THE DISTANCES TRAVELLED BY BIRDS
Not only do the distances of the migration paths of different species
vary considerably, from a trip of a few miles to a voyage from the
Arctic to the Antarctic, but the individuals of one and the same species
do not all travel to the same degree.
The familiar swallow, _Hirundo rustica_, though subject to certain
geographical variations, is found throughout the Palæarctic and Nearctic
regions, nesting throughout Europe to between 63° and 70° north and in
Africa north of the Sahara, where, however, Canon Tristram found it
also wintering in the oases. South of the Sahara to the Cape it is a
winter visitor. In Asia it breeds, according to Seebohm, in Asia Minor,
Persia, Afghanistan, and western Siberia, and winters in Scinde and
western India. One form breeds in and north of the Himalayas, eastward
to China and Japan, and winters in India and Burma, and another ranges
from eastern Siberia across Behring Strait throughout North America,
so far south as Mexico. This form winters in Burma, in Central America
and Brazil, but the Mexican birds are more or less stationary at all
seasons.
Our swallow and its congeners have an almost cosmopolitan range,
summering in the Northern and wintering in the Southern Hemisphere or
comparatively near to the Equator in the Northern. Towards the centre of
its range its migrations are either short or the bird is non-migratory.
Mr W. L. Sclater, addressing the South African Ornithologists' Union
(42), stated that the swallow arrives at Cape Town at the end of
October, and is common from November to March; practically all have
left by the middle of April. Swallows begin to arrive from the south
in Africa north of the Sahara in the latter half of February; early
in March they reach southern Europe, later in the same month they are
in Central Europe and by the middle of April large numbers arrive in
England. Thus swallows leave South Africa actually after they have
arrived in England; the South African birds cannot be the same which are
in North Africa a month earlier! The swallow supports Seebohm's thesis
that the individuals which go farthest to the south in winter, breed
farthest north. A day-migrant and by no means a rapid one, the swallow
may be timed from place to place, and it is not presumption to suggest
that the birds which reach Britain to nest came from lands little south
of the Sahara and well north of the Equator, and that those which pass
through England and along our shores in May and even June are on their
way from Southern Africa to the northernmost limits of their range.
Mr Sclater points out one very interesting fact; when the swallow
reaches South Africa it is in ragged worn plumage, before it begins its
northward journey it passes through its one annual moult.
Waders and shore birds which reach South Africa in autumn--the spring of
the Cape--are moulting into winter dress; before they leave they have
often assumed or partially assumed the breeding dress. When they arrive
the native South African birds are breeding, but though Mr Sclater
thinks that some nest a second time in the south, no satisfactory
evidence has ever been brought forward to support the suggestion.
These long-distance travellers not only move from a zone of moderate
temperature to a warmer one, but many of them pass through the hotter
zone to a country having a similar temperature to the one in which they
bred, thus enjoying summer but not torrid heat all the year round.
There are birds in which the northern and southern forms are distinct.
The wheatear, _Saxicola oenanthe oenanthe_, reaches us early, sometimes
during the second week in March, and speedily settles down to nest.
Towards the middle or end of April a brighter larger bird appears, the
Greenland wheatear, _Saxicola oenanthe leucorrhoa_, which was recognised
in Greenland, Iceland and eastern North America before it was seen
that both forms occurred in Britain. This larger bird loiters through
Britain, for its northern home is not ready for it until the Arctic
spring. We know it breeds farther north than our wheatear, but its
winter range is not fully worked out. The smaller bird is found in north
and north-western Africa, and the larger form farther east, even south
of the equator on the eastern seaboard, and probably, when we know more
about the range of the two we shall find that the form breeding farther
north, winters farther south.
