170 



BRITISH BIRDS. 



They are gregarious in their habits^ and often congregate into enormous 

 flocks to perform their migrations. Swallows perch but little^ and spend 

 most of their time in the air, except when incubating or sleeping. Their 

 powers of flight are very great ; but on the ground they can only progress 

 in a very awkward manner, owing to the shortness of their tarsi. Their 

 usual note is a twitter, but many species are also capable of singilig very 

 sweetly. Their nests are built o£ mud, straw, feathers, &c., and placed on 

 beams or ledges in buildings, under eaves, in caves, and on rocks ; but 

 some species burrow deep into earthy cliffs, and make very slight nests at 

 the end of the passage. Their eggs are white, sometimes unspotted, but 

 more frequently marked with rich brown and lilac-grey. Their food consists 

 entirely of insects. 



