ON THE MIGRATION OF BIRDS. 471 
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 conditions. When such is the case the 
immigrants arrive in Britain in a correspondingly exhausted condition, 
and, no doubt, many sometimes perish during the journey. An exa- 
mination of the weather data for such occasions reveals a very simple 
explanation of this peculiar, and partially unfavourable, phase in 
Migration-Meteorology. It is as follows :—Though the weather-condi- 
tions at the area of departure be entirely favourable for emigration, and 
induce the birds to move southwards, the conditions prevailing on the 
British coast are unfavourable, owing to the too close proximity or the 
depth of the western low-pressure centre. On the location and character 
of this cyclonic centre entirely depends the nature of the weather in the 
immediate neighbourhood of our shores. If the western cyclonic system 
is too close to Britain, or if the depression is exceptionally deep, then 
unfavourable conditions for migration, with strong winds, prevail beyond 
our eastern shores, and the birds perform the latter portion of their 
journey under trying conditions. On the other hand, if it is off our 
western shores and shallow, then fine-weather gradients entirely bridge 
the North Sea. 
Between these extremes of autumnal migration-weather there are 
intermediate phases, whose influences are easily determined by a study of 
the two sets of phenomena. 
The autumnal immigration from the east by the ‘East and West 
Route’ across the narrows of the North Sea to the south-east coast of 
England remains to be considered in connection with its meteorological 
aspects. Concerning this, however, there is not much to be explained. 
‘It has been ascertained that the movements take place during favourable 
weather conditions, and that they are most pronounced when the per- 
fectly calm conditions and cold of anticyclonic periods prevail. They are 
interrupted by rough weather, to be renewed with increased momentum 
when the cyclonic spell is broken. 
Simultaneous Autumn Immigration and Emigration—It has been 
mentioned in the Seasonal Section of this Report, that under certain con- 
ditions in the late autumn decided immigratory and emigratory movements 
are witnessed in progress simultaneously. On these not very frequent 
occasions, it has been clearly ascertained that the anticyclone in North- 
western Europe covers an unusually wide area. This is due to the gentle 
character of its gradients, which, having their centre over Scandinavia, 
extend in a south-westerly direction to and beyond the limits of the 
British Isles. Thus there prevail over this exceptionally extensive region 
all the conditions already described as favourable for great emigratory 
movements. The result is a great simultaneous inpouring of birds on our 
east coast and a general outpouring from all British coasts of migrants of 
many species. 
Autumn Emigration —The autumnal emigratory movements are con- 
trolled, so far as they may be affected by meteorological phenomena, by 
weather-conditions prevailing in the British area. 
The chief feature in migration during the earlier autumn days is the 
departure of British summer birds, including those which have been 