The folly of laying down the law on the strength of the knowledge of
the habits of a few species is shown by the study of the movements
of American birds. Mr Cooke shows that as a rule "the migration is a
synchronous southward movement of the whole species" in autumn, "the
different groups of individuals or colonies retaining in general their
relative position." The black and white creeper _Mnistitta varia_ breeds
from South Carolina to New Brunswick, nesting in the south in April and
reaching the northern limits in the middle of May. In the middle of July
old and young birds have been seen at Key West, 500 miles south of the
breeding range, and towards the end of August they have reached the
north coast of South America. The New Brunswick birds cannot be ready
to leave before the middle of July, and Mr Cooke allows them fifty days
for the trip, bringing them to the Gulf States in September; he argues
that this is proof that the earlier migrants must have been birds from
the southern part of the range. Black-throated blue warblers, _Dendroica
coerulescens_, reach Cuba at about the time that others of the same
species are arriving in North Carolina; the first, he concludes, are
birds from the southern Alleghanies and the others from northern New
England or beyond (20). Other species illustrate the same order which
he calls "normal," but show that it is not an invariable rule.
Southern-bred Maryland yellow-throats, _Geothlypis trichas_, reside
throughout the year in Florida; those in the middle districts of the
range migrate for a short distance only, whilst the Newfoundland
birds pass over the winter home of their southern relatives to the
West Indies. The palm-warblers of the interior of Canada travel 3000
miles to Cuba, passing through the Gulf States early in October; those
from north-eastern Canada travel later and slowly and settle in the
Gulf States, after a journey of only half the distance. He sums up
wisely--"No invariable rule, law, or custom exists in regard to the
direction or distance of migration.... Each species presents a separate
problem, to be solved for the most part only by patient, pains-taking
observation and by the recognition of sub-species."
The order in spring is yet unproved. "With many birds ... the first
individuals to appear in spring at a given locality are supposed to
be old birds that nested there the previous year." These are followed
by those which nested a little farther north, followed later by those
whose homes are in the most northerly part of the range. "If, then,
for any species, the southern nesting birds lead the van in both fall
and spring migration, and the near guard in each case is composed of
northern breeding birds, it follows that some time between October
and April a transposal of their relative positions occurs; and that
the more southern birds pass over the more northern ones, which delay
their migration, knowing that winter still holds sway in their summer
dominions." It is not known where this transposal takes place, nor
whether the northern birds remain in winter quarters till the southern
birds have passed, or start a slow migration, during which the southern
birds pass over them. Later another transposal occurs; the northern
birds cross the southern part of the range, passing birds which are
already nesting. "Spring migration seems to be therefore for some
species a game of leapfrog--the southern birds first passing the
northern, and the northern passing them in turn" (20).
The custom, now fortunately becoming widespread, of marking birds
by affixing a numbered metal ring to one leg, may help to elucidate
this and many other problems, but until a large number of results are
collected it is unwise to draw conclusions. Almost every month the
recovery of some of these marked birds is noted in the scientific
journals, but so far, beyond indicating the minimum distance travelled
by individuals, little can be proved. It is, however, plain that birds
do not invariably act as they ought to do if they obeyed all the laws
which have been invented for them. A few records or results may be
quoted, but any suggestions from these must be treated as suggestions
only; many more must be forth-coming before we can say, proved.
The white stork, _Ciconia alba_, has been systematically ringed in
Rossitten in East Prussia, in Denmark and in Hungary for some years, and
Mr A. L. Thomson gives a brief summary of the interesting results up to
date, in "British Birds" for May, 1911. Ten young birds, taken during
their first autumn journey, show a general south-easterly trend through
Europe. Three east Prussian storks were obtained in Syria, one in the
April after it was marked, the other two in April and July of the second
year; another was taken in Palestine, and one Hungarian stork in Syria.
In the May of the year following that in which it was marked in Prussia
one was obtained at Alexandria. In their first autumn Prussian storks
have been recorded from near Lake Chad in October, from Rosaires on the
Blue Nile in the same month, and from the Victoria Nyanza at the end of
November. A ringed bird is reported from German East Africa, but full
details are wanting, but one shot at Fort Jameson in north-east Rhodesia
in December 1907 had only been hatched in Pomerania a few months before;
it left the nest on August 19th and began its journey south on or about
the 26th. In its first winter a Prussian bird was shot in the Kalahari
Desert.
