CHAPTER IV 



THE DURATION OF YOUTH IN BIRDS AND 

 LOWER ANIMALS 



Birds show Nature in her most luxuriant and inventive mood. 

 There seems to be an infinite variety in size, habits, disposition and 

 colouring, and yet these many difierences conceal a similarity of 

 structure so close that there is a smaller gap between the ostrich 

 and the humming-bird, or between the flamingo and the wren, than 

 exists between many members of the same order of mammals. 

 For our present purpose they may be considered as a single group, 

 without reference to the divisions into which systematists have 

 been able to place them. I have already said that attempts have 

 been made to find some relation between the duration of the period 

 of youth and the whole Ufe. Such attempts would fail completely 

 in the case of birds. It is a curious fact that in proportion to their 

 size, birds are longer lived, or at least have a higher potential 

 longevity, than mammals. If we compare them, group by group, 

 mammals are much larger than birds, herbivorous mammals than 

 herbivorous birds, frugivorous mammals than frugivorous birds, 

 omnivorous and carnivorous mammals than omnivorous and car- 

 nivorous birds. And yet, group by group, birds approach or 

 surpass mammals in longevity. Passerine birds, which range in 

 size from minute creatures which, stripped of their feathers, are no 

 larger than the tiniest shrew-mouse, to the large ravens, have a 

 potential longevity ranging from twenty to sixty years. Owls and 

 parrots certainly can live for half a century, and eagles and vultures 

 much longer. Pelicans and storks may live for from fifty to thirty 

 years, ducks and geese much longer, pigeons and gulls for thirty 

 years, ostriches for fifty years. Compared with these figures the 

 duration of youth is always short, and ranges from about two to 

 three or four years. Ostriches, which are the largest living birds, 

 take from three to four years to become full grown and adult, but 

 birds-of-paradise take nearly the same time. Condors and the 

 larger birds-of-prey are as big as a hen when they are a month old, 

 but take rather more than three years to reach their full size. The 



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