THE RETURN OF THE felRDS 71 



be taken by other birds who come to Britain from the 

 Continent. 



The broad fact that birds breed in the coldest part of 

 their range implies that the Spring immigrants into a North 

 Temperate country come, on the whole, from the south 

 or south-east, and that the Autumn emigrants fly, on the 

 whole, towards the south. This very general statement is 

 not inconsistent with more detailed statements, for instance, 

 that many immigrants come to Britain from the Continent 

 in Autumn and may swell the throng of our partial 

 migrants. 



We have little secure knowledge of the precise winter- 

 quarters of those birds which spend their summer with us, 

 but the general fact of an autumnal movement from Arctic 

 and North Temperate Zones to lands nearer the Equator, 

 and of a reverse vernal movement, is certain. Of particular 

 interest are those cases, like that of the stork already 

 mentioned, where the birds " keep always on the summer 

 side of the Equator," where, in other words, they enjoy the 

 northern summer, and, fly ing across the Equator, find a second 

 summer in the Southern Hemisphere. 



It must be clearly understood that migration is more 

 than a mass-movement, such as is familiar in the case of 

 Pallas' s Sand Grouse, which has repeatedly invaded Britain 

 from the East in considerable numbers. Migration is to be 

 distinguished from gradual extensions of the range of 

 distribution, from tentative explorations in search of food, 

 and from invasions of new territory under urgent stress of 

 over-population. True migration is a regularly recurrent 

 oscillation between a place of breeding and nesting and a 

 place of feeding and resting ; it is an adaptation to the 

 seasons and to the conditions of reproduction ; the impulse 

 which prompts it is now instinctive, but the trigger is 

 pulled by external stimuli. 



