The Migration of Birds

by T. A. Coward

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.