67 



It will be opportune here to quote Mr. W. Eagle Clarke's 

 remarks on the meteorological conditions affecting migration, as 

 presented in his " Digest of the Observations on the Migration of 

 Birds, 1880-1887." Mr. Clarke states at the outset that observa- 

 tions he has utilised have been constantly conducted and reported 

 from 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. Mr. Clarke remarks : — " When studying bird migration 

 in connection with meteorological conditions, it is essential that 

 the weather peculiarities synchronous with the setting in of the 

 migration, and prevailing in the particular area in which the 

 movement had its origin, should be considered. This alone has 

 any true bearing upon migration, not the weather prevailing 

 upon the shores reached after an extended migratory flight. . 

 The weather influences are of two kinds. (I.) Ordinary weather 

 influence. It is found that in both 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 in- 

 difference 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 (1) as barriers to the ordinary movements, or (2) in 

 diametrically the opposite direction, as to incentives to great 

 movements, or " rushes," as they have been termed. 



The weather barriers to bird migration, are unfavourable 

 conditions of a pronounced nature, which interrupt and make 

 impossible, during their prevalence, the ordinary seasonal 

 movements. 