Seven Prussian and about a dozen Hungarian birds have been obtained
in winter quarters in the Transvaal, Natal, and other parts of south
Africa, and one in German south-west Africa; one, recovered in the July
following the summer in which it was marked, was possibly a weakling
bird which had failed to make the return journey. Storks which had
returned are recorded in their first, second and third summer; most of
them having been found within a few miles of their birthplace. One bird,
marked as a nestling near Brunswick in 1906 was reported in June 1908
from Sorquitten in East Prussia, over 430 miles away. Mr Thomson, from
his reference to this being roughly in the same latitude but reached
by a different "line of flight," suggests that it is an exception;
this will be shown when some hundreds more cases have been collected.
It may, however, be found that some birds deliberately launch forth in
search of new homes, though at present it looks more like a bird which
on approaching the breeding area had banded itself with the wrong local
body of travellers.
A bird marked at Cassel in western Germany has been recorded from
Barcelona in Spain; this so far single record may indicate that storks
get lost, that there is no regular direction, or that there is more than
one, a south-westerly as well as a south-easterly route. That too, we
hope, will be shown in the future.
That much will be learnt from storks is evident, but the lessons will
be only general; each migratory species must be treated separately, and
to some extent this is being done. In the opening chapter I mentioned
the uncertainty about the behaviour of any individual song thrush,
merely as an example. So far, the few records of marked song thrushes
add to rather than solve this problem. Years ago the song thrush was
looked upon as a permanent resident so far as Britain was concerned.
Then it was found to be migratory even in Britain, and it was suggested
that each song thrush performed a short migration, southern British
birds leaving the country, northern British retiring to winter in the
south of England, and northern European birds replacing these as autumn
immigrants. The study of local races quickly altered this opinion; it
was guessed that in Britain there was a sedentary insular race and a
migratory passage race; others, however, saw that some of our home-bred
birds left in autumn. What do we find? A song thrush, marked as a
nestling in July in Northumberland, is found in November in Durham;
another one, marked in Berkshire travels to Norwich and is recorded in
November, but a third, born in Aberdeen takes an autumnal flight of at
least 1500 miles and is found in Portugal. Evidently we cannot yet frame
any rule for our British-bred birds.
It is said that home-bred lapwings are somewhat sedentary, and that the
large winter flocks are composed of Continental immigrants. The frequent
westward migration of lapwings during exceptionally severe winter
weather has led to the supposition that these birds fly for refuge,
under these circumstances, to Ireland. This is true, so far as it goes,
but a lapwing marked as a nestling near Stirling has been found in the
south of France, and two others in Portugal, whilst five have been
recovered in Ireland.
The results of marking sea-birds are interesting, showing that the
young birds often wander northward in search of food before there is
any marked autumnal southward migration. Terns and black-headed gulls
have been found a month or more after they have left the nest to the
north of their breeding colonies in Cumberland and mid-Wales. A bird
from Ravenglass was taken in its first January in Brittany. Rossitten
black-heads have been shot in the Isle of Wight and in Breydon in
Norfolk.
This may only mean that the young blackhead is a confirmed wanderer
in search of food, but the few results with woodcocks, marked as
British-bred nestlings, are puzzling. They have been known to linger in
the neighbourhood of their home until November, and have been found in
Portugal only a month later. Birds marked at Tyrone have been found so
far apart as Cornwall, Harrow and Inverness; what route for the Irish
birds can be guessed at?
Birds marked as adults present further problems, but also provide
interesting evidence. Hooded crows, captured on migration in spring at
Rossitten and then released, have been recovered in autumn actually
in the same place and in other localities in Germany, and one marked
in October was taken two years later, in spring, in Finland. The sum
of these records of crows proves one thing conclusively--the fallacy
of Gätke's due east to west and west to east flight, and supports a
coastwise migration for this species.
