THE PRINCIPAL BIRD GROUPS 81 



in size, the largest being the Crowned Pigeons, 

 which are nearly as big as a Goose, and the 

 smallest are included in the Doves, and are 

 scarcely as large as a Skylark. The Pigeons 

 all bear such a " family resemblance " that none 

 of them can be readily confused with any other 

 birds. Perhaps their most striking external 

 feature is the somewhat Plover-like bill, enlarged 

 and hardened at the tip (which is modified into 

 a decided hook in the Tooth-billed Pigeon) and 

 covered at the base with soft skin, in which the 

 nostrils, partly concealed by an incumbent valve, 

 are situated. The metatarsus is more or less 

 feathered (except in some of the small Doves) ; 

 the anterior toes are moderate in length, but 

 the hallux is small. The plumage is dense and 

 compact, and the hyporhachis is absent from 

 the contour feathers or rudimentary only. It 

 is impossible here to give the many features of 

 the various families and genera. All that we 

 can say is that the predominant colour is grey 

 or brown, often loricated on the neck and 

 breast with metallic sheen, but in some genera 

 the birds are brilliantly arrayed in purple, 

 gold, green, and red. Pigeons as a group are 

 arboreal in their habits, but some species show 

 a partiality for rocky or even barren districts, 



