R3LATIVR VIADILITY IN MAMMALS AND BIRDS. 53D 



shorter lives. Owls liave lai'ge cfeca,, Eagles and their allies have 

 vestigial cteca, and Owls in propoi-tion to their size appear to 

 have lower potential and average longevities. 



Many large birds such as Eagles, Yultui'es, Pelicans, Flainin- 

 goes, and Cranes, can attain great ages, but similar ages can be 

 attained by much smaller birds such as Parrots and Crows. In 

 proportion to their size, it is quite clear that Passeres have the 

 longest potential and average durations ; and that Parrots, 

 Pigeons, and Limicolous birds, similarly in proportion to size, 

 come next. There are few generalizations mox-e difficult to 

 support with convincing argument than those which relate to the 

 I'elative degrees of specialization to be assigned to natural groups 

 of animals, but I think that many ornithologists would agree with 

 ine if I were to say that Passeres, Parrots, Pigeons, and Limicolous 

 birds were relatively high types, staii ding faraway from whatever 

 we may imagine the primitive bird-type to have been. And I 

 think that the most certain general conclusion which I may 

 draw regarding the relative longevity and viability of the groups 

 of birds is, that in proportion to size, longevity and viability 

 increase with perfection of organization. The further a bird has 

 advanced along the lines of evolution of the bird gi'oup, the more 

 viable it is. No doubt shortening of the hind-gut, reduction of 

 the number of eggs in a clutch, increased care of the young, may 

 all be taken as stages in the perfection of the bird-type, and so 

 have a general associa,tion with increased longevity, apart from any 

 direct influence which any one of them may have. 



Viahiliiy, Temperature, and Open Air. 



I cannot see that there is any relation between the climate 

 from which birds come and their viability in captivity. Nor 

 would such a relation be expected on general grounds, for birds, 

 by their covering of feathers and the perfection of their vascular 

 and respiratory systems, are well adapted to resist exposure to 

 changes in temperature. If food be abundant, and if there be 

 protection from draught, they seem remarkably indifferent to the 

 temperature of the air. The interpretation, however, of their 

 viabilities is obscured by special factors which existed in the period 

 with which the figures deal, and which exist in most menageries. 

 The comparison that is to be desired is a direct contrast between 

 the viabilities of birds kept in the interior of a warmed house, and 

 birds kept with little or no artificial heat and free exposure to the 

 open air. The numbei'S of species and of individuals usually kept 

 in captivity is much greater in the case of birds than in the case 

 of mammals, and from the smaller size of birds and their more 

 gregarious habits, it is usually possible to keep numbers of 

 individuals of the same or of different species in the same aviaiy. 

 The most fainiliar conti-ast in the conditions under which birds 

 are kept, is that between one or two individuals in a small cage in 



