Age-specific reproductive performance of black grouse Tetrao tetrix females

As part of a demographic study of Black Grouse ill the southern French Alps , we Estimated variations in reproductive parameters ill relation to age of females. Both yearlings and adults initiated a clutch, but adults raised a bout eight times more young (yearlings 0.25 chicks/females, adults 1.85 chi cks/female). Yearlings laid significantly fewer eggs and initiated incubati on later than older females. Success of nests did not differ between the tw o age classes, but yearlings tended to re-nest less frequently and exhibite d higher chick loss than older females. The lower reproductive success of y earlings was primarily due to their lower ability to raise young. At nest s ites, nest concealment, as well as vertical and horizontal structure and de nsity of the vegetation, did not differ between yearling and older females, which is consistent with the similarity in nesting success in the two age classes. Results support the hypothesis that the extent of the difference i n number of young raised by yearling and older females increases with preda tion pressure.