THE MYSTERY OF MIGRATION.
THE "sense of locality" awakens more interest when shown by the bird than by the bee, because the former has a mind of an order more human, and more cognisable by our senses. Yet the mystery of migration remains a mystery still ; and this year's arrival of the tens of thousands of spring migrants from Asia, Africa, and the Mediterranean, has not found us much better informed as to the means which guided them on their way. When the movements of birds on migra- tion were first noted and set down, the results were unexpected and bewildering. Movement, not rest, seemed the general law of their being. The greater number of species were found to pass their lives in travel, though the motive and the routes chosen for their journeys were less accountable as the facts were multiplied. The simple and easy conclusion of the older naturalists as to migration, from the days of Virgil to those of Gilbert White, was that their own particular country was the natural home of all the birds which we call " spring migrants," such as the swallow and the nightingale ; and that when winter came, bringing cold and hunger, these deserted their home and sought a temporary shelter in warmer climates, whence they hurried back to their dulces nidos, like ships' crews impatient to revisit a frozen port which was nevertheless their home. This accounted for the departure of the birds in autumn and their return in spring. The visits of northern birds in winter were only a result of the same causes, working in more northern lands. It was a theory as simple as Homer's notion of the ocean stream circling in endless flow round the rim of the inhabited world. When the naturalists grew dissatisfied, and demanded more data, and began to sit in lighthouses through the winter nights, to note the migrants coming and going, and sift reports from those whose duty makes them watchers by the shore, the apparent simplicity of birds' travel could no longer be admitted. There seemed neither limit nor law in the incessant and perplexing streams of bird-movement. The problem of the real ocean currents compared with the ancient belief in the constant ocean stream, was simple in comparison with the crossing and recrossing, the apparent contradiction and want of purpose in the journeys of the birds.
The first task undertaken was to note and separate these lines of flight, and to determine whence and whither the birds were going. We now know whence the birds come to us for the summer, and the roads by which they come. Africa, sends perhaps the greatest number. Some of these start for England from Algiers and the Mediterranean coast. Others have win- tered in the Nile Valley, and descending to its mouth—for the Sahara desert is almost as dreaded by the birds as the Atlantic Ocean—skirt the shore towards the Straits of Gibraltar. Some fly across to the Balearic Islands, and thence through Catalonia, and down the Garonne Valley to the Biscayan Coast. Others pass by Sardinia and Corsica to the valley of the Rhone. But the main body cross at the Straits of Gibraltar, follow the East Coast of Spain, pass up the valley of the Ebro, cross the Pyrenees near Pampelnna, and then follow the Bay of Biscay, till they reach Cape Finisterre, and fly across the Channel to the Lizard or the Start. France
contains the three other main roads of the spring migrants. The most southerly stream passes up the Rhone Valley to Lyons, crosses to the Loire Valley at Orleans, and passes thence yid Cape La Hogue to the Start Point. The stream from Turkey and Greece passes up the Italian coast, through the Swiss passes, into Central France, down the Seine Valley, and across to Selsey Bill. The most northerly route is that of the birds that are coming from the Danube Valley. These pass through Zurich on to Rheims, and reach the Channel Coast near Boulogne and Calais, whence they cross the Channel at the Straits of Dover. In these journeys, the land- routes are preferred, and the straits are as a rule chosen for sea.
passage ; but some birds fly straight across the Mediterranean at its widest part, from Algeria to Marseilles; while birds making for Ireland, though mainly crossing via England and that part of the Irish Channel which lies opposite Milford Haven, often cross directly from Brest to Cape Clear.
