MIGRATION IN THE BRITISH ISIANDS. 265 



ings will unerringly foretell a snow-storm hours in 

 advance, and commence a nomadic migration to 

 escape its dangers. From all this constant Passage 

 one vastly important fact may be deducted. It 

 shows us how the individuals of a species are kept 

 well intermixed, and thus furnish those all-necessary 

 facilities for interbreeding that are the dominating 

 influences in preserving those species, and keeping 

 them up to their normal standards of constancy. 



We would fain have entered more fully into the 

 Migration of British birds, but we are compelled 

 with reluctance to confine our chapter to the limits 

 it has now reached. It would have been a very 

 easy matter to fill our volume with the stirring 

 story of their movements alone ; but we trust that 

 sufficient has been said to make the whole subject 

 of Migration reasonably and tolerably complete, 

 notwithstanding its unavoidable meagreness. The 

 following tables will give the reader some idea of the 

 general movements of birds in the British Islands. 



The student will also find appended Blank Tables 

 for the next eight years in which he may record, in 

 his own particular district, the Migration Flight of 

 certain common or fairly well distributed species 

 whose movements in Spring and Autumn may be 

 remarked by the most casual observer of natural 

 phenomena. 



perience of the Sky Lark (Alauda arvensis\ is that the bird 

 always shuns a valley, and frequents the most elevated pastures 

 during winter. 



