174 STUDIES IN BIRD-MIGRATION 



The greatest movements, or "rushes," as they have 

 not inappropriately been designated, in both spring and 

 autumn, are those which follow the advent of favourable 

 weather on the passing away of a more or less prolonged 

 spell of adverse conditions. These unfavourable periods 

 in the autumn are not unfrequently characterised by 

 great ungeniality, and this, no doubt, gives the summer 

 visitors warning that the time has arrived for seeking 

 milder climes in which to pass the winter. Upon the 

 duration and severity of the unfavourable conditions for 

 migration depends the magnitude of the exodus which 

 follows when the anticyclone removes the barrier, 

 releases the flood of pent-up migrants, and furnishes 

 ideal conditions for fliofht over the North Sea. When 

 such a sequence of weather changes takes place, it is not 

 surprising that vast rushes southwards follow, and that 

 our shores receive great waves of migrants, sometimes 

 for several successiv-e nights. 



The gentle barometric gradients for east to south 

 winds, with their fine weather, do not always, however, 

 entirely bridge, as it were, the North Sea between 

 the Continent and Britain. In the autumn, the favour- 

 able conditions which induced the birds to quit North- 

 Western Europe may not extend to the British shores 

 (nor to those of Scandinavia in the spring). Indeed, 

 it not unfrequently happens that the bird-voyagers pass 

 into more or less adverse conditions — high winds or 

 heavy rain, or both^ — ere our shores are reached, and 

 arrive in a correspondingly exhausted state. Occasion- 

 ally many perish during the latter stages of the passage, 

 and their bodies are cast upon our eastern coasts in 

 considerable numbers. In connection with these unfor- 

 tunate autumn flights across the North Sea, an examina- 



