November 2, 1896.] 



KNOWLEDGE. 



255 



Autumn Immiyration. — Birds which have migrated north 

 for the summer, seem to begin to return south towards the 

 end of July, when a few reach our shores. Immigration 

 does not set in in earnest, however, until Augu3t, while in 

 September it increases, and in October the Hood of 

 migratory birds reaches its highest level, and there are 

 experienced those vast "rushes" upon our shores so 



often described "The immigratory movements 



occurring iu November are not only on a very much reduced 

 scale, but after the middle of the month the immigration of 

 such birds as spend the summer in the north entirelij ceases, 

 with the exception of those of certain marine species (duck, 

 gulls, grebes, swans), whose late movements to the south 

 are dependent upon severe weather conditions. This is 

 entirely contrary to the views hitherto propounded regard- 

 ing the limits of the^e movements, but it is, nevertheless, 

 a fact well established by this inquiry." 



Except that they do not begin until the end of September, 

 the immigration from the west by the east and west route 

 nearly coincides with that from the north. 



'• Daring immigration our shores are reached during 

 the late night or early morning on the part of migrants 

 from the north. On the contrary, the immigratory move- 

 ments from the east, across the narrows of the North Sea, 

 appear to be performed during the daytime." 



Autumn Emiijration. — A tew of our summer visitors 

 leave us in July {e.g., swifts and adult cuckoos). In 

 August emigration increases, while " September witnesses 

 the height and close of the emigration of the bulk of the 

 smaller British summer visitors. The movements of fony- 

 two of these emigrants appear in the records for the month ; 

 ■whi)e those of the partial migrants are also considerable, 



over forty species being recorded The October 



emigrants among the summer birds are not numerous. 

 The partial migrants, on the other hand, are much on the 

 move." These movements of partial migrants are often 

 pronounced, and " rushes " are recoided during October, 

 but they cease by or during the first half of November. A 

 partial migrant is a bird which, although it may be 

 sedentary in our islands as a species, yet many individuals 

 of the species are strictly migratory. Tlius many birds, 

 such as the thruah, blackbird, robin, leave us in the 

 autumn, and their places are taken by Continental in- 

 dividuals, and so we do not miss them. 



Winter movements are of a very different nature, and 

 are entirely due to a fall in temperature, as is proved by 

 the fact that " in mild winters the only movements 

 recorded are a few local migrations, which strictly coincide 

 with the occasional periods of cold from which hardly any 

 season is entirely exempt." 



Sprimi Immiiiration. — The first bird-harbingers of spring 

 are recorded for February, and a considerable number 

 return in March. During both these months, however, 

 but few summer visitors appear, the immigrants being 

 chiefly partial migrants, "which had fled the country 

 through the winter cold." "April is a month of pronounced 

 immigration on the part of summer visitors. In con- 

 nection with the arrival of these earliest immigrants 

 among our summer visitors during March or April, a 

 remarlsable and interesting fact reiiiaius to bo mentioned — 

 namely, that the great majority of these birds are recorded 

 first lor the south-western area of the British region — the 

 south-west coast of Eugland and Ireland. It thus seenn 

 probable that the first arrival of the spring migrants not 

 unnaturally occurs oq tiiosa parts of our isles which are 

 the warmest so early in the season." During May the 

 stream flows on, while "during the first half of June 

 several species whose breeding range extends to tbe 

 Polar regions, appear in considerable numbers on our 



shores on their way to the far north ; a few appear even 

 still later. In connection with the spring immigration, 

 it has to be remarked that the observations are all in 

 fivour of the theory that the earliest arrivals among the 

 summer visitors to our islands are British-breeding birds, ' 

 and that tbe " miicrants bound for the north are the last of 

 their kind to appear in the British area." 



Sprimj Emi'iratiiin. — Birds that have wintered with us, 

 or further south, begin to leave our shores in February. 

 " The chief emigratory movements of this month are the 

 departure of larks and rooks along the east and west route 

 to the Continent. During March these south-easterly 

 movements become more pronounced, and emigration for 

 the north also commences. During April and May, 

 emigration is in full swing, and even in June a few birds 

 leave us for their northern breeding stations. 



Meteorological. — Special attention has been bestowed 

 upon this section of the report, since the actual relation- 

 ships between migrational and meteorological phenomena 

 have not hitherto received the attention they deserve. 

 The daily weather reports for Western Europe Lave been 

 closely studied in connection with migration. Ordinary 

 weather has no influence on migration. Extraordinary 

 weather may influence bird migration in two ways. It 

 " may act either (1) as barriers to the ordinary movements, 

 or (2) in diametrically the opposite direction as incentives 

 to great movements or ' rushes," aa they have been 

 termed." 



During a cyclonic spell, a weather barrier "dams back, 

 as it were," the ordinary seasonal migratory stream. 

 The formation of an anti-cyclone removes the cyclonic 

 weather barrier, and so releases tbe flood of migration. 

 " Tbe movements just described take place when gentle 

 pressure-gradients bridge, as it were, the North Sea with 

 fine weather between Scandinavia and Britain. Such an 

 extension, however, of the favourable conditions does not 

 always prevail for the entire journey — that is to say, they 

 do not always reach to the British side of the North Sea. 

 Indeed, it not unfrequently happens that the birds reach 

 our shores under more or less unfavourable weather con- 

 ditions. When such is the case the immigrants arrive in 

 Britain in a correspondingly exhausted condition, and, no 

 doubt, many sometimes perish during thejourney." 



There are other meteorological conditions to be con- 

 sidered iu connection with the subject. Outbursts of 

 ungenial weather in summer or winter often produce 

 migration, but this form of migration is only of a partial 

 kind. It is iu early autumn and early spring that migra- 

 tion is hastened or held back by extraordinary weather. 



" Winds. — The importance attached to winds in con- 

 nection with bird migration has hitherto been much over- 

 estimated by popular writers, and their influence, such as 

 it is, mi-understood. " The conclusions to be drawn 

 from a careful study of the subject are: (l)that the 

 direction of the wind has no influence whatever us an 

 incentive to mitirntum : 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 



It is, however, a fact that particular winds almost in- 

 variably 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 Cise, and 

 it may be at once stated that these supposed favourable 

 breezes are simply another direct result of the pressure 

 distribution favourable to the movements." 



In conclusion, I may say that Mr. Eagle Clarke's report 

 is merely a summary of the results obtained with regard 

 to migration as a whole. For this the author deserves the 



