Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/787

Rh MIGRATION.] BIRDS 769 tory Wagtail will build her nest in the accustomed spot, and year after year the migratory Cuckow will deposit her eggs in that nest, and yet in each interval of time the former may have passed some months on the shores of the Mediterranean, and the latter, absent for a still longer period, may have wandered into the heart of Africa. 1 The writer cannot offer an approach to the solution of this mystery. There was a time when he had hopes that what is called the &quot;homing&quot; faculty in Pigeons might furnish a clue, but Mr Tegetmeier and all the best authorities on that subject declare that a knowledge of landmarks ob tained by sight, and sight only, is the sense which directs these Birds, while sight alone can hardly be regarded as affording much aid to Birds and there is reason to think that there are several such which at one stretch transport themselves across the breadth of Europe, or even traverse more than a thousand miles of open ocean, to say nothing of those and of them there are certainly many which perform their migrations by night. That particular form of Bluethroat which yearly repairs to breed upon the mosses of the Subalpine and Northern parts of Scandinavia (Cyane- cula suecicci) is hardly ever seen in Europe south of the Baltic. 2 Throughout Germany it may be said to be quite unknown, being replaced by a conspicuously different form (G. leucocyana), and as it is a Bird in which the collectors of that country, a numerous and well-instructed body, have long taken great interest, we are in a position to declare that it is not known to stop in its transit from its winter haunts, which we know to be Egypt and the valley of the Upper Nile, to its breeding-quarters. Other instances, though none so crucial as this, could be cited from among European Birds were there room here for them. In New Zealand there are two Cuckows which are annual visitors : one, a species of Chrysococcyx, is supposed to come from Australia, the other, Eudynamis taitensis is widely spread throughout Polynesia, yet both these birds yearly make two voyages over the enormous waste of waters that sur rounds the country to which they resort to breed. But space would utterly fail us were we to attempt to recount all the examples of these wonderful flights. Yet it seems impossible that the sense of sight should be the faculty whereby they are so guided to their destination, any more than in the case of those which travel in the dark. Dr von Middendorff (op. cit. p. 9), from the conclusions he has drawn, as before mentioned, as to the spring-move ment of all birds in the Russian Empire being towards the Taimyr Peninsula, the seat of one of the magnetic poles, has suggested that the migrating Bird is always aware (he does not sufficiently explain by what means) of the situa tion of this point, and thus knows how to steer its course. Not only is this hypothesis unsupported by any considera tions known to the writer, but it is not at all borne out by the observed facts of Migration in North America, where Birds as has been shewn by Professor Baird (op. cit. p. 347) do not migrate in the direction of the magnetic pole. Other authors there are who rely on what they call &quot; instinct &quot; as an explanation of this wonderful faculty. This with them is simply a way of evading the difficulty before us, if it does not indeed remove the question alto gether from the domain of scientific inquiry. Rejecting such a mode of treatment, Herr Palmdn meets it in a much 1 Absolute proof of the identity of the particular birds is of course wanting, but if that objection be raised the circumstance becomes still more puzzling, for then we have to account for some mode of com municating precise information by one bird to another. 2 It has occurred indeed as a straggler in about a dozen instances in England, and it arrives twice a yea? in greater or less numbers in Heligoland as reported by the ever-watchful observer on that island, Mr Gatke, to whom ornithologists are so deeply indebted for his long- continued and intelligent scrutiny of the extraordinary number of wandering birds which alight there. fairer spirit. He asserts (op. cit. p. 195) that migrants are led by the older and stronger individuals among them, and, observing that most of those which stray from their right course are yearlings that have never before taken the- journey, he ascribes the due performance of the flight to &quot;experience.&quot; But, granting the undisputed truth of his Expericr.ee observation, his assertion seems to be only partially proved. That the birds which lead the flock are the strongest is on all accounts most likely, but what is there to show that these are also the oldest of the concourse ] Besides this, there are many Birds which cannot be said to migrate in flocks. While Swallows, to take a sufficiently evident example, conspicuously congregate in vast flocks and so leave our shores in large companies, the majority of our summer-visitors slip away almost unobserved, each appa rently without concert with others. It is also pretty nearly certain that the same species of Bird does not migrate in the same manner at all times. When Skylarks arrive on our north-eastern coast in autumn they come flitting over in a constant, straggling stream, not in compact flocks; yet a little later these same birds collect in enormous assemblages which prosecute their voyage in company. It is indeed possible that each bird of the stream intentionally follows that which goes before it, though in a long sea- passage it must be hard to keep the precursor in sight, and it may perhaps be granted that the leader of the whole is a bird of experience. But then we must consider not these cases only, but also those of Birds which do not migrate in. company, and we must also have regard to what is implied in the word &quot; experience.&quot; Here it can only signify the result of knowledge acquired on former occasions, and obtained by sight. Now it was stated by Temminck 8 many years ago, and so far as would appear the statement has not been invalidated, that among migrants the young and the old always journey apart and most generally by different routes. The former can have no &quot;experience,&quot;, and yet the greater number of them safely arrive at the haven where they would be. The sense of sight, essential to a knowledge of landmarks, as we have above attempted to demonstrate, is utterly insufficient to account for ths success that attends Birds which travel by night, or in a single flight span oceans or continents. Yet without it the idea of &quot; experience&quot; cannot be substantiated. We may admit that inherited but unconscious experience, which ia really all that can be meant by instinct, is a factor in the whole matter certainly, as Mr Wallace seems to have proved, in originating the migratory impulse, but yet every aspect of the question is fraught with difficulty, and we must leave to time the discovery of this mystery of mysteries. There yet remain a few words to be said on what may Excep- be termed Exceptional Migration, that is when from some tional cause or other the ordinary practice is broken through, migration, This differs from the chance occurrence of the waifs and strays with which this section of the article began in that the Birds subject to it keep in a great measure their cus tomary habit of migrating, and yet are compelled to indulge it in an irregular, or perhaps an altogether novel, manner, though they are not entirely the sport of circumstances. The erratic movements of the various species of Crossbill Crossbills, (Loxici) and some allied forms afford perhaps the best- known examples. In England no one can say in what part of the country or at what season of the year he may not fall in with a company of the Common Crossbill (L. curvirostra), and the like may be said of many other lands. The food of these Birds consists mainly of the seeds of conifers, and as its supply in any one locality is inter mittent or precarious, we may not unreasonably guess that 3 Manual a&quot; Ornithologie, iii. Introd. p. xliii. note. III. - 97