ROUTES
The migrating bird, when passing between the breeding home and the
winter quarters, travels by what is termed its Route. The definition of
the route has caused more controversy than perhaps any other incident
of migration; the chief point at issue is whether the bird uses a
particular high road, along which all its fellows from the same area
travel, or if all birds move in what has been called a "Broad Front."
Ornithologists have been, and to some extent still are, divided into two
camps, one upholding defined routes and the other the extended or broad
front movement.
After all the difference is merely one of degree. Even the widest
notion of the broad front, that of Gätke, who insisted, as dogmatically
as he did on most points, that the width or breadth of the migrating
host corresponded with the extent of the breeding range (29), is of
a route, bounded on the one hand by the northern or eastern and on the
other by the southern or western lines of latitude or longitude which
marked the limits of the range. The idea of a route may be narrowed down
to the extent of a wide river valley, or to a fly-line represented on
a map by a ruled line, which passes over certain ascertained places.
The absurdity of Gätke's arguments are proved by the study of his truly
remarkable book. According to him the island of Heligoland was only
remarkable in that it possessed an observer, himself, who saw marvels
unobserved elsewhere, though the same number of birds were every year
passing over any particular spot in an area which, for many species,
must have been many degrees in extent.
Had not so much weight been placed upon, and so many arguments based
on Gätke's extraordinary statements by, unfortunately, many of our
leading British ornithologists, his theories might have been ignored.
Unfortunately he is looked upon as an authority, even an oracle,
whereas, as Dr Allen pointed out, on many points which he treats with
great positiveness his knowledge is obviously as limited as the little
field which was the scene of his life-long labours (2). Glibly he
tells of hooded crows "in never-ending swarms of hundreds of thousands"
passing across and for many miles on either side of the island; of
"every square foot of the island" teeming with goldcrests, and of "dark
autumn nights" when "the sky is often completely obscured" by the
migrants, which pass thousands of feet overhead. How did he observe the
obscured sky? Indeed he again and again declares that migration passes
unseen yet calculates the numbers observed on the darkest nights; the
illumination of the lighthouse could not be sufficient to enable him to
even guess at the numbers he mentions. After stating that "the whole
vault of heaven was literally filled to a height of several thousand
feet with these visitors from the regions of the far North," and that
a certain east to west passage extended from the Faroes to Hanover, he
concludes that "the view--that migrants follow the direction of ocean
coasts, the drainage areas of rivers, or depressions of valleys as fixed
routes of migration can hardly be maintained."
As emphatically he maintains that most observable migration over
Heligoland is due east to west or west to east, though the birdstuffer
Aeuckens, who supplied him with much of his information, told Seebohm
that it was north-east in spring and south-west in autumn (45). Is
it not perfectly evident that the geographical position of Heligoland
makes it a convenient resting place for large numbers of migrants, for
it is certainly true that large numbers are observed there, which pass
southward and westward along the Baltic, crossing Schleswig-Holstein and
the mouth of the Elbe, or coast south along Denmark, and cross the Elbe
diagonally, _en route_ for the Dutch and French coasts and to a lesser
extent the south-east coasts of Britain?
Coasting undoubtedly exists; birds, day migrants especially, may be
observed following coast-lines in steady flight, though a mile or less
inland no passage is visible. On the North Norfolk coast I have seen
little parties of swallows passing along the shore in spring, coasting
slowly but steadily from east to west. All day long and almost every day
for more than a week this steady flight was continued, though I never
saw any passing within sight more than a few yards out at sea, nor any
at all more than a few hundred yards inland. Evidence which cannot be
refuted shows that this habit of coasting is general, though a deeply
indented bay, an estuary or strait, is usually crossed, and by no means
always at the narrowest point. The same careful observations prove that
both narrow and wide river valleys are followed by migrating birds in
greater numbers than are ever observed passing beyond the limits of
these valleys.
Seebohm's experience in Siberia led him to doubt the existence of
routes, but his later studies of migration in autumn at Arcachon and in
spring at Biarritz, caused him to modify his ideas. He found a gentle
but continuous stream of migrants following the coast of the Bay of
Biscay, arriving from over the Pyrenees on their northward journey, but
moving "only within a mile or two of the coast." He contrasts island
and coastwise migration; in the latter the travellers can rest at night
or take short journeys during bad weather, but in the former they must
await favourable conditions before attempting a perilous passage (45).
On the other hand many birds undoubtedly pass over inland localities
independently of any river valley or mountain range which might
indicate a route. Even such typical coast-lovers as the maritime waders
constantly cross or pass through inland England. They are heard at
night, or met with resting or feeding on inland waters, or their bodies
are found when, on a dark night, they have collided with telegraph or
telephone wires.
