38 STUDIES IN BIRD-MIGRATION 



conditions of the season. Usually during each winter 

 there are outbreaks of cold of a more or less severe 

 nature and of longer or shorter duration. Such out- 

 breaks compel certain resident species, and also some 

 of our winter guests, to shift from the areas affected 

 to others where more favourable conditions prevail, 

 and where food, the main incentive for the movements, 

 can be obtained. Some of these evicted birds find refuges 

 in milder districts, if such there be, within the British 

 Isles, especially in the western districts and Ireland, while 

 others proceed to Southern Europe. During winters of 

 exceptional severity, and particularly when the entire 

 British area is affected and food therefore almost un- 

 obtainable, many birds which have endeavoured to brave 

 the storm with us perish in great numbers, even those 

 which have sought what are usually the mildest portions 

 of our islands — namely, the south-western portions of 

 England and Ireland. The species which are most 

 affected by these cold snaps include, among others, 

 the Mistle Thrush, Song Thrush, Redwing, Fieldfare, 

 Blackbird, Greenfinch, Starling, Skylark, Water Rail, 

 Lapwing, Curlew, Snipe, and Woodcock. 



These winter movements do not occur in the con- 

 tinental areas to a like extent. There the climatic 

 conditions of the season are more stable, and are not 

 subject to the ups and downs to be found with us. 

 Consequently such emigrations are more or less ex- 

 ceptional, and it is only in winters of remarkable severity 

 that the birds accustomed to winter in Scandinavia 

 and in Central Europe are compelled to quit their 

 retreats. When this does occur there is a renewal 

 on our shores of movements similar to the autumnal 

 influx of winter visitors from the north-east and east, 



