520 EVOLUTION OF BIRDS xvm. 7 



any one individual female lays mostly in the nests of a single foster 

 species, in England often the meadow-pipit or hedge-sparrow. She 

 watches the building of the nest and lays her egg on the same day as 

 the foster parent, removing one of the clutch before she does so. 

 Often about twelve eggs are laid in this way, each in a different nest ; 

 even more have been recorded. The eggs are usually strongly mimetic 

 with those of the host, variable in colour, more so when varied host 

 nests are available. The young hatch before the host eggs, which are 

 then ejected from the nest by the young cuckoo. 



Order 13. Psittaciformes. Parrots 



The parrots are birds found mainly in warm climates, living among 

 the trees and having many special characteristics. With the crows, 

 they are usually reckoned to be the most 'intelligent' birds and cer- 

 tainly have considerable powers of memory. They are predominantly 

 vegetarian and some, though by no means all, make use of the beak 

 for breaking open hard shells. The eggs are usually laid in holes and 

 are white and round. The period of parental care after hatching is 

 unusually long (2-3 months). 



Order 14. Strigiformes. Owls 



The owls, specialized for hunting at night, resemble the hawks, by 

 convergence, in their beaks, claws, and in other ways. The food is 

 swallowed whole. They probably detect their prey mainly by sound, 

 and show various specializations in the ears. The eyes contain mostly 

 rods and are directed forwards ; they are very large and they cannot be 

 moved in the orbits, the movements of the neck compensating for this 

 restriction. The feathers are so arranged as to make very little noise 

 in flight. The eggs are white and laid in holes or in the old nests of 

 other birds, some on the ground. Many genera are recognized from 

 all parts of the world, examples being the barn owls (Tyto) and the 

 eared owls (Asio). 



Order 15. Caprimulgiform.es. Nightjars 



The nightjars (Caprimulgus) are a rather isolated group of cre- 

 puscular birds, feeding on insects taken on the wing. Two mottled 

 eggs are laid on the bare ground. 



Order 16. Micropodiformes . Swifts and Humming-birds 



The swifts (Apus) and humming-birds (Trochilus) are perhaps more 

 fully adapted to the air than are any other birds. The wings are very 



