should outstrip the weaker by a very perceptible distance, and it
can hardly be doubted that in most species the males are stronger,
as they are bigger than the females. Some observers assert that
the same thing takes place in the return journey in autumn—Seebohm,
for instance, says that, from Europe, first go the young,
then the males, having finished their moult of autumn, and lastly
the females—but on this point others are not so sure, which is not
surprising when we consider that the majority of observations have
been made towards what is the northern limit of the range of the
Passeres, to which the remark is especially applicable—in the
British islands, France, North Germany and the Russian empire—for
it is plain that at the beginning of the journey any inequality in
the speed of travelling will not have become so very manifest. There
is also another matter to be noticed. It has been suspected that
where there is any difference in the size of birds of the same species,
particularly in the dimensions of their wings, the individuals that
perform the most extensive journeys would be naturally those with
the longest and broadest remiges, and in support of this view it
certainly appears that in some of the smaller migrants—such as the
Wheatear (Saxicola oenanthe) and willow-wren (Phylloscopus trochilus)—the
examples which reach the extreme north of Europe and there
pass the summer possess greater mechanical powers of flight than
those of the same species which stop short on the shores of the
Mediterranean. It may perhaps be also inferred, though precise
evidence is wanting, that these same individuals push further to
the southward in winter than do those which are less favoured in
this respect. It is pretty nearly certain that such is the case with
some species, and it may well be so with individuals. H. B. Tristram
has remarked (Ibis, 1865, p. 77) that, in many genera of birds,
“those species which have the most extended northerly have also
the most extended southerly range; and that those which resort
to the highest latitudes for nidification also pass further than others
to the southward in winter,” fortifying his opinion by examples
adduced from the genera Turdus, Fringilla, Cypselus and Turtur.
For many years past a large number of persons in different countries have occupied and amused themselves by carefully registering the dates on which various migratory birds first make their appearance, and there is now an abundance of records so compiled. Still it does not seem that they have been able to determine what connexion, if any, exists between the arrival of birds and the weather; in most cases no corresponding observations have been made about the weather in the places whence the travellers are supposed to have come. As a rule it would seem as though birds were not dependent on the weather to any great degree. Occasionally the return of the swallow or the nightingale may be somewhat delayed, but most sea-fowls may be trusted, it is said, as the almanac itself. Foul weather or fair, heat or cold, the puffins (Fratercula arctica) repair to some of their stations punctually on a given day as if their movements were regulated by clockwork. Whether they have come from far or from near we know not, but other birds certainly come from a great distance, and yet make their appearance with scarcely less exactness. Nor is the regularity with which certain species disappear much inferior; every observer knows how abundant the swift (Cypselus apus) is up to the time of its leaving its summer-home—in most parts of England, the first days of August—and how rarely it is seen after that time is past.
It must be allowed, however, that, with few exceptions, the mass of statistics above spoken of has never been worked up and digested so as to allow proper inferences to be made from it, and therefore it would be premature to say that little would come of it, but the result of those exceptions is not very encouraging. E. von Middendorff carefully collated the records of the arrival of migratory birds throughout the Russian Empire, but the insight into the question afforded by his published labours is not very great. His chief object has been to trace what he has termed the isepipteses (ἶσος = aequalis, ἐπἰπτσις = advolatus) or the lines of simultaneous arrival, and in the case of seven species these are laid down on the maps which accompany his treatise. The lines are found by taking the average date of arrival of each species at each place in the Russian dominions where observations have been regularly made, and connecting those places where the dates are the same for each species by lines on the map. The curves thus drawn indicate the inequality of progress made by the species in different longitudes, and assuming that the advance is directly across the isepiptesial lines, or rather the belts defined by each pair of them, the whole course of the migration is thus most accurately made known. In the case of his seven sample species the maps show their progressive advance at intervals of a few days, and the issue of the whole investigation, according to him, proves that in the middle of Siberia the general direction of the usual migrants is almost due north, in the east of Siberia from south-east to north-west, and in European Russia from south-west to north-east. Thus nearly all the migrants of the Russian empire tend to converge upon the most northern part of the continent, the Taimyr peninsula, but it is almost needless to say that few of them reach anything like so far, since the country in those high latitudes is utterly unfit to support the majority. With the exception of some details this treatise fails to show more. The routes followed by migratory birds have been the subject of a very exhaustive memoir by J. A. Palmén, but it would be beyond our limits to do more than mention his results concisely. He enters very fully into this part of the inquiry and lays down with much apparent probability the chief roads taken by the most migratory birds of the palaearctic region in their return autumnal journey, further asserting that in the spaces between these lines of flight such birds do not usually occur. Broadly speaking, the birds of Europe, Russia and Western Siberia go for the winter to Africa, those of middle Siberia to Mongolia, and those of Siberia east of the Lena go towards Japan.
But lay down the paths of migratory birds, observe their comings and goings, or strive to account for the impulse which urges them forward as we will, there still remains for consideration the most marvellous thing of all—how do the birds find their way so unerringly from such immense distances? This seems to be by far the most inexplicable part of the matter. Year after year the migratory wagtail will build her nest in the accustomed spot, and year after year the migratory cuckoo 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. That particular form of bluethroat which yearly repairs to breed upon the mosses of the subalpine and northern parts of Scandinavia (Cyanecula suecica) is hardly ever seen in Europe south of the Baltic. Throughout Germany it may be said to be quite unknown, being replaced by a conspicuously different form (C. 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 cuckoos 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 surrounds 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. J. A. Palmén asserted (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 ascribed the due performance of the flight to “experience.” There are many birds which cannot be said to migrate in company. 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 apparently without concert with others. Experience here can only signify the result of knowledge acquired on former occasions and obtained by sight. Now it was stated by C. J. Temminck (Manuel d’ornithologie, III. Introd., 1820) 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 “experience,” 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, is utterly insufficient to account for the success that attends birds which travel by night, or in a single flight span oceans or continents. Yet without it the idea of “experience” cannot be substantiated. We may admit that inherited but unconscious experience, which is really all that can be meant by instinct, is a factor in the whole matter—certainly, as Wallace seems to have proved, in originating the migratory impulse, but yet every aspect of the question is fraught with difficulty.
Less than nothing is known about the speed at which birds fly during their long stretches of migration. Gaetke, in his otherwise very interesting book, has startled ornithologists by various statements, but his calculations were based upon such crude observations that the results are ridiculous. For instance, he proved to his satisfaction that the grey or hooded crow, Corvus cornix, which notoriously is not a fast bird, flies from Heligoland to the coast of Lincolnshire in England at the rate of one hundred and twenty miles an hour. To the little bluethroat he assigned a velocity of two hundred and forty miles an hour, a statement as silly as that made by some fanciful observer in Portugal who convinced himself that “Turtle-doves leaving Kent or Surrey at dawn might easily be the very birds that a few hours later were skimming over the Portuguese pine forests on their way to Central Africa.” Fifty miles an hour would be a high average speed for most migratory birds, and there are no reliable data to tell how long such birds can continue their flight without interruption. All we seem to know is that not a few kinds manage, in various parts of the world, to cross enormous distances without the chance of a break. It was Gaetke’s notion that migration was for the most part carried on at such a height in the air as to be beyond our ken, and that what
