Heritability of Ecologically Important Traits in the Great Tit

Abstract

Ecologically important characters in birds, such as clutch size, date of laying and body size, vary within and between populations and species. The mean values of these characters are con- sidered to be fixed by natural selection on values ensuring a maximum reproductive output over a life-time. For clutch size this implies that more eggs may result in less offspring. Lack (l954a) showed that mean clutch size is fixed at an op- timal number: when this number is exceeded the number ofyoung dying from starvation is so large that the productivity ofthe brood decreases.