THE HEIGHT AND SPEED OF MIGRATION FLIGHT
In the last chapter reference was made to 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 across 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_ 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, says
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 also be 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
be 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 he is contending
with, but he cannot make headway if the speed of the water exceeds his.
He may, however swiftly 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).
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