406 



THE BIRDS 



rounding temperature ; if the air is cold the blood will be cold, if the air 

 is warm the blood will be warm. 



Warm-blooded animals, on the other hand, have a constant high body 

 temperature, and the blood and the rest of the body remain at about the 

 same temperature regardless of the surroundings. If the air is cool, 

 extra heat will be generated through metabolism to keep the body warm. 

 Birds will have a normal body temperature of about 105 degrees, which 



FILO 

 PLUME 



BARBULES 



DOWN FEATHER 



BARB' 



SH/^FT FLIGHT FEATHER 



Biology, the Science of Lije, McDousall and Hegner, Mc-Graw-Hill Book Company, Inc. 



Fig. 27.1. Types of feathers. 



is nearly always above that of their surroundings, so much of their food 

 is burned for heat rather than energy. Since the reptiles, as an example 

 of cold-blooded animals, burn their food primarily for energy, the birds 

 must eat much more than reptiles. An average snake will not eat oftener 

 than once a week, but birds seem always to be hungry ; chickens seem to 

 be scratching and pecking around constantly in search of food. 



Birds have streamlined bodies. They are flying animals as a group, 

 and we find nearly every part of the body modified in some way to ac- 

 commodate flying. Their streamlined bodies offer a minimum of re- 



