T. A. Coward

“ During the westward rushes in winter when exceptionally severe weather has cut off the food-supply of ground-feeding birds, observers who have seen the birds moving in front of the storm have maintained that they had felt its approach and retreated in time. The truth seems to be that the birds start so soon as the supply is cut off but in many cases speedily outstrip the storm. When these exceptional winter migrations take place the birds in the lowlands of Lancashire and Cheshire move westward towards Ireland, and are observed at different points along the North Wales coast. ”

“ Cyclonic spells are not always unfavourable to migration In spring, when they are of a mild type with soft rain and warm winds following after a cold anticyclonic period, a northward movement is frequent.
Mr Eagle Clarke says that the importance of winds is overstated, but as an incentive only. The direction of the wind has no influence as an incentive but its force is an important factor; in a strong wind a bird may be blown out of its course. ”

“ In the autumn migration to Britain, the chief movements take place when a large and well-defined anticyclone has its centre somewhere over Scandinavia, with gentle gradients in a south-westerly direction over the North Sea. Coincident with this we usually find cyclonic conditions prevailing to the west of the British area, with low-pressure centres off the west or south-west of Ireland. The weather is clear and cold, with light variable airs over Scandinavia, but in Britain the sky is overcast, and the wind easterly and moderate to strong ”

“ Even violent storms move at varying rates, and it is conceivable that a bird leaving Scandinavia on favourable anticyclonic winds might at once come into the influence of a large, slowly-moving, circular storm, with a low-pressure centre to the West of Ireland, and might, if the air currents were strong, be carried westward at first, then south and finally eastward, so that it would actually pass round the British Islands. ”

“ Slightly unsettled conditions have little effect, but an increase of the irregularities accelerates migration. Sooner or later cyclonic disturbances interrupt regular movements, and, if these are extraordinary, act as barriers, either holding the birds in one place or forcing a hurried departure or "rush." Favourable weather immediately following a check or "hold up" often causes a rush; a sudden fall in temperature may force large numbers of birds on in autumn or retard them in spring. ”

“ If the birds leave Scandinavia under favourable conditions they may be met by the approaching cyclonic system, which usually, though by no means always, travels in a north-easterly direction across the Atlantic. Migration is thus checked, but a return of favourable anticyclonic conditions starts the birds again, oft-en with a fresh impulse in the shape of falling temperature. ”

“ 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. ”

“ The oft-repeated assertion that birds can foretell the nature of approaching weather—that they are living barometers—is not supported by any satisfactory evidence, but it is certain that on many occasions the weather into which they have passed in moving from one zone to another has not only retarded, checked, or exhausted them, but has proved fatally disastrous. ”

“ During a long overland journey, winds will probably have less influence, though for rapid passages high flights certainly appear to be not uncommon. There is, however, another aspect of the connection between migration and weather which we have hardly touched, migration synchronal to the change of season. Mr Cooke shows that. in North America the push forward in spring is not in most species so soon as the weather permits; they do not actually move on the spring wave. ”

“ The suggestion I can offer at present is that there are ornithologists directing their attention to these spots which, through geographical position and isolation, are the liker refuges for wind-borne migrants. Also that the accidental departure from the directions aimed at by the birds is, where wind and barometric systems are so Variable, far more frequent than is usually suspected. Direct routes are doubtless aimed at, but only accomplished under favourable conditions for the whole journey; migration is less infallible than we have been led to think. ”

“ The direction of the wind may he the same up to a great height, many thousand feet, or it may vary within 500 feet of the earth. Nearly fifty years ago, when Glaisher made his great ascents, he sometimes met with three or four currents moving in opposite directions. The more recent upper air investigations show that though as a rule the wind at various elevations is in the main from one point of the compass, its degrees vary considerably, and its force at the various heights shows remarkable differences, Generally the force rises to about 5000 feet, but there is no invariable rule. ”

“ In autumn the journey varies according to the time of starting, Early fall migrants, and indeed the majority of autumn migrants all the world over, travel more slowly than in spring; they are neither impelled by sex-impulses nor the need to escape from failing food supplies. A little later the supply does slacken and With it the temperature cools, and if the changes are sudden southward migration is accelerated. Migration, however, is such an advantageous and well-established habit that it usually begins before hurry is necessary, and the birds loiter southward, feeding as they go. ”

“ I have endeavoured to show that often the force of wind is greater at a high than st 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 ”

“ The bird may have dropped into most adverse currents.
In Hungary, where migration has been very carefully studied, we find evidence supporting Mr Clarke's theory, and yet giving it a slightly different complexion. Low atmospheric pressure, depression (the warm cyclonic conditions of spring) very often shows the greatest rate in the arrival of the swallow. If there is a centre of depression west of Hungary, and its path is directed north or north-east, swallows appear in crowds. The fair side of the depression, with its warm southerly winds, is therefore favourable. ”

“ Is it fair, if I am right that the meteorological observations are founded upon only those observed at comparatively low altitudes, to lay down laws as to the particular conditions which are favourable or unfavourable, or the particular winds which are used or avoided? ”

“ The arrows indicate the actual directions and force of wind at the times marked during a slow-travelling circular storm in autumn 1901. Speed of bird about twenty-five miles per hour. conflicting force 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. ”

“ Under ordinary circumstances are we justified in saying that birds make use of the winds blowing with a certain force at the point of departure, or that they ignore them? Certainly we cannot judge by either the force. or direction of the wind at the point of arrival, as Mr Clarke points out. ”

“ 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 ”

“ When the anticyclonic area is exceptionally large, extending from the Scandinavian peninsula in a south-westerly direction and embracing the whole of the British Islands, simultaneous immigration and emigration may be witnessed. ”

“ 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 earning 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 all first retarded by the ”