Although it was thought for a long time that migratory flights were made at normal cruising speeds, Harrison (1931) and Meinertzhagen (1955) showed that migration speeds were in between cruising speeds and escape speeds. The theory that migrating birds attain high speeds received encouragement from the German ornithologist Gatke (1895) who, for many years, observed birds at the island of Heligoland. He postulated that the bluethroat, a species of thrush smaller than the American hermit thrush, could leave African winter quarters at dusk and reach Heligoland at dawn; this flight would mean a sustained speed of 200 miles per hour! He also thought the American golden plover flew from the coast of Labrador to Brazil in 15 hours at the tremendous speed of 250 miles per hour. Most ornithologists now consider these conclusions to be unwarranted.
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. A thermal draft may induce an almost imperceptible air movement at the Earth's surface, but a good glider with motionless wings may make 35 miles per hour on a current of air that is rising vertically at less than 2 miles per hour. If the bird coasts downhill at a slight angle in still air, it can attain a similar speed.
For sustained flight, it may be generally concluded that larger birds 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 the speedometer of an automobile, 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 200 miles per hour during a swoop from a greater height onto its prey.
The speed of migration is quite different from that attained in forced flights for short distances. A sustained flight of 10 hours per day 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 (without the aid of a tail wind). 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. It is probable that individual birds do make flights of the length indicated and that barn swallows seen in May on Beata Island, off the southern coast of the Dominican Republic, may have reached that point after a nonstop flight of 350 miles across the Caribbean Sea from the coast of Venezuela.
Radar has given us some of our best estimates of ground speeds for migrating flocks, especially at night. 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 (Drury 1960). Some birds appear to reduce flight speed in proportion to the degree of assistance or resistance. The literature is in some disagreement on the flight speed of birds and the influence of wind, but good radar observations coupled with accurate measurements of winds aloft will help give us a more accurate estimate of migrating speeds for different species under varying wind conditions.
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.
|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.
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.
|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.
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.