Birds and Saliva. 331 



general colours of the trees which they select. If it is an 

 alternation of green moss, yellow lichen, and ruby-tinted 

 cups, with here and there a spot of black, then the green 

 woodpecker comes in charge; but if it is the black and 

 white lichens of the alpine forest, then we may look for the 

 spotted race upon the bark." 



The glutinous secretion which we find in practical service 

 in so many birds deserves attention. The swallow uses it 

 in nest-building, the nightjar uses it to assist it to keep in 

 its mouth the moths or beetles it has caught in flight till 

 with them it can feed its young ; the woodpecker uses it 

 for the purpose we have just seen, and the nuthatch with 

 it gums the clay with which it reduces to true proportions 

 the entrance to its nest. The edible nests of China, which 

 are an article of commerce, are chiefly composed of this 

 glutinous secretion which the birds use to supply the lack 

 of other materials for their nests ; and evidently the king- 

 fisher uses something of the same kind to unite together, 

 however flimsily, the fish bones of which he forms his nest. 

 And is it not likely that the chaffinch uses something 

 of this in supplement to the spiders' webs, in so neatly 

 cementing the " lichen " over the outside of its nest ? Is 

 it not possible that before differentiation this was even a 

 more important element than it is now ? 



