12 BIRD-LIFE OF THE BORDERS. 



class whose annual movements it is difficult to ascertain with 

 precision, owing to the overlapping, in our country, of the 

 belts or zones of land which form respectively their summer 

 and their winter quarters. In order to arrive at a general 

 understanding of 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 it — in order to ascertain the 

 limits of their summer and winter ranges. 



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

 spread during summer over the belt of land from Siberia 

 to Northumberland, while its winter range extends from 

 Shetland to Morocco, it follows that the average annual 

 range of the species in question amounts to some 2,000 

 miles. This of course assumes — and it appears to be a 

 reasonable inference — that the range of each individual is 

 approximately equal to that of the general body. Regarded 

 in this light, it is easily demonstrable that many (or most) of 

 the species which are popularly regarded as resident Britons, 

 are in reality foreign migrants to the extent of a thousand, or 

 perhaps two thousand miles twice every year. Many people 

 will perhaps find it difficult to believe that their common 

 homely Thrushes and Starlings are quite as strictly (and 

 almost as extensively) migratory as are the Swallow and 

 the Cuckoo ; but the logic of ascertained facts shows that 

 this is unquestionably the case. 



Now, why do birds migrate ? The question, at first 

 sight, appears a simple one, and several answers will pro- 

 bably at once suggest themselves. In reality, however, it 

 will be found a many-sided biological problem, and one of no 

 small complexity and mystery. 



Suitable climatic conditions, temperature, food-require- 

 ments, and distribution in proportion to food supply, are 

 among the more obvious answers to the question. These 

 and similar circumstances iiijiuence and, no doubt to a 

 great extent, rcriuhite migration. But, on examining a little 

 more deeply into the subject, it is clear that, though in many 

 cases they form main regulating factors of migration, yet 

 they are not its primary cause. 



