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 singing very 
sweetly. Their nests are built of 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. 
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