Mammals and Birds : Lifespans of Wild Populations 99 



shows the survival curve of a cohort of these terns which were 

 banded as chicks in 1934. 



In this sample, the first year mortality was over 94 per cent 

 and the mean annual mortality rate from the fourth to the 

 18th year averaged 26 per cent. Beyond the 18th year the 

 curve continues downwards, showing a steady increase in a 

 mortality rate that had remained a straight line during the 

 previous 14 years. The continuous rebanding of all the older 

 birds handled during this study reduces the possibility that 

 this sudden increase in the death rate can be explained by 

 band loss ; the Austins therefore consider that it might be due 

 to senility. The composite life-table based on all the returns 

 of common terns banded as chicks and trapped in nests by 

 these observers, 1940 through 1955, shows the same increase 

 in the death rate after the 18th year. 



This tendency towards an increased mortality rate in old 

 long-lived birds may be counteracted by a progressive im- 

 provement in the survival rate as the birds get older, at least 

 in the species which are heavily hunted by man. Inexperienced 

 immature or young adults appear to be shot much more 

 frequently than older ones. Such an improvement in the 

 survival rate of older individuals is quite apparent in the 

 survival curve of Scandinavian buzzards (Buteo huteo)^ com- 

 mon herons (Ardea cinerea) and tawny owls {Strix aluco) 

 drawn by Olsson (1958) and shown in Fig. 5. 



In small and short-lived passerines, the situation is very 

 similar to that of the small rodents and shrews, and the 

 population turnover is very rapid. The mortality rate is 

 always very high, especially at the nestling stage and in the 

 first four months of life. Summers-Smith (1959) has found, for 

 instance, a mortality rate of about 87 per cent for the juveniles 

 (one to four months old) and about 40 per cent for the adults 

 in the urban populations of house sparrows {Passer domesticus) 

 he has studied. We have found even higher figures in a tropical 

 population of the red-billed fire finch {Lagonosticta senegala) 

 now under study in the lower Senegal valley. Kluijver (1951) 



