100 



F. BOURLIERE 



has found an average annual adult mortality of 49 per cent 

 for great tits (Parus major) in Holland. 



The lowest adult mortality rate for small birds is found 

 among swifts, where it averages only 18 to 20 per cent per 



1000 



500- 



100 - 



5 10 



AGE IN YEARS 



Fig. 5. The number {Ix) of surviving buzzards, 

 herons and tawny owls at beginning of age group 

 {x). Abscissa : Age in years. Ordinate : Numbers 

 of surviving individuals, on a logarithmic 

 scale. 



year. It should be remembered that swifts, like bats, have a 

 very poor temperature regulation. 



The lowest figures in the whole class of birds are those of 

 two sub-antarctic birds. In the yellow-eyed penguin (Mega- 

 dyptes antipodes) population studied by Richdale (1957), the 

 adult mortality rate was only 12-9 per cent per year; in the 



