FAIR ISLE, THE BRITISH HELIGOLAND 47 



arrest during stressful periods, or, again, its even flow 

 under ordinary conditions : in other words, the island 

 has afforded the opportunity of correlating the divers 

 movements with the weather conditions, and ascertaining 

 what the meteorological incentives, checks, and barriers, 

 as the case may be, to migration are — a knowledge of 

 the relations existing between the two sets of phenomena 

 which was highly desirable. 



This superiority of Fair Isle for these important 

 researches arises from its isolation and its small size ; 

 otherwise it has no advantages over the other Isles of 

 the Shetland and Orkney groups. In both these archi- 

 pelagos, however, the islands are many, not a few of 

 them are large, none are far apart, and hence the 

 migrants visiting them are widely and thinly scattered, 

 and in this way the great majority of their bird-visitors 

 entirely escape notice. Fair Isle itself, with its 2J 

 square miles of varied surface, and its extensive belt of 

 lofty cliffs, Is too spacious even for several observers ; and 

 there were many days, when migrants were abundant, 

 that we were conscious that In spite of strenuous and 

 unremitting endeavours much had been missed — Indeed, 

 that the great majority of the visitors had not come 

 under notice. 



The experience gained during many vacations spent 

 in some of the most favoured observlng-stations In the 

 British Isles and elsewhere, has convinced me that we 

 see an infinitesimal portion of the migrants which visit 

 our shores. This is especially the case on the mainland, 

 with Its vast extent of coastline, its enormous acreao-e 

 of enclosed ground, and its extensive woodlands and 

 other forms of cover. On reaching the mainland, the 

 migrants, particularly the Passerine birds, seek suitable 



