102 F. BOURLIERE 



We therefore find very much the same dichotomy in birds 

 as in mammals, between small and fast-breeding species on the 

 one hand and large and slow-reproducing ones on the other. 

 The highest ratio of mean to potential lifespan is indeed 

 found in a bird, the royal albatross, which reproduces only 

 every second year and may reach an age of at least 25 years in 

 the wild. 



Reproductive and mortality rates are thus closely adjusted. 

 Both vary mainly with ecological conditions and an increase 

 in mortality rate in older individuals, which may be due to the 

 onset of old age, is apparent only in large, slow-reproducing 

 and long-lived birds and mammals. 



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