nebraska, and in eastern South Dakota and southeastern Manitoba. Normally, however, 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 is a slow migrant, taking an average of 78 days to make the 3,000-mile trip from Iowa to Alaska. The same stretch of country 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 between the Gulf of Mexico and the Arctic Ocean. The Blackpoll Warbler again furnishes an excellent example (Figure 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 (Figures 2, 4, and 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 corresponds to 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 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.


Migratory Flight Altitude
While factors regulating the heights at which birds migrate are not clear, there are many obvious reasons why flying at higher altitudes may be advantageous. 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 thermoregulatory balance.

In general, estimates of bird heights based on direct observation are quite unreliable except under special conditions. A Eurasian Sparrowhawk could be distinguished at 800 feet but disappeared from site 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. An interesting experiment with an inflated model of a vulture painted black with a wing span of 7 feet 10 inches illustrated similar limitations. 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 power binoculars were used. Radar studies have demonstrated more accurately than human vision that 95 percent of the migratory movements occur at less than 10,000 feet, the bulk of the movements occurring under 3,000 feet.

Yet birds do fly at higher altitudes. 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 northward about two miles over his head honking as they went. At 20,000 feet a man has a hard time talking while running, but those geese were probably flying at 27,000 feet and even calling while they traveled at this tremendous height. Numerous other observations have come from the Himalayas. 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 Northern Pintail and a Black-tailed Godwit at 16,400 feet on Khumbu Glacier. Bar-headed Geese have been observed flying over the highest peaks (29,000+ feet) even though a 10,000-foot pass was nearby. Probably at least 30 species regularly cross these high passes. Other accurate records on altitude of migratory flights are scanty, although altimeter observations from airplanes and radar are becoming more frequent in the literature. For example, a Mallard was struck by a commercial airliner at 21,000 feet over the Nevada desert. Radar observations have revealed that birds on long-distance flights fly at higher altitudes than short-distance migrants. It has been hypothesized that advantageous tail winds of greater velocity are found higher up and that the cooler air minimizes the demand for evaporative water loss to regulate body temperature under the exertion of flight. Radar studies also have shown that nocturnal migrants 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. Thus, there is considerable variation, but for most small birds the favored altitude appears to be between 500 and 1,000 feet. Some nocturnal migrants (probably shorebirds) fly over the ocean at 15,000 or even 20,000 feet. Nocturnal migrants also fly slightly higher than diurnal migrants. Observations made from lighthouses and other vantage points indicate that certain migrants commonly travel at altitudes of very few feet to a few hundred feet above sea or land. Sandpipers, Red-necked 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 rising above 200 feet over the waves.


Segregation During Migration

As Individuals or Groups of Species
During the height of northward movement in spring, the woods and thickets may suddenly be filled in the morning 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 Great Plains 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. Common Nighthawks fly in separate companies, as do American Crows, Cedar Waxwings, Red Crossbills, Bobolinks, and Eastern Kingbirds. And 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 species-specific as to make traveling with other species incompatible. 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.

Even if different species 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 coincide with peaks in the numbers of individuals (mostly perching birds) traveling through the area. Therefore, in the latter part of March and again in April, there are not only more birds in the area, but also more species.

Figure 7. Average number of species captured daily in mist nets during spring migration at Eilat, Israel, in 1968. The number of species passing through an area on migration will rise and fall similar to the number of birds counted in the area. In this case two major movements came through about 1 month apart.

Closely related species or species that eat the same food are not often found migrating through the same area at the same time. In North America, peaks in the migration of the five species 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, the processes involved in evolution have 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 during 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 end of the migration period, the Black-headed Bunting appears (Figure 8). Many groups of migrating species like shorebirds, blackbirds, waxwings, and buntings maintain a close flock formation. Other species like Turkey Vultures, hawks, swifts, Blue Jays, swallows, and warblers maintain a loose flock. And still others, like shrikes, Belted Kingfishers, grebes, and Winter Wrens, ordinarily travel alone.

Figure 8. Average number of three species of buntings captured daily in mist nets during spring migration at Eilat, Israel, in 1968. Closely related species that migrate through the same area often appear at different times. Thus species that may eat the same foods do not compete with each other.

Just as flocking among resident birds provides group protection against predators and facilitates food finding, flocking of migrants probably serves the same purposes. The V-shaped flocks associated with Canada Geese and Double-crested Cormorants have a definite energy conserving function by allowing members of the flock to gain an aerodynamic advantage from the wing-tip vortices of the bird ahead. It has also been observed from radar studies that day migrants fly in tighter formations than flocks migrating by night. This again may reflect a strategy to deter the effect of aerial predators.

By Age
The adults of most birds abandon the young when they are grown. This gives the parents an opportunity to renew their plumage and gain fat stores before starting for winter quarters. The young may move south together ahead of their parents, as has been documented in a number of species including Mourning Doves and the Common Swift and White Storks in Europe. In Sharp-shinned Hawks passing through Wisconsin, the immatures are much in evidence during mid-September while the adults come through a month later. Far to the south in Antarctica, young Adelie Penguins depart for coastal wintering grounds much earlier than adults.

In a few species, adults depart south before the young. Adult American 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 this segregation does not occur in the closely related Hammond's Flycatcher. In Europe, adult Red-backed Shrikes are known to migrate ahead of their young. In contrast, 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 family groups. When these flocks separate into small V-shaped units it is probably correct to assume an older 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.

By Sex
Males and females may migrate either simultaneously or separately. Although there are exceptions, generally passerine males arrive before females. Thus, in spring great flocks of male Red-winged Blackbirds reach a locality several weeks before any females. The first American Robins are usually found to be males, as are the first Song Sparrows, Rose-breasted Grosbeaks, Dickcissels, 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 the establishment of territories in which each male defends a definite area from trespass by other males of his own kind, while announcing his presence to rival males and later arriving females by song or other displays. The female then selects the site where she wishes to nest. 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. In the Chicago area, male Hermit Thrushes, Swainson's Thrushes, Gray-cheeked Thrushes, and Veerys arrive before any females and predominate during the first week of passage.

In a few species the males and females arrive at the breeding grounds together and proceed at once to nest. In fact, among shorebirds, ducks, geese, and the Osprey courtship and mating often takes place while the birds are in the South or on their way north, so that when they arrive on the northern nesting grounds, they are paired and ready to proceed at once with raising their families. Mallards and American Black Ducks may be observed in pairs as early as December, the female leading and the male following when they take flight.

In the Pacific-slope Flycatcher, the sexes appear to migrate in synchrony during the spring in contrast to migration of the closely related Hammond's Flycatcher in which the adult males usually precede the females. Both sexes of the Common Blackcap of Europe appear to migrate together at least across the eastern end of the Mediterranean during the spring (Figure 9).

Figure 9. Numbers of male and female blackcaps captured daily in mist nets during spring migration at Eilat, Israel, in 1968. At this point in their migration the sexes are passing through the area at the same time. In other species (e.g., the buntings in Fig. 8), the males often precede the females.

Previous Section
Next Section