The influence of female age on breeding in the Eider Somateria mollissima

Abstract

The effect of female age on the breeding biology of the Eider was investigated at the Sands of Forvie, Scotland, between 1977 and 1979 using marked birds of known age. Laying dates were estimated from the relationship between fresh egg weight and egg volume index. Female weights at the start of incubation were estimated from regressions of female weight on number of days of incubation. No females bred when one year old; 26% of two year old females and 42% of three year olds did so. Most females over three years of age attempted to breed. Young females laid later, had smaller clutches and were lighter in weight at the start of incubation than older birds. Mean egg weight and hatching success did not vary with female age, but the latter result must be treated with caution as successful females were more likely to be identified than unsuccessful ones. No decline in the breeding performance of older birds was detected, but the oldest brids in this study were only 17 years old, which is below the average adult life expectancy for this species.