THE VERNAL MIGRATION 17 



example, the mallard), there also co-exist locally-resident 

 races which, to that extent, vary the general rule. I will, 

 later in this book, endeavour categorically to distinguish 

 each of these two classes — that is to say (i) those which 

 are absolutely stationary, and (2) those species which 

 have both a resident and a migratory race. 1 



As already pointed out, we have in this latitude 

 a numerous class whose annual movements it is less 

 easy to follow with precision, owing to the overlapping 

 in our country of the belts of land which form respectively 

 their summer and their winter quarters. Thoroughly to 

 understand the movements of such birds, it is necessary 

 to ascertain their geographical distribution at the different 

 seasons. In other words, we must go outside our own 

 country — often far outside of it — in order to ascertain 

 the limits of their summer and winter ranges. 



Thus, if, for example, we find that a given species 

 occupies during summer the whole area from Siberia to 

 Northumberland, while its winter range extends from 

 Shetland to Morocco, it follows that the average annual 

 range amounts to some 2000 miles. Assuming — and it 

 appears to be a reasonable inference — that the range of 

 each individual is approximately equal to that of the 

 general body, it is easily demonstrable on these lines that 

 most of those species which are popularly regarded as 

 resident British birds are in reality foreign migrants to 

 the extent of 1000 or perhaps 2000 miles twice every 

 year. Many people refuse to believe that their common 

 homely thrushes and starlings are quite as much migrants 

 as are the swallow and the cuckoo ; but the logic of 

 ascertained facts shows that such is the case. 



Thoroughly to realise the universality of migration, I 



1 See Chapter XII., p. 147. 



B 