So long ago as 1886 Mr W. Brewster maintained that the breadth of the
fly-line varied according to the character of the country which was
being crossed. The migrating column, he said, might be hundreds of
miles in length, "a continuous but straggling army," which only became
a "solid stream" when travelling through some narrow pass (8). This
solid stream or army passage is, however, seldom observed when the
birds are crossing continents, especially if they are traversing a wide
area in which food is equally plentiful for miles on either side of the
direction of flight. The consolidation of their numbers appears only to
take place when, either on account of the indifferent food-supply or of
unsuitable weather conditions, the speed is accelerated.
In America Mr Cooke proves that the Mississippi Valley is undoubtedly
utilised as a fly-line by a large number of species, but by no means
all, and his evidence, though proving the use of routes, is that
these are seldom constricted pathways but broad areas crossed in a
generally coincident direction by the birds which make use of them.
This main fly-line is however formed in America as in other places by
the convergence of subsidiary streams, and it is these tributaries,
as Herr Herman points out, which have in many instances led to error;
they have been mistaken for main routes. The main route may be compared
to the trunk of a tree, the birds following the roots from the area
in which they have been nesting or wintering, and at the end of the
journey splitting off in various directions, like the branches, to their
temporary winter or summer homes.
The contrast in the method of travelling of different species or of
the same species under different conditions, may be realised by taking
two examples. Firstly, Mr Eagle Clarke's experiences at the Eddystone
and Kentish Knock Lightship, when birds passed during the daytime at
varying elevations, sometimes close to the waves, in twos or threes or
scores, and at night in large numbers. The other is an observation of a
"bird wave" by Mr P. Cox, during a snow storm in 1885 at Newcastle, New
Brunswick. The birds passed eastward in a column about twenty-five yards
wide, some just above the trees, others hardly visible, but the bulk in
a massed column directly over the margin of the shore, and not over the
river or meadow on either side. The movement was continuous for about
two hours.
Dr I. A. Palmén was the great upholder of routes in the Old World, but
his routes were largely speculative; they were founded on a considerable
knowledge of migratory birds, but not sufficient to cover the vast
area mapped out (39). Until a very large band of workers, working
on similar lines all the world over, accumulate a sufficient mass of
evidence as to which birds do or do not pass their various stations,
with the times at which they appear, accurate knowledge of the routes of
birds is impossible.
Von Middendorf collected statistics of the passage of birds in the
Russian Empire, and by reckoning the average date of arrival of a few
species at certain points of observation, worked out a number of curves
or lines which he calls "isepipteses," or lines of simultaneous arrival
(35). The result was, according to his argument, a general convergence
northwards; the birds passing through Central Siberia travelled roughly
in spring from south to north, in Eastern Siberia from south-east to
north-west, and in Europe from south-west to north-east; they converged,
in fact, upon the Taimyr Peninsula. This to some extent is doubtless
true, but Middendorf goes on to prove that the magnetic pole is situated
in this Peninsula and that the birds are drawn thither by magnetic
influence, "in spite of wind, weather, night or cloud." He calls them
"sailors of the air," possessed of an internal magnetic influence.
He supports his argument by the statement that there is a similar
convergence in North America towards the magnetic pole of the western
hemisphere.
But all birds do not go in the direction of the magnetic poles, and many
of those which do, stop short at suitable breeding places long before
they have travelled so far north. The Taimyr Peninsula, a vast area in
the extreme north of Siberia, is each spring a "zoological vacuum";
towards this desirable spot migrants will stream.
Herr Otto Herman cleverly shows the absurdity of many of the reputed
routes by cartography; his map is crossed in all directions by the
routes upheld by various theorists. Birds could not possibly follow all
the directions "which authors invented for them," most of which he adds
are founded on mere supposition (31). Dr Palmén, he shows, usually
managed to avoid districts where there were no observers, but Mr Dixon
and M. Quinet made their routes follow rivers and coast lines, whether
there was evidence to support this idea or not.
Only to a certain extent can it be safely contended that the present
route of a species is an indication of its earlier journeys, or that
the direction of original dispersal is recapitulated in the present
line of migration. Heredity, experience, and imitation would certainly
tend to preserve and confirm the general direction; the shortest and
easiest passage from food-base to food-base would become an hereditary
route, unless circumstances arose which caused a change. Mr Cooke
shows how there has probably been evolution of the route as well as
of everything else concerned with a mutable animal. The fly-line
across an arm of the sea may be lengthened if this lengthening means a
corresponding advantage in reaching the desired haven. Thus the birds
which now cross the Gulf of Mexico at its widest part, at one time
probably coasted round the Gulf, as many do still, by the land-bridge
of Mexico and Central America. The gradual straightening of this curve
would shorten the journey both in time and distance, though lengthening
the actual single flight across a portion of the sea. We can imagine
a bird arriving in autumn at the mouth of the Mississippi, at first
passing from Louisiana to Mexico, so as to save the time of travel
through Texas. Generations later the shortening of the journey, through
lengthening of the short cut, would lead the birds to Vera Cruz and
later still to Yucatan. It may be questioned, what object could the
birds have in risking an oversea voyage, away from chance of food
and hope of rest, when the land-bridge remained open for them? Each
individual or group of individuals which arrived at any particular place
a little in advance of the migrating multitudes of its own species,
or others which fed upon the same kind of food, would certainly gain
advantage, and would be the most likely to develop strong flight and the
power of endurance in its descendants; it would indeed be a winner in
life's race.
