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 Pires 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 
nN 
Fig. 1104.—1, Lesser Tern (Sterna minuta), and 2, Ringed Plover (2 gialitis 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 
