ON THE MIGRATION OF BIRDS. 475 
movements naturally are. That such is the case is manifest on a com- 
parison being instituted between the migrational and the meteorological 
data for spring. Here, as abroad, it is found, other conditions being 
equal, that increase in temperature is the main influencing factor, and 
also that upon it depends, to a considerable degree, the extent of the 
movement. 
The emigratory movements from Great Britain and Ireland naturally 
take place at later dates than the corresponding movements into our 
Islands from the south. Thus it is not until April, and especially May, 
that the decided or great departure movements are recorded which are 
relevant to the particular investigation under consideration. 
In Aprit the fine weather or anticyclonic periods have varying emigra- 
tional values, depending entirely on their temperature. They are favourable 
if characterised by high, or moderately high, temperatures ; or they may 
be distinctly unfavourable through their decided cold. There is, however, 
a medium even in the influence of anticyclonic spells, and thus during 
periods which are moderately cold but calm, some emigration, of a 
straggling nature it is true, is recorded. 
In spring, too, cyclonic periods vary also in their influences on emi- 
gration. They are, as a rule, unfavourable owing to their high winds 
and ungeniality. On the other hand, when they are of a mild type and 
characterised by warm rain and soft breezes, following a cold anticyclonic 
spell in April, they are found to be distinctly favourable to a northward 
movement from our Islands. 
The great spring emigratory flights, and most of the lesser ones too, 
are embarked upon under precisely the same type of pressure distribution 
as that described as being so markedly favourable for the autumn passage 
of birds across the North Sea to our Islands, namely, the presence of a 
high pressure centre to the north-east of our Islands over Norway and 
Sweden, with gentle gradients to the south-west. Under such circum- 
stances of pressure distribution the North Sea between our Islands and 
the Scandinavian peninsula is spanned by fine weather, and moderate 
easterly or southerly breezes prevail. Such highly favourable periods, as 
in the autumn, usually follow spells of weather decidedly ungenial for 
bird migration. 
Some of these spring movements to the north are occasionally 
undertaken during somewhat unfavourable weather. Even in May 
there are a few records of emigration during sleet, cold rain, and north- 
east breezes, but it has to be explained that these flights followed 
prolonged spells of ungenial weather, with decidedly low temperature, 
late in the season, and were genial when compared with the preceding 
conditions. 
Late in the spring—at the end of May and in JunE—it is not surprising 
to find that meteorological influences do not play an important part in 
the last movements to the north. That this should be the case is due, 
no doubt, to the advanced state of the season and its settled, or .com- 
paratively settled, weather. 
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 : 
(1) that the direction of the wind has no influence whatever as an 