It remains for naturalists to find a probable reason for this northern migration of birds, and to explain the means by which, given the desire to do so, they find their way across seas and lands, unknown to many of their number, to a goal which, in the case of young birds of the year retreating to their winter homes, must be unknown, and which, in the case of their first return to the land in which they were bred, can at best be but an imperfect memory. The reason for the northern migration of birds has been found in the desire of species to return to districts from which they were formerly driven by the advance of ice in the glacial age. Mr. C. Dixon, in his recently published book on the Migration of British Birds,* modifies this theory, and suggests one which covers less ground and makes fewer assumptions. He notes that all migration has for its object to increase the breeding area of a.
species, and that the tendency of all birds is to move in the direction of the Poles, which tendency he calls the "Law of Dispersal." Thus, in oar hemisphere, there is no known migration-route which goes south in summer or north in winter. Hence, our migratory birds winter in the countries in which their ancestors escaped extermination in the glacial epoch, and go north to extend their breeding area at a time when the individuals need more space and food than at any other time, in obedience to instinct used strictly in Paley's sense, of a "propensity prior to experience and independent of instruction," for Mr. Dixon can hardly mean that the "constant endeavour to regain and repeople the area once occupied during pre-glacial times" is a conscious effort on the part of birds, though his language almost suggests it.
This he calls the "impulse to migration," though why he should not call it the " instinct of migration" is not very clear. The " propensity " in all the young birds must be prior to actual experience, and independent of instruction in many cases, such as that of the young cuckoo, in which the parent- bird leaves the young to find their way back to Central Africa from Kew Gardens or Richmond Park.
Mr. Dixon seems in difficulties between the theory that the "impulse" to migrate is a transmitted instinct due to descent from ancestors who once lived in the northern regions, and a " corollary " from his Law of Dispersal, that birds never retreat before adverse conditions," but, "if overtaken, perish, so far as the species is exposed to them." If the members of a species, say, of the swallow tribe, which were exposed to the glacial period, died, and did not migrate, and only those were left which were secure in " refuge areas" (parts which the ice did not reach) survived, these latter could have had no recollec- tions of their old home to suggest migration to their descend.
ants. Mr. Dixon is not lucid either in argument or statement ; but this is the difficulty which his remarks on the " Law of Dis- persal " suggest. This " law," that species all tend to migrate
northwards in spring to regions from which they are supposed once to have been driven by the advancing ice, is not new either in idea or form. But whether " impulse " or " instinct " sends them northward, Mr. Dixon attempts no explanation of the exceptional power which enables them to achieve their journey. He even suggests that they need none. "Once a bird begins its migration," writes Mr. Dixon, "all instinct as a guiding medium ceases ; memory and knowledge of locality, in fact experience, assist it to per- form that long journey." Until this conclusion is supported by more evidence than is contained in some two or three pages of very incoherent evidence set down by Mr. Dixon, it • The Mviration of British Birds. By C. D.xon. London: Chapman and may safely be neglected. Whatever the "impulse" which sends them out, it cannot yet be shown that it is not instinct that shows them the way. It is not instruction; for no parent bird could leave with its young in England a way- bill for the Nile Valley, though the young birds may possibly find their way here in the wake of the old ones. It is not example, for though many species travel in flocks with old and young together, such as the swallows, others force their young to migrate separately, and Temminch states that the flocks of old and young always travel separately. Other birds force the young to migrate while they remain on their breeding-ground, or, as in the case of the cuckoo, leave England a month before the young. The "follow my leader " instinct, accounts for much of the marvel in the case of some species on certain routes of migration, where the flocks of species as different as chaffinches and cranes, pass on in such quick succession that the flocks are always in sight, while the lateral width of the travelling multitude feels the way, like a fleet extended over a great width of sea. But this does not account for the safe passage of isolated species to lonely spots, such as the crossing of the wheatears from Cape Finisterre to the South of Ireland. The assumption that migration is due to a mere automatic impulse guided by experience, does not take into account the conscious end proposed by many birds as the object of their journey. The sentimental motive which induces many birds to seek the old nesting-place, is one of the strongest factors in migration. Without some such eager desire, those birds which have once faced the dangers of one journey might refuse to persevere in a second, or to call to their aid those astonishing faculties which direct their path. To conclude that, because a domestic species of homing pigeon performs its flights better after experience, the Australian cuckoo, which traverses a thousand miles of open ocean to nest in New Zealand, or that the midnight flights of birds so tiny as the golden-crested wrens across the North Sea are guided by experience alone, is too hasty and imperfect a generalisation. The journey of the birds is guided by some faculty not yet understood, but indicated by its results. Our daily growing knowledge of what is done on these migrations only magnifies the faculty which accomplishes it.
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