286 A HISTORY OF BIRDS 



this we might infer that this species at say from fifteen to twenty- 

 years has become worn out and shortly afterwards dies ; since 

 among the lower animals the exhaustion of the reproductive 

 period is more or less speedily followed by death, a necessary 

 sequence to prevent the competition of the now useless — to the 

 species — individuals with the younger and fruitful members. 

 But it is known that the Eagle is a long-lived bird, attaining an 

 age of from sixty to one hundred years, and requiring, according 

 to some authorities, ten years to reach maturity. So far then, 

 the value of the pigmentation of the shell, in so far as it con- 

 cerns this species, does not seem to be of any very reliable 

 character, though it must not be forgotten that this calculation 

 is based on a single instance. But it is doubtful whether in 

 any case it will ever prove of value. 



So far, then, once the adult plumage has been assumed, there 

 is no possibility of estimating the age to which a bird lives, save 

 the rough index afforded by the number of young produced 

 annually by each species. 



Though but few of us realise it, the death-rate among birds 

 is a very high one, and is especially high during the first few 

 weeks of life, as we shall show presently (p. 288). So then, for 

 any given species to maintain itself one of two courses must 

 be followed. The members thereof must annually produce a 

 large number of offspring during a few years and die; or 

 they must live long and produce annually but few offspring. 

 It has been calculated that, at the lowest figure, the life of the 

 Golden Eagle is sixty years, and that it will retain its repro- 

 ductive activity for fifty years. During this time, of three nest- 

 lings produced annually by any given pair more than two will 

 rarely, if ever, leave the nest ; so that, estimating the number of 

 young which they succeed in launching on the world during 

 fifty years at a hundred, not more than one pair will arrive at 

 maturity ! The fecundity of a species is an approximate index 

 of its longevity in short : while the estimated longevity and the 

 number of young produced annually give a rough index of the 

 mortality which any given species has to provide against. 



Few grasp, probably, the huge numbers of the birds which 

 die violent deaths, compared with which the inroads made by 

 disease are insignificant, while death from senescence is probably 

 rare indeed. 