Adult teal, captured in decoys, ringed and released in South Denmark
in September and October, were taken in November and December in
Hampshire, Suffolk and the Moray Firth, whilst others from the same
place were recorded from other parts of England and Ireland, from
western France, Holland, the south of Spain and the north of Italy.
Fly-lines, if followed, are divergent and complicated. Four young herons
were marked in one nest in Denmark; one was recorded in Holstein in
June, and another in Mecklenburg in July; the third was killed near
Salisbury in Wiltshire in October, and in the following February the
last was obtained in the north-west of France. Two from another nest
were recovered in Denmark, one in July and the other in February, twelve
months after birth. Another heron reached Andalusia by August. In each
case where there was indication of a direction it was south-westerly.
Many more records might be mentioned, but these are sufficient to show
the value of the method and the present insufficiency of results.
Many of these records show that the speed of the migrating birds, even
in spring, is not great. Mr Cooke proves that most species in North
America travel slowly through the districts where food is plentiful
and during the earlier part of the journey northwards only a few miles
are covered per day; they travel with the slowly advancing vernal
wave, but, as we shall show in the next chapter, many species actually
outstrip it, and travel from warmer to colder climates.
[Illustration: Map showing the range of the American Golden Plover,
with its known migration route.
(From _The National Geographic Magazine._)]
By the kind permission of Mr Cooke I am able to reproduce three of his
maps, illustrating the longest known distance travelled by any bird in a
single flight, and the probable evolution of this extraordinary oversea
voyage (21). This long journey, roughly 2500 miles at a flight, is
used in autumn by several species of American shore birds, and the
particular species most easily recognised, is the American golden
plover, _Charadrius dominicus_, which differs but little from our _C.
pluvialis_. An important point to notice is that the route followed in
the fall is not the one used by the bird in spring, an undoubted proof
that all routes are not identical with the original line of dispersal
of the species. Nor is the route directly from the north to the south,
though there is plenty of evidence to show the fallacy of the notion
that all birds move in this one direction.
The golden plover nests along the Arctic coasts of North America from
Alaska to Hudson Bay. So soon as the young are able to take care of
themselves the birds migrate south-east to Labrador, where for some
weeks they fatten on the autumn harvest of fruits. A short journey
across the Gulf of St Lawrence brings them to Nova Scotia, where they
gather before starting on their oversea flight. The eastward trip to
the food-supplying districts is support of the idea that a route is
originated by passage from food-base to food-base, rather than by any
hasty rush from the dangers of approaching winter. The birds start south
from Nova Scotia for South America!
During this long oversea journey, which Mr G. H. Mackay thinks, with
reason, may be undertaken under favourable conditions at a speed of from
150 to 200 miles an hour by birds with such magnificent power of flight,
the plovers may meet with many different winds. The Cape Cod sportsmen
look for them if the wind is strong from the north-east; the Barbados
gunners expect them when there is squally weather from the south-east,
but when westerly breezes are blowing they will pass so far as 400 miles
east of the Bermudas. Only when the wind is adverse and strong do the
plovers visit the Bermudas or even stop at any of the northern Lesser
Antilles, 600 miles from the coast of South America. In favourable
weather they neglect any of these "emergency stop-overs" and hasten on.
In the Guianas the birds rest and feed, but they soon move on. Across
the Brazils their actual route is uncertain, but they have been met
with in Amazonia, and are known to winter in Argentina, and, it is
suspected, in eastern Patagonia.
[Illustration: Map showing the evolution of the migration route
of the American Golden Plover.
(From _The National Geographic Magazine._)]
The return migration is, so far as it is known, in a steady northerly
direction, rather north-west across Bolivia towards Central America.
