36 THE AGE OF BIRDS 



of cage-birds succumb to the ills engendered by a 

 sedentary life, but that any of them are able to retain 

 good health. The cases cited above indicate a life-span 

 considerably longer than that of mammals of corre- 

 sponding bulk. Considering how greatly nutrition is 

 interfered with by the decay, attrition or displacement 

 of teeth, it would seem that birds have gained rather 

 than lost in discarding the mouth-armature of their 

 reptilian ancestry. 



