ii8 



The Bird 



changing starch to sugar and in many other ways making 

 ready the food, that the important changes which take 

 place in the stomach may begin at once. In birds, how- 

 ever, the sahva has but Httle chemical effect on the food, 

 its principal use being to moisten the substances before 

 they are swallowed. 



It is not often that Nature, when she has produced 



Fig. 90 — Nest of Chimney Swift; twigs glued together with sah'va. 



an organ or special tissue by the elaborate synthesis of 

 evolution, confines its use to an}^ one function. If birds 

 were provided with salivary glands intended only for the 

 purpose mentioned above, they soon found other uses for 

 them. In a woodpecker we will find very large salivary 

 glands on each side of the mouth. These secrete a sticky 

 liquid w^hich covers the long, many-barbed tongue and is 

 an efficient aid in picking out insects from their holes in 

 the bark and wood of trees. 



