SCIENCE-GOSSIP. 



331 



quarters of the spring birds of passage and of the 

 winter visitors to Britain takes place from our 

 eastern coasts and the northern isles ; a few only 

 of the species, such as the redwing, wheatear, 

 white wagtail, barnacle goose, swans, whimbrel, 

 etc., passing up our western coasts, possibly f« ivnte 

 for Iceland. 



Meteorological. 

 " Special attention has been bestowed upon this 

 section of the Digest, since the actual relationships 

 between migrational and meteorological phenomena 

 have not hitherto received the attention they 

 deserve, no doubt because the necessary sets of 

 data for a satisfactory investigation of the problem 

 were not obtainable. The material collected by 

 the Committee has proved in all respects most 

 valuable for establishing a useful comparison 

 between these two sets of phenomena, and for 

 determining, to a certain extent, the precise 

 influence exercised by the weather upon bird 

 movements. The standard for the weather has 

 been the ' Daily Weather Reports ' issued by the 

 Meteorological Office. . . . It may be well to state 

 that these ' Daily Reports ' are based upon obser- 

 vations made at fifty-four stations, distributed over 

 Western Europe between Haparanda and Bodo 

 in the north, and Toulon, Biarritz and Corunna in 

 the south ; as well as all parts of Great Britain and 

 Ireland. 



"The weather influences are of two kinds, as 

 treated of separately below : 



" I. — Ordinary Weather I iifluences. — It is found that 

 both in the spring and autumn migratory periods 

 there are spells of genial weather without marked 

 features other than those favourable for migration. 

 During these the movements of the various species 

 are of an even-flowing and continuous nature. If 

 the weather should prove slightly unsettled during 

 such periods, it is a matter of indifference to the 

 migrants; if more pronouncedly so, their move- 

 ments are slightly quickened thereby. This may 

 be termed normal migration under ordinary weather 

 conditions. The duration of such favourable spells, 

 however, is sooner or later broken by the advent 

 of a cyclonic period of a more or less severe type. 

 This interferes, to a greater or lesser degree, with 

 the progress of the migratory movements. 



" II. — Extraordinary Weather Influences. — These are 

 exerted by the prevalence of particular weather 

 conditions, which may act either (i) as barriers to 

 the ordinary movements, or (2) in diametrically 

 the opposite direction, as incentives to great move- 

 ments or ' rushes,' as they have been termed. . . . 

 The weather incentives to migration are widely 

 different in their nature, and may take several forms. 

 First, there may be favourable weather periods imme- 

 diately following unfavourable periods. Secondly, 

 they may be due to weather in certain respects 

 unfavourable to the birds, such as a decided fall 

 in temperature, which either compels the birds to 

 move, or acts as a warning that the time has 

 arrived for their departure southwards. Such cold 

 spells are characteristic of anticyclonic periods, 

 when the weather is calm and highly favourable 

 for a prolonged flight. Thirdly, and on the other 

 hand, the advent in spring of a genial spell, 

 especially if accompanied by a rise of temperature, 

 is an incentive to a move to the northward for the 

 summer haunts. The weather influences thus 

 vary considerably ; but temperature plays the 

 most important part in the various seasonal move- 

 ments, and is the main controlling factor in all 

 extraordinary movements, other meteorological 



conditions being suitable. Each movement, how- 

 ever, has its peculiarity, and the conditions 

 controlling it are often due to meteorological 

 phenomena of a more or less complex nature, 

 most of which, perhaps, admit of explanation. 



" Winds. — The importance attached to winds in 

 connection with bird-migration has hitherto been 

 much over-estimated by popular writers, and their 

 influence, such as it is, misunderstood. The 

 conclusions to be drawn from a careful study of 

 the subject are : (i) that the direction of the wind has 

 no influence whatever as am incentive to migration ; 

 but that (2) its force is certainly an important factor, 

 inasmuch as it may make migration an impossibility, 

 arrest to a greater or lesser degree its progress, or 

 even blow birds out of their course. We have the 

 clearest proof, indeed, that birds do not emigrate 

 when the winds are exceptionally high, though they 

 sometimes pass into high winds and gales when 

 en route, under the meteorological conditions which 

 have already been described and explained. Ordi- 

 nary winds — that is, winds not too strong — appear 

 to be of small concern to the birds, for they are 

 recorded as migrating with winds blowing from all 

 quarters. It is, however, a fact that particular 

 winds almost invariably prevail during the great 

 autumnal movements, and these have hitherto 

 been considered by some as the direct incentives 

 to such migrations. Such is not the case, and it 

 may be at once stated that these supposed favour- 

 able breezes are simply another direct result of the 

 pressure distribution favourable to the movements. 

 This peculiar type of weather has already been 

 fully described and its effects discussed ; the winds 

 prevailing and dependent upon these barometric 

 conditions are easterly, chiefly south-easterly, 

 breezes. There is really no reason why westerly 

 (west, north-west, and south-west) winds, not too 

 strong, of course, should not, other things being equal, 

 be in every way as suitable for migratory move- 

 ments as those varying between such divergent 

 points as north-east to south. When, however, we 

 come to inquire into the meteorological conditions 

 producing these westerly winds, the reason for 

 their unsuitability becomes at once apparent. 

 These winds are the result of types of pressure- 

 distribution which are fatal to migration between 

 north-western Europe and Britain, namely, the 

 presence of cyclonic areas to the north-east or east 

 of the British Isles. This means that the area 

 under disturbed conditions would be the very 

 region from which we derive our autumn immi- 

 grants and render emigration from such sources 

 impossible. Such areas of disturbance, with their 

 high westerly and north-westerly winds, indeed, 

 often extend to and influence the weather in our 

 islands, and interfere with the British emigratory 

 movements in both autumn and spring. Strong 

 winds have a curious effect on the flight of gulls, 

 compelling them to move in a direction more or 

 less directly heading the wind. Thus a strong 

 westerly wind causes great numbers of gulls to 

 seek the estuaries and bays of our east coast. On 

 the other hand, strong easterly winds will fill the 

 estuaries and sea-lochs of the west coast with these 

 birds. The lee side of islands is also sought under 

 similar conditions of the wind. A south-easterly 

 wind, for the same reason, causes considerable 

 numbers of gulls of various species to pass 

 southward along the eastern coast of Britain. 

 Large parties of gulls are also recorded as passing 

 north — sometimes for a whole day — with a 

 north-north-west wind. These movements are 



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