SEPTEMBER. 145 



LARGE FAMILIES MEAN SHORT LIVES. 



There is, however, a sad side to this advantage of 

 rearing several broods, for in spite of it the swallows 

 and martins no more than hold their own among 

 other species in the struggle for existence, and it is 

 therefore evident that more of them die between 

 autumn and spring than of other birds which rear 

 fewer young in summer. Indeed, in the case of all 

 living things in a state of nature, you may be sure 

 that their rate of mortality corresponds to the number 

 of young which they produce. Thus a guillemot, 

 which lays only one egg, must live at least ten times 

 as long, on the average, as a wild duck, which lays a 

 whole nestful ; while the risks of the swallow, with 

 two or three broods, must be several times as great 

 as those of the butcher-bird, with only one. 



WHY SOME BIRDS ARE RARE. 



That it is a disadvantage, however, in the long 

 run for birds to rear only one child in a season is 

 plain, from the fact that this is the class of bird 

 which tends to become extinct whenever man or any 

 other enemy reaches its breeding-haunts. Living 

 and breeding on almost inaccessible precipices, the 

 guillemot is still abundant in places ; but all over 

 the world the common birds are those which rear 

 several broods of four or five each in a season, show- 

 ing that, other things being equal, this is the best 

 system ; though we have still to learn why some 

 creatures are very common, while others, differing 

 hardly at all in structure and habit, are very rare. 



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