Great weight has been placed upon the use of land-bridges and the
hereditary habit of crossing seas where these land-bridges once existed
but have been submerged during the great geological changes in the
earth's surface. Many have insisted that wherever migrants cross the sea
they do so along submerged coast-lines or over submerged land-bridges,
arguing that the gradual evolution which has made the advantageous
adoption of a habit of migration possible was unable to eliminate
the hereditary tendency to follow the exact route by which their
ancestors passed from place to place. That there have been considerable
alterations in coast-lines and in the general distribution of land
and water since the time when birds began to be migratory is indeed
probable, but unless crossing the sea means a distinct advantage it
implies the retention of a habit which would not only be useless but
might be a positive danger to the species.
In the Gulf of Mexico and the Caribbean Sea there is evidence of perhaps
the most recent land-bridge in the chain of islands from Florida to
Venezuela, collectively known as the West Indies. Although vast numbers
of North American birds winter in South America only a few of the
species which annually pass from one continent to the other make use of
this comparatively easy passage. One might naturally conclude that the
final severance of England from the Continent was in the neighbourhood
of the Straits of Dover, yet this short passage is only used by a
comparatively small number of our migrants.
Mr Dixon indeed argued that there is no greater barrier to migration
than even a narrow arm of the sea (26). He refers to many Continental
species which are common breeders in France but are unknown as nesting
species in the British Islands, and others which are found in England
but not in Ireland. But this is surely but an incident of distribution;
the narrow strait or even river may for a time mark the limit of
expansion of a species, just as at the present time the westward and
northward unseen barrier prevents the range of the nightingale from
spreading to districts apparently well suited for its home, and until
recently the turtle dove and great crested grebe were checked in their
northward advance.
In the evolution of some routes land-bridges certainty appear to have
played their part, but once those bridges have ceased to influence
direction the shortening of the time occupied by the lengthening of
the single oversea flight is only a question of generations when an
advantage to a species is to be gained.
This subject will be further dealt with in connection with the actual
passages performed by certain birds.
The study of migration, based on observations at our lighthouses and
lightships, shows that even in the comparatively small area of the
British Islands there are certain routes followed with regularity. The
birds which pass along our western coasts of England and Wales do not as
a rule follow the shores round Cardigan Bay or along the eastward tidal
scoop of the Irish Sea towards the coasts of Lancashire; the main body
passes from Pembroke to the Lleyn Peninsula, and thence to Anglesey and
the Isle of Man, on its way to the southern Scottish shores.
A source of possible error in the method of deduction from these results
must be taken into consideration. The observations at lightships and
lighthouses are mostly made when untoward circumstances bring the birds
within range of vision, and on dark and foggy nights cause them to
strike the light in great numbers. What is their normal course when
no great migration wave or "rush" is observed? Are the few passing
stragglers noted all that go by this route in fair weather? The same
uncertainty must be applied to the observation of passing birds in
inland localities. The immense numbers which do pass is shown by the
observation of large movements, when as occasionally happens some
check to normal migration leads an army of birds to a dangerously low
altitude, or when high winds hold up a portion of the host on our
coasts; but even these multitudes must be small compared with the
millions of birds which annually pass from zone to zone unseen. The
few or many birds we meet with, either on the coast or inland, resting
on passage, may represent a lost or wandering party of stragglers or
weaklings from a vast army which has passed over; they may or may
not be on the route or course normally followed by the majority. The
cartography of bird migration is a study in itself.
Mr Abel Chapman, describing his experiences in the Mediterranean,
says--"For forty hours we were passing across (or beneath) the lines
of an army of migrants--say 500 miles in width; yet not a sign did we
see, save only the wreckage--the feeble that fell out by the way." On
April 10th a sudden bitter northerly gale sprang up, and two hours later
the steamer was the goal of hundreds of birds, no longer able to face
the adverse wind. These were blue-headed wagtails, swallows, martins,
pipits, wheatears, nightjars, and lesser kestrels. He thinks that the
strong ones may have passed on but all the others perished (12).
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