I have endeavoured to show that often the force of wind is greater at a high than a low altitude, and there is ample evidence to prove that birds fly at a great height when conditions are favourable. Birds usually leave Scandinavia when there are descending currents flowing outwards from the centre of high pressure; is it wild speculation to suggest that it is the southward flowing currents, which are also deflected westwards, upon which the birds intend to travel? Thus the bulk of the Scandinavian birds might not touch Britain at all, but those which started upon light to moderate north-east to easterly winds from the western shores of Norway would be helped to Britain. Mr Clarke mentions that when he was at Fair Island, north-west to westerly winds did not stop migration from the north, but is it certain that the birds did travel in or against these westerly winds? May they not actually have travelled on the "good side" of the cyclonic system, with these very winds carrying them towards Fair Island? their actual visible approach from the north does not prove that they had travelled all the way in this line.
On September 22nd, he says—"The favourable meteorological conditions of yesterday—fine weather and moderate south-east breezes,—has had a marked effect, for to-day goldcrests are swarming everywhere." But what does he mean? Favourable to him as an observer or to the goldcrests? Surely the birds did not aim for Fair Island; were not these weak-winged birds probably making for the south, when the south-east wind caught them and drifted them to the west? Fair Island was a refuge, but hardly the objective of their flight [(17)].
Compare this with Cordeaux's notes of another goldcrest immigration, this time to the Lincolnshire coast [(23)]. On October 13th the wind was north to north-east in the afternoon, light but increasing in force, the weather clear and bright—a few birds arrived. They had started under favourable circumstances. Shortly after midnight on the morning of the 14th, the wind got full east, with quite half a gale and heavy beating rain, continuous to the morning of the 16th; the nights were very dark. "During this time the immigration was immense," and most of the birds were goldcrests. Cordeaux's idea that these were not normal immigrants but birds which were passing probably from north-east to south-west, when the easterly gale caught them, is probably correct.
I have referred to birds starting at a high elevation. Service says that in normal departure from the Solway, most birds mount to a high altitude, but "a strong beam wind will bring the birds—even those of strongest power—down to 200 to 500 feet of the surface, and it is interesting to see whole flocks with heads turned almost completely to wind, and yet travelling along at nearly their normal speed, at right angles to their position" [(46)]. Mr Tomison mentions rooks, daws and hooded crows driven to Sule Skerry by south-east winds in March, leaving two days later in a westerly gale. They, at any rate, did not object to a strong wind which was in the right direction.
I have mentioned Mr F. J. Stubbs' paper on the "Use of Wind" [(50)], and I believe that there is much more in it than is actually proved by low-level observations. I doubt if birds always intentionally make use of strong winds, currents which would carry them for great distances at a considerable speed, but the preliminary ascent may be to search for these currents. Cyclonic and anticyclonic winds, even when at an altitude of some thousands of feet, would carry them easily, and probably it is the wind-borne individuals, parties, or even hosts, which drop for a refuge to the first island they see when carried far from their migratory path. They are carried rather than drifted from their pathway, borne in the moving current whether they wish it or not. Provided that the cyclonic winds are fairly steady in direction and force, sweeping round and inwards towards their centre, we may in imagination trace the pathway of our so-called lost wanderers to far distant islands; without many more upper-air observation stations, we cannot actually prove the route.
But even putting aside the high altitude idea, and confining our route-tracing to the known courses of air currents, we shall find immense difficulty in mapping out the actual course of any bird on any particular day. The study of some of the publications of the Meteorological Committee, such, for instance, as the "Life History of Surface Air Currents," by Shaw and Lempfert, published in 1906, shows the great variation in the pathways, speeds, and formation of these systems; a bird which accidentally entered a cyclone would unconsciously alter its actual track and speed very many times before it passed beyond the area of influence.
I am indebted to Mr Stubbs and Mr Herbert Taylor of King's College, London, for some interesting mathematically worked-out routes of birds, travelling at a given speed in a cyclone rotating at given speeds and moving at a fixed rate; these show great variation both in direction and speed according to the time and place of entering the system. The track of the bird is, of course, influenced by its own rate of progress, by the speed of the rotating currents, and by the rate at which the whole system moves in any direction. Thus a migrant passing south and coming within the influence of a cyclone which is moving north-east at a high rate of speed, say 40 miles per hour, will, if it enters towards the northern limits of the system, be at first retarded by the conflicting forces of the easterly winds, the trend towards the north-east of the rapidly travelling cyclone and its own southward flight. If it is flying faster than the speed of the cyclone it will drift westward but gradually approach the low pressure centre. After passing this its course will at once change and its speed will be accelerated towards the east.