COMMON SWALLOW. 



87 



and within a day or two of retiring they assemble in 

 vast flocks on house-topSj the leads of chuixhes^and 

 on trees, especially by the water side, which latter 

 circumstance may have induced many persons to 

 think they went into the water : many of thero are 

 observed to stay behind for some time after the 

 general flight, but they are only the young and 

 weak birds, that are incapable of undertaking the 

 journey at that time ; but most probably the few 

 that are obliged to remain behind perish in the 

 ensuing winter through a want of food. 



They generally make their first appearance in 

 the early part of April, and occasionally thousands 

 die for want of food, should the weather prove 

 cold after their arrival. It has been observed by 

 Colonel Montagu, that all the Swallows in his neigh- 

 bourhood disappeared of a sudden, in the month 

 of April, when the thermometer was at 42 ; but 

 in a day or two he discovered them collected to- 

 gether hj hundreds in a valley, close to the sea- 

 side, near a large sheltered pool. 



This species is sometimes found entirely of a 

 snowy white colour, and is described by Brisson 

 as distinct on that account. 



