BIRDS 



297 



through the lungs into hollow spaces filled with air, which are 

 found between the organs of the body. 



The high temperature of the bird is a direct result of this rapid 

 oxidation and also because the feathers and the oily skin form an 

 insulation which does not readily permit of the escape of heat. 

 This fact is of much use to the bird in its flights at great altitudes, 

 where the temperature is often very low. 



The Nervous System and the Senses. — The central nervous system is well 

 developed. A large forebrain is found, which, according to a series of 

 elaborate experiments with pigeons, is found to have to do with the 

 conscious life of the bird. The cerebellum takes care of the acts which are 

 purely mechanical and are concerned with the living (digestion, absorption, 

 beating of the heart, etc.) of the bird. 



Sight is probably the best developed of the senses of a bird. The keen- 

 ness of vision of a hawk is proverbial. It has been noticed that in a bird 

 which hunts its prey at night, the eyes look toward the front of the face. 

 In a bird which is hunted, as in the dove, the eyes are placed at the side of 

 the head. In the case of the woodcock, which feeds at night in the marshes, 

 and which is in constant danger from attack by owls, the eyes have come to 

 lie far back on the top of the head. Hearing is also well developed in most 

 birds; this fact may be demonstrated with any canary. 



The sense of smell does not appear to be well developed in any bird, and 

 is especially deficient in seed-eating birds, most seeds having little odor. 



Nesting Habits. — Among the most interesting of all instincts 

 shown by birds are the habits of nest building. We have found 

 that some invertebrates, as spiders and ants, protect the eggs when 

 laid. In the vertebrate 

 group some fishes (as the 

 sunfish and stickleback) 

 make nests for the depo- 

 sition of the eggs. But 

 most fishes, and indeed 

 other vertebrates lower 

 than the birds, leave the 

 eggs to be hatched by the 

 heat of the sun. Birds 

 incubate their eggs, that 

 is., hatch them by the 

 heat of their own bodies. 



Nest of a pho-be under tin- b;irii tl<>'>r. 



