There are some birds occurring on our east coast year by year with tolerable regularity, which, during the autumn of 1882, have been remarkable for their scarcity. This has been the case with all the large raptorial birds, and especially with the Short-eared Owl, and Common Linnet and Twite. Their absence on migration has also been remarked upon in Heligoland. The Short-eared Owl also appears to have been specially scarce on the east coast of Scotland. (See East Coast of Scotland Report.)
Our returns show very clearly that the spring lines of migration, followed by birds leaving our shores, are identically the same as those followed in the autumn, but of course in the reverse direction from W. and N.W. to E. and S.E.
As this is the fourth report issued by the Committee, we may, perhaps, with the mass of facts at our disposal, be expected to draw deductions, which, if they do not explain, will serve at least to throw some light on the causes influencing the migration of birds. We might reasonably reply that the work undertaken by us was not to theorise, or attempt explanations, but simply to collect facts and tabulate them. This we have endeavoured to do in the shortest and simplest manner consistent with accuracy of detail. There is, however, one circumstance which can scarcely fail to present itself to those who have gone carefully into the reports issued by the Committee, namely, the marvellous persistency with which, year by year, birds follow the same lines of migration when approaching or leaving our shores: the constancy of these periodical phenomena is suggestive of some settled principle or law governing the movement. It is clearly evident, from the facts already at our disposal, that there are two distinct migrations going forward at the same time; one, the ordinary flow in the spring and ebb in the autumn, across the whole of the western Palæarctic regions, which of course includes the British Isles, of a great migratory wave moving to and from the nesting-quarters of the birds in the coldest part of their range, N.E. in the spring and S.W. in the autumn. Quite independent of this there is a continual stream of immigrants, week by week and month by month, to the eastern shores of these islands, coming directly across Europe from E. to W., or more commonly from points S. of E. to others N. of W., and the reverse in the spring. These are mainly composed of those common and well-known species which annually make these islands their winter resort, and take the place of our summer birds: they come in one broad stream, cutting the line of ordinary migration at nearly right angles; one flank brushes the Orkneys and Shetlands, pouring through the Pentland Firth, even touching the distant Faroes; the southern wing crosses the Channel Islands, shaping its course in a north-westerly direction to the English coast. In our explanation of the causes which first induced, and perhaps still influences, this E. to W. migration, we must probably go back a long way in the history of the world, when the distribution of the land and water of continental Europe was very different to what it now is; when there was no North Sea, and the western coast-line of Europe was represented by what is now known as the hundred-fathom line off' the West of Ireland, a coast which on the one side touched Scandinavia, and on the other was linked with the Spanish peninsula. Great as is now the contrast between the winters of Central Russia and those of these islands, the difference would then be much more marked,—arctic cold on one hand, and semi-tropical warmth on the other.[11] It requires then no stretch of imagination to believe that great flights of birds would on the approach of winter be driven before the intense cold of Eastern and Central Europe to seek refuge and find food in the warm regions of the west, regions which then would feel the full effects of the warm equatorial currents, and enjoy an almost perpetual summer. This movement once begun would, by the very necessities of existence, and in time by an hereditary instinct, be continued. Gradually the land now occupied by the North Sea has been withdrawn from beneath the migrating flocks; year after year the middle passage became wider and more difficult; yet the habit once formed would be continued, and hereditary instinct, or whatever other name we choose to give it, supply the rest.
[11] There are ninety species of plants, all told, common alike to Southwestern England and Ireland, and to the Pyrenean and Italian region. They represent an old flora no longer adapted to the country,—a flora of warmth and sunshine,—and now dying out under the advance of hardier, more vigorous and congenial species. They may be regarded as the last floral relics of the submerged land, that semi-tropical western land whose plants and flowers are not of Scandinavian origin, but derived from Southern Europe.
Mr. Wallace has told us how, in the Eastern Archipelago, comparatively narrow, and probably very ancient, straits of water divide and wholly separate distinct races of birds; and we have instances of this in Europe, where species, common on the opposite coast of the Continent, rarely or never occur in the British Islands.
Small birds, like the Goldcrest, do not cross great breadths of water from choice; they doubtless would prefer a migration over land, from field to field, or hedge to hedge; or at the most closely following some old established coast-line. Why, except on some such hypothesis as stated, should they attempt the North Sea, not alone at the narrowest part, the straits of Dover, or from Ostend to the coast of Kent, but in the very widest parts also, from the Elbe to the Humber, or Danish coast to the Pentland Firth and Scotch islands? What impels our autumn visitants, the young weeks in advance of their parents, to launch westward across what, for anything they can possibly know to the contrary, may prove an Atlantic, an ocean without a further shore?
There are doubtless several causes, working separately or together, which influence migration, and we must not look for an explanation of the phenomena attending these great periodical movements to one cause only, for by doing this we lose sight perhaps of other equally powerful incentives. I have spoken in previous reports of the probability of birds following ancient coast-lines once linking now distant lands, impelled by what we call, for want of a better term, hereditary instinct, that is, an instinct derived through ancestors. It is, perhaps, an open question whether the young, which undoubtedly arrive in the autumn weeks in advance of the great mass of old birds, depend entirely on this, or whether they are in any way dependent on guidance and direction. It is a curious fact, which we have frequently remarked, that the very earliest of their kind are frequently a few old birds,—flocks of young, too, often contain a sprinkling of old female birds,—such as may be supposed have made the journey before; but it must be also borne in mind that on dark or even starlight nights, when these movements mostly take place, any guidance, even that of call-note, would be futile at any but a very limited range.
WEST COAST OF SCOTLAND.
Schedules, &c., were sent to forty stations, the same number as in previous years. We have received filled-in schedules from thirteen stations.[12]
[12] But the Isle of Man stations—five in number—have been taken over by Mr. P. Kermode, as he is on the spot, and as they really lie more into his district,