Genetic and Environmental Variation in Clutch Size of the Great Tit (Parus major)

An analysis is made of genetic and non-genetic components in the number of eggs in the completed first clutch of the Great Tit.1) It is doubtful whether a causal relation between density and clutch size exists.2) There is hardly a systematic effect of age on clutch size.3) There is an annual variation in clutch size with similar changes in individual females in the same population, but it is hardly correlated between populations.This emphasizes a lack of genotype-environment interaction.4) Within populations there is no detectable variation in clutch size that can be attributed to differences in habitat quality.5) About 40% of the total phenotypic variation in clutch size is genetic variation.Several ways of eliminating a possible resemblance through correlated environments yield the same result.6) Selection for clutch size is demonstrated in several years.7) The implications for rapid evolutionary change in mean clutch size are discussed.