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Migration of Birds

Flight Speed and Rate of Migration

There is a widespread misconception concerning the speed at which birds can fly. One often hears stories of birds flying "a mile a minute." While undoubtedly some birds do attain this speed, such cases are exceptional; and it is safe to say that, even when pressed, few can develop an air speed of 60 miles per hour. Birds, except for the heavy-bodied, small-winged species such as auks, grebes, and other divers, generally have two different flight speeds. There is a normal rate for ordinary purposes, and an accelerated speed for escape or pursuit that may be double the normal rate.

Reliable data on the speed of birds are accumulating slowly. Accurate measurements are difficult to obtain unless the bird travels over a measured course and wind conditions at the level of flight are known. Several subtle factors, besides wind and pursuit, can influence the speed of a flying bird. For instance, species that have a courtship flight often reach their maximum speeds then. Small woodland birds often fly faster across an open area where they might be attacked by a bird of prey than under cover where there is less danger. Birds in flocks generally fly faster than when flying alone.

In general, flight velocity of birds ranges from 20 to 50 miles per hour. For sustained flight, larger birds typically fly faster than smaller birds. A common flying speed of ducks and geese is between 40 and 50 miles per hour, but among the smaller birds it is much less. Herons, hawks, Horned Larks, ravens, and shrikes, timed with an automobile speedometer have been found to fly 22 to 28 miles per hour, whereas some of the flycatchers fly at only 10 to 17 miles per hour. Even such fast-flying birds as the Mourning Dove rarely exceed 35 miles per hour. A Peregrine Falcon will have difficulty catching a pigeon during a level chase at 60 miles per hour, but this predator can probably exceed 100 miles per hour during a stoop from a greater height onto its prey, although this velocity has never been accurately measured.

The rate of migration is quite different from that attained in forced flights for short distances. A sustained flight of 10 hours per day in still air would carry herons, hawks, crows, and smaller birds from 100 to 250 miles, while ducks and geese might travel as much as 400 to 500 miles in the same period. Measured as straight line distances, these journeys are impressive and indicate birds could travel from the northern United States or even from northern Canada to winter quarters in the West Indies, Central, or South America in a relatively short time, especially if they took advantage of tail winds. It is probable that individual birds do make flights this long and that Barn Swallows seen in May on Beata Island, off the southern coast of the Dominican Republic, have reached that point by a nonstop flight of 350 miles across the Caribbean Sea from the coast of Venezuela.

Radar has provided some of our best estimates of ground speeds for migrating flocks. Radar echoes identified as shorebirds migrating off the New England coast moved steadily about 45 miles per hour for several hours; songbird echoes typically traveled around 30 miles per hour. Some birds appear to reduce flight speed in proportion to the degree of assistance from a tailwind, thus conserving energy.

The intensity of migration depends not only upon extrinsic environmental conditions but also on intrinsic circumstances affecting the drive motivating the birds' behavior; birds travel faster when hurrying toward the breeding grounds. Radar investigations along the eastern coast of the United States and in England indicate spring migration is several miles per hour faster than in the fall. Also, directions of the migrants in the spring were less diverse than in the fall, suggesting less time lost in passage. Furthermore, fat stores in the spring are greater than in the same species during their fall migration. This would provide vernal migrants greater energy reserves for longer flights at that season. In fall, the flights are more leisurely, so that after a few hours of flying, birds often pause to feed and rest for one or several days, particularly if they find themselves in suitable 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 usually takes a month or more to cover a straight-line distance of a thousand miles. For example, an American Black Duck banded at Lake Seugog, 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, evidence of rapid migrations is decidedly scarce, for with few exceptions, all thousand-mile flights require 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 two or three 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 one 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 a slow gate of flight. 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 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 (Figure 5). In fact, the isotherm of 35F (16C) appears to be a governing factor in the speed at which the these geese move north; from an evolutionary viewpoint we might expect this. If the geese continually advanced ahead of the freezing line, they would find food and open water unavailable.

Figure 5: Map showing Isotherms of 35°F (March 1 to April 30) and Isochronal Migration Lines (Feb. 10 to April 30) for the Canada Goose
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: Map showing Isochronal Migration Lines (April 25 to May 25) for the Gray-cheeked Thrush
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.

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