Role of Heredity in Egg Size Variation in the Great Tit Parus major and the Pied Flycatcher Ficedula hypoleuca

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Abstract
Phenotypic correlations between body size and egg dimensions were poor in the great tit and pied flycatcher populations studied in northern Finland. Heritability of egg volume was high in the great tit, 0.86 .+-. 0.29. The phenotypic effect of a female upon egg dimensions was great, about 60% of total variance, and this was interpreted as the approximate effect of additive genetic factors. The age of the female had slight effects on egg dimensions. Clutches of different years and the 1st and 2nd clutches of the same year had similar egg dimensions, although clutch size decreased significantly during the season in the great tit. Egg size as well as the effect of non-heritable factors on egg size increased in replacement clutches laid shortly after the 1st clutch. The high heritability of egg dimensions implies that egg size should rapidly respond to differential selection pressures, but geographical variation in egg size has not been found in the species studied. Selection pressures affecting the adaptive variation of egg size in the English [UK] great tit population differ from those operating in northern Finland.