CHAP. XXIII. 



MIGRATORY BIRDS. 



301 



Ust-Zylma district, leaving out those to which, from their 

 rarity or localness, considerable doubt attaches as to their 

 date of arrival : — 



This list is necessarily very imperfect. In addition to the 

 difficulty of ascertaining the date of arrival of rare or local 

 birds, we had a still greater difficulty to contend with. There 

 can be no doubt that Ust-Zylma lies somewhat out of the 

 line of migration, which is probably determined largely by 

 the direction of the great valleys. Birds from the Mediter- 

 ranean might fairly be supposed to reach the Volga via, the 

 Bosphorus, the Black Sea, the Sea of Azov, and the river 

 Don to Sarepta. The natural course of birds from India 



