CHAPTER IV
THE HEIGHT AND SPEED OF MIGRATION FLIGHT
In the last chapter reference was made no the great height at which birds may fly on migration. Certain species, even comparatively weak-winged ones, appear normally to fly high, Whilst others, Often birds with pointed Wings and great aerial powers, usually proceed at low elevations; but there is still much conjecture as to the actual altitude reached by any migrants.
Gätke was of opinion that we do not see much of real migration, which is certainly correct, but there is no reason for his statement that it normally takes place at anything like the altitudes he mentions, 30,000 feet or more, or that birds at the time of migration undergo physiological changes which enable them to fly at immense heights and speeds and to see clearly in the dark (29). Nor need much weight be placed on the speculation of Lucanus, who contends that the height of travel must be less than 1000 metres, for above that elevation aeronautical observations show that perspective lessens. There are actual observations which, though liable to a margin of error, are proof of migratory flight at very high altitudes.
Most of these observations were at first made by accident; birds were seen through astronomical telescopes passing acres»; the face of the moon or sun; but recently this method of observation and several ingenious plans of measuring height and speed have been made use of expressly to study migration. W. E. D. Scott, at Princetown in 1880, thought that by shape and size he could even recognise two species, Chrysomitris tristris[1] and Quiscalus purpureus, which passed across his field of vision at a height of at least half a mile above the earth (43). In 1888 Mr Frank M. Chapman published an account of similar observations; he calculated that the birds passed at distances varying from one to five miles, and that their altitudes must therefore have been between 600 to 1000 feet: and 3000 to 15,000 feet. He adds an important note: "A number of birds were seen flying upwards, crossing the moon, therefore, diagonally, these evidently being birds which had arisen in our immediate neighbourhood, and were seeking the proper elevation at which to continue their flight," but the direction of flight of most of the birds which were observed was parallel to the earth's surface and southerly. The average height was certainly far above the inferior limit (13).
Mr F. W. Carpenter, reviewing these astronomical calculations, that Verey compared the apparent size of birds with lunar features, and considering that the small birds noticed would average 6 inches in length, calculated a height of 2000 feet above sea-level. At Detroit, Winkenwerde obtained estimates of a little over half a mile, and R. A. Bray, in England, saw birds crossing the sun in September 1894 which were invisible to the naked eye and must have been 2 or 3 miles away (11).
In December 1896 Mr H, H. Clayton, at Blue Hill Observatory, saw ducks flying at a height of 958 feet. He was at the time engaged in measuring the height and velocity of clouds, and was able to estimate the speed of the birds at nearly 48 miles per hour. In March geese passed at over 900 feet at 44.3 miles per hour. In 1905 Prof. Stebbins and Mr Carpenter worked out a scheme for ascertaining heights by simultaneous observation from different points. They based their calculations only on birds which were Observed by them both, and found that these passed at various altitudes, ranging from 1200 to 2300 feet; in the following October the lowest altitude observed was 1400 feet, and the highest 5400. Allowing the 25 per cent possible error, these results are of great value.
Mr Chapman's remarks about the upward flight of some of the birds are enlightening, for when birds start on oversea journeys they frequently ascend to a great height. Dr Allen and others think that the ascent is to increase the visible distance, but it may
D also he to reach a zone or stratum of atmosphere in which flight may be more easily accomplished. Robert Service's account of the departure of migrants from the Solway shores, gives suggestion of high flight. They arrive often one by one and "seem to drop literally from the clouds," but when they actually departed it was easy to see their method. They "fly upwards and onwards, then they hesitate, fly sideways once or twice, again attempt an upward and onward flight, hesitate again, and down they come once more to earth." After repeating this manoeuvre several times, "away they go over the sea." One morning he counted sixty blackbirds in one hedge, and others kept arriving, but, however closely he watched, he failed to see whence they came. "They came down from the upper air, becoming suddenly visible, sometimes three at a time." "I saw about a dozen birds thus drop into view, but I quite failed to see any indication of the point of the compass from whence they had come " (46)
Gätke frequently mentions birds raining down from the sky, appearing first as mere specks, and dropping vertically to the island, and others when departing "with breasts directed upwards and rapid powerful strokes of the wings; fly almost perpendicularly upwards."
On May 24th; 1911, I watched the departure of a spoonbill from Easton Broad on the Suffolk coast. The bird rose and soared in ever-widening circles until it was a mere speck, even when seen through powerful prismatic glasses; the great stretch of its wings alone enabled me to watch it for so long. When at a great height—I will not guess what elevation—it ceased its circling flight and made straight for the north.
In October I saw several flocks of redwings leave the Spurn. They rose to a great height before directing their flight southwards, although the Lincolnshire coast was plainly visible.