From Yucatan they cross the Gulf to Texas, then slowly travel up the
great Mississippi highway and across Canada to their northern breeding
grounds. "Its round trip has taken the form of an enormous ellipse with
a minor axis of 2000 miles and a major axis stretching 8000 miles from
Arctic America to Argentina."
The following is Mr Cooke's suggestion of the origin of this great
ellipse. Towards the close of the glacial era, when the ice began to
recede, the Florida peninsula was submerged and only a small area in the
south-east of the States was free from ice. Plover attempting to follow
up the retreating ice were confined to an all-land route from Central
America through Mexico to the western part of the Mississippi Valley.
As the east gradually became uncovered the route would be extended to
the north-east, until the area stretching to the Great Lakes was fit
for bird-habitation. As the route lengthened and the power of flight
developed, there would be a tendency to shorten the line by cutting off
some of the great curve (No. 1) through Mexico and Texas, and a short
flight across the Gulf (No. 2) would be gradually lengthened, until the
present spring route, then also the autumn route (No. 3), was attained.
As Canada opened out, the routes in spring and autumn diverged; in
autumn the fruits of Labrador were an attraction, but the Chinook winds
made the country east of the Rockies more suitable for spring migration;
the fall route tended eastward (No. 4), the spring route remained
unchanged. When the fall route had worked eastward to the Gulf of St
Lawrence (No. 5), shortening took place in the same way from the great
westward curve, culminating in an ocean flight, short at first (No. 6)
and later extended, the total distance shortened, until the present
route was attained (No. 7).
This reasoning, sound enough, helps to a more difficult problem--how
the Pacific golden plover, _Charadrius fulvus_, found its way to the
Hawaiian Islands, where numbers of the birds winter annually. Roughly
the islands are 2000 miles from California, 2400 from Alaska, whence
the birds fly, and 3700 miles from Japan. Mr Cooke scouts the idea that
any bird flies aimlessly out to sea to find a new winter home, and the
chance colonisation by a storm-swept party is as improbable; if this did
occur it is hardly likely that they would at once depart, in a single
season, from ancient habits and carve out an entirely new migration
route. Probably the origin of the route is as follows. The bird breeds
on the northern shores of eastern Siberia from the Liakof Islands to
Behring Strait, and on the Alaskan side south to the northern base of
the Alaska peninsula. It winters on the mainland of south-eastern Asia,
in eastern Australia, and throughout the Oceanic Islands from Formosa
and the Liu Kiu Islands on the north-west to the Low Archipelago in the
south-east.
[Illustration: Map showing the evolution of the migration route
of the Eastern or Pacific Golden Plover.
(From _The National Geographic Magazine._)]
It is fairly certain that the original route would be roughly north and
south, between Siberia and southern Asia. In time the species spread
eastward in winter, to Australia and to islands farther east, whilst the
breeding area extended to Alaska. If these extensions took place before
any cutting off of corners in the route, Alaska birds would travel
11,000 miles to reach the Low Archipelago, only 5000 miles in a direct
air-route (No. 1). Probably shortening began early among the Pacific
islands, from the northern islands to the Asiatic coast, and finally to
Japan (No. 2). From Palmyra the flight to the nearest of the Marshall
Islands is 2000 miles; thence a journey, provided with several possible
rests, of 3000 miles would bring them to Japan. A thousand-mile drift
through strong winds might cause the birds to reach Hawaii, whence they
would find a chain of islands which would help them, and render the
last flight to Japan no longer than the one they had been accustomed to.
Having once reached the Midway Islands the shortening of the route would
be carried on again by lengthening the oversea journey northwards until
the Aleutian Islands were discovered (No. 4). The present route, now
followed in spring and autumn (No. 5), would be the natural climax of
this long evolution. The two golden plovers, sub-specifically distinct,
nest little more than a hundred miles apart; their migrations and winter
homes are as different as they could be in any two widely divergent
species. It is one of the most striking of the ascertained facts in the
distribution and habits of birds.
* * * * *