ON THE MIGRATION OF BIRDS. 469 
arrival of immigrants in the British Isles in spring and autumn, and our 
home records referred to for an explanation of the movements of emi- 
grants during the spring, autumn, and winter. 
As the result of an extensive series of comparisons instituted between 
the two sets of phenomena, it has been ascertained that they are most inti- 
mately associated, and that a knowledge of the meteorological conditions 
prevailing during the movements in most instances contributes in no 
small degree to a correct interpretation of their precise nature and the 
seat of their origin. 
_ The weather influences are of two kinds, as treated of separately 
below — 
I. Ordinary Weather Inflwences.—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 indifference to the migrants ; if more pro- 
nouncedly so, their movements are slightly quickened thereby. 
This may be termed normal migration under ordinary weather con- 
ditions. 
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. ” 
Il. Lxtraordinary Weather Influences.—These are exerted by the pre- 
valence of particular weather conditions, which may act either (1) as 
barriers to the ordinary movements, or (2) in diametrically the opposite 
direction as 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. 
The weather incentives to migration are widely different in their nature 
and may take several forms. First, there may be favourable weather- 
periods immediately 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 south- 
wards. 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 toa 
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 movements, and is 
the main controlling factor in all extraordinary movements, other 
meteorological conditions being suitable. Each movement, however, has 
its peculiarity, and the conditions controlling it are often due to meteoro- 
logical phenomena of a more or less complex nature, most of which, 
perhaps, admit of explanation. 
Meteorology and Autumn Immigration.—The immigratory movements 
of the early autumn are those already mentioned as normal migration 
under ordinary weather conditions, and need no further notice. 