Great differences of opinion have been expressed about the speed of migrating birds, and here again Gätke's estimates, on account of the weakness of his arguments and his immense presumption, cannot be seriously considered. There are but few measured speeds, and most of these, except perhaps the ducks and geese referred to already, are of birds travelling at low elevations
Many birds, especially day-migrating swallows, hooded crows and other birds, frequently travel at slight elevations; it is not unusual to see birds at sea flying a few feet only above the waves Mr W. Eagle Clarke, whose systematic observations demand the profoundest respect, again and again urges that the direction of the wind has little effect upon migration, but that the force of the wind may make migration impossible. At the Eddystone, where he spent a month in the autumn of 1901, he noticed birds passing at heights varying from 20 to 200 feet, all flying southwards, He concluded that "the Wind is certainly the main factor in migration meteorology I am convinced that the direction of the wind is, in itself, of no moment to the emigrants, for they flitted across the Channel southwards with winds from all quarters" (16). When the velocity of the wind, however, was above 28 miles per hour (a fresh breeze, force 5 on the Beaufort scale), no migration was observed.
Allowing this, and also taking into consideration the undoubted fact that birds are frequently held up by strong winds on the shore before starting oversea journeys, is there any proof that they do not actually avail themselves of fairly steady strong winds when they reach the upper air?
Mr F. J. Stubbs makes some useful suggestions (50). He points out that Gätke and others imagine that a bird flying with the wind would suffer inconvenience through the wind ruffling up its feathers. It is surely evident that a bird supported in a moving medium could not progress at any slower rate than that medium; the bird is not flying on one stratum of air with another moving with a different speed immediately above it; it is actually in a current, not on it. If the bird flies at 20 miles an hour, and the wind or moving air is progressing at 10 miles an hour, the bird will cover a distance of 30 miles in one hour, though the force exerted by the bird is the force necessary for 20 miles in a calm. Conversely, if a 10-mile air current meets it, it will unconsciously he carried only 10 miles. If the speed of the bird is the same as the opposing force of the wind, it will remain stationary; I have seen ducks in a blizzard rise head to wind and fly rapidly, making no progress but maintaining their position over the water, to which they dropped again when the storm passed. Some black-headed gulls on the same water did not attempt this manoeuvre, and in a few seconds had vanished down wind. The swimmer, in a swiftly-flowing river, may hold himself in position so long as he can swim at the same rate as the stream ho is contending with, but he cannot make headway if the speed of the water exceeds his. He may, however swift1y the stream moves, swim in any direction, but his actual progress will be down stream; if he aims to swim directly across, his real course will be diagonal.
The fact that birds fly in any direction in a wind, and when at low elevation pay little heed to the direction of the wind, when the breeze is light, simply means that they can fly faster than the medium they are in; if the medium travels faster than they do, they will be carried in it to their advantage or disadvantage.
Even in these days of upper-air investigation we really know very little about the. speed, direction and steadiness of upper-air currents; but we do know that at a moderate elevation—some two or three thousand feet—the strength is usually greater than nearer the earth.
Mr Abel Chapman, in "Wild Norway," makes this pertinent remark—"Except by aid derived from the operation of physical laws, the nature and extent of which are unknown to me, and by taking advantage of 'Trade-wind' circulation in the upper air, I believe that migration is impossible for short-winged forms of sedentary habits—but that aid, and those advantages, may facilitate, and perhaps vastly accelerate, a process which is otherwise impossible."
In "Bird Life of the Borders" he goes further. "Birds are warmer-blooded than ourselves or other mammalia, and are capable of sustaining life in rarified atmospheres where these could not. By a simple mechanical ascent, they can reach, within a league or two, regions and conditions quite beyond human knowledge: where, selecting favouring air-strata, they may be able to rest without exertion; or find meteorological or atmospheric forces that mitigate or abolish the labours of ordinary flight, or possibly assist their progress. . . . It is in the upper regions of open space where, I suggest, the final clue will be found" (12).
A warbler flying leisurely, say at 10 miles per hour, in a current of air which was travelling at 20 or more miles an hour, could accomplish the journey across the North Sea—say 300 miles, in ten hours. Allowing much higher rates of speed for strong-winged species, and greater force of wind, some of the marvellous distances covered by migrating birds cease to be mysterious Prof. J. Stebbins and Mr E. A. Fath made careful calculations from observations with the telescope, and found that birds passed at rates varying from 80 to 130 miles per hour, and these were the minimums, for if the birds were not flying absolutely at right angles with the line of observation, they must have travelled a greater distance in the time occupied between their passage of the observation points (47).
- ↑ American Goldfinch, now Spinus tristis (Wikisource contributor note)