264 



HOW DO WE CLASSIFY ANIMALS? 



uses in flight, a large quantity of fuel which will oxidize quickly 

 must be used. Birds are large eaters, and the digestive tract is 

 fitted to digest the food quickly. As soon as the food is absorbed 

 by the blood, it may be sent rapidly to the places where it is 

 needed, by means of the strong four-chambered heart and large 

 blood vessels. 



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

 oxidation; furthermore, the feathers and the oily skin form an 



insulation which 

 does not readily 

 permit the escape 

 of heat. This in- 

 sulating cover is 

 of much use to the 

 bird in its flights 

 at high altitudes, 

 where the temper- 

 ature is often very 

 low. 



The nervous 

 system and the 

 senses. The cen- 

 tral nervous sys- 

 tem of a bird is 

 well developed. 

 Attached to the 

 fairly large brain 

 is the spinal cord 

 which extends the length of the body. From this cord nerves are 

 given off. Sight is probably the best developed of the senses. 

 The keen sight of a hawk is proverbial. Hearing is also well 

 developed in most birds. The sense of smell does not appear to 

 be well developed, and is especially deficient in seed-eating birds. 

 Nesting habits. Among the most interesting of all instincts 

 shown by birds are those of nest building. Birds incubate their 

 eggs, that is, hatch them, by the heat of their bodies. Hence 

 a nest is needed. The ostrich is an exception; it makes no 



Wright Pierce 



The sharp-edged, chisel-like bill of the woodpecker made these 

 holes in the tree. Red-headed woodpeckers also have the un- 

 usual habit of storing nuts of various kinds in the crevices and 

 holes they make in the bark of certain trees. 



