182 STUDIES IN BIRD-MIGRATION 



rule, in March, and it may be remarked that the 

 thermal pecuHarities of the British area play an 

 important part in determining their geographical dis- 

 tribution. The remarkable fact that the great majority 

 of the summer visitors to our islands are first observed 

 on the shores of the south-west of England and 

 Ireland, has already been mentioned. This holds 

 good even in genial seasons, but in cold ones it is 

 almost entirely the case. Thus in March 1887, with 

 its monotonously low temperatures, the arrival of six 

 species, on twelve occasions, was recorded in the returns 

 made to the British Association's Committee, all for 

 the south-west. Again, in the cold March of 1885, 

 every record but one of the fourteen chronicled was 

 made in this same mild region of the British area. 

 During the exceptionally cold and stormy March of 

 1883, "^"^y one species — the Wheatear — was observed 

 on two occasions, both at stations on the west coast 

 of Ireland, where the temperature was highest. 



The Spring Emigratory Movements of birds which 

 have spent the winter in our islands and are returning 

 to their northern native homes, are influenced by the 

 weather conditions which prevail in the British area. 

 Here, it is found, other conditions being suitable for 

 the sea passages to the north or east, that increase in 

 temperature is again the main influencing factor, so 

 that upon it depends, to a considerable degree, the 

 extent of the departure movements. Thus it is not until 

 April, and especially May, that the decided departures 

 are recorded. In April the fine weather (anticyclonic) 

 periods have varying emigrational values which depend 

 entirely on their temperature. They are favourable if 

 characterised by high, or moderately high, temperatures ; 



