--------+-------------+-------------+-------------+-------------+ | 4000 mtrs. | 4500 mtrs. | 5000 mtrs. | 6000 mtrs. | Date. |(13,320 ft.).|(15,000 ft.).|(16,700 ft.).|(20,000 ft.).| --------+-------------+-------------+-------------+-------------+ Jan. 2 | NE by N | | | | | 23 | | | | | | | | | " 2 | | | | | | | | | | | | | | | " 3 | | | | | | | | | | | | | | | " 4 | | | | | | | | | | | | | | | " 11 | E1/2N | E by S | E by NE | ENE | | 8 | 14 | 13 | 14 | | | | | | April 9 | NW by W | W by N | | | | 18 | 20 | | | | | | | | " 8 | NW | SSW | NNW | SW | | 1 | 3 | 5 | 7 | | | | | | " 30 | | | | | | | | | | | | | | | May 16 | | | | | | | | | | | | | | | Sept. 5 | | | | | | | | | | | | | | | " 7 | | | | | | | | | | | | | | | " 10 | | | | | | | | | | | | | | | " 20 | | | | | | | | | | --------+-------------+-------------+-------------+-------------+
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. 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. A list of twelve other birds, which also appear in spring under these conditions in greatest numbers, is added. The "bad" side, with cool northerly winds causes delays in the arrival of these thirteen species. The depressions often have a sphere of influence extending so far as North Africa, so that birds, on the fair side, can cross the Mediterranean with southerly winds all the way (31).
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.