CHAPTER XIII 



Special physiological peculiarities of birds — ^Longevity — Tempera- 

 ture of body — Change of colour in bare skin of some, such as 

 Turkey — ^The phenomena of the moult — Gradual change in 

 colour of biU and feet according to age and sex or season 

 — Changes in iris colour — Beak-sheath shedding as in Puffin. 



Birds are by far the most perfect of living machines 

 that evolution has as yet produced, for although 

 they do not nearly equal reptiles in longevity, they 

 live much more intensely and exhibit incomparably 

 more activity, vv^hich must fairly be taken into 

 account. In length of life, however, they are 

 vastly superior to most of the mammals, though, 

 as in so many other cases, length or shortness of 

 life is a group-character, and in birds of the same 

 group is related in many cases to the size of the 

 species. The evidence goes to show that the 

 longest-lived birds are the larger species of Parrots, 

 birds of prey, and waterfowl, but it must be 

 remembered that the data are necessarily obtained 

 from birds in captivity or domestication, and that 

 Igrge species are generally easier to keep and less 

 liable to accidents than small ones. 



But there is no record of any small bird of the 

 above groups attaining even half the age that is 

 recorded, for instance, for the Lemon-crested 



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