Introduction 1 1 



lines were more or less followed has, in the minds of 

 most people, given way to the certataty that the flocks 

 gradually spread over large areas, and that considerable 

 deviations occur, for which at present it seems im- 

 possible to account. Many birds, such as our Swallows, 

 are seen to collect together some time before they leave 

 us, others, such as Woodcocks, arrive simiiltaneously in 

 large flocks, while close observation soon shews that a 

 very great number of other species act similarly, and 

 that " rushes " continually occur, which are most 

 strikingly witnessed at Lighthouses. Migration, how- 

 ever, takes place largely at night. 



The causes of migration have been a fruitful source 

 of discussion, but there is a general agreement that 

 changes of temperature and the available food-supply 

 are the most effective. Exceptionally hardy birds, 

 such as Penguins in the southern oceans and the 

 Spitsbergen Ptarmigan in the north, especially if they 

 live on the sea or in districts thinly populated by their 

 kind, need hardly migrate at aU; but, as regards the 

 more delicate forms, the colder weather that begins in 

 autumn might of itself be sufficient to drive the birds 

 from their breeding quarters. This colder weather also 

 diminishes the supply of insect food, while the season 

 of berries and other fruits soon comes to an end, and 

 even the smaller mammals on which some birds feed 

 almost cease to be seen. It does not follow that 

 northern species stand in need of very high tempera- 

 tures during the winter; if so, they would probably 

 stay in great numbers near the equator, instead of so 

 constantly passing further to the south, and, as already 

 stated, the more adaptable species, specialized to that 

 effect, may remain throughout the year in Arctic or 



