SOCIAL BACKBONED ANIMALS 133 



usually consist of well-contrasted markings on the wings and tail, 

 which are concealed during repose, but become fully visible when 

 the bird takes flight. Such markings are well seen in our four 

 British species of shrikes, each having quite different white marks 

 on the expanded wings and on the tail-feathers; and the same 

 is the case with our three species of Saxicola the stone-chat, 

 whin-chat, and wheat-ear which are thus easily recognizable on 

 the wing, especially when seen from above, as they would be by 

 stragglers looking out for their companions. . . . Those birds 



Fig. 1104. i, Lesser Tern (Sterna minuta], and 2, Ringed Plover (sEgialitis hiaticola] 



which are inhabitants of tropical forests, and which need recogni- 

 tion marks that shall be at all times visible among the dense 

 foliage, and not solely or chiefly during flight, have usually small 

 but brilliant patches of colour on the head or neck, often not inter- 

 fering with the generally protective character of their plumage. 

 Such are the bright patches of blue, red, or yellow by which 

 the usually green Eastern barbets are distinguished; and similar 

 bright patches of colour characterize the separate species of small 

 green fruit-doves. To this necessity for specialization in colour, 

 by which each bird may easily recognize its kind, is probably due 

 that marvellous variety in the peculiar beauties of some groups 

 of birds." If we admit the truth of the view just set forth, it 

 follows that, in many birds, comparatively slight differences in 

 plumage will often prove a safe guide in the discrimination of 

 species. Though trivial in themselves they may nevertheless 



