Genetic variation in offspring condition: An experiment

1. Body condition, reflecting size of the subcutaneous fat reserves at the end of the nestling period, is a good predictor of subsequent survival in nestling passerines. While the condition has traditionally been viewed as a highly plastic trait reflecting the nutritional state of individuals determined by the quality of parental care, nothing is known about the genetics of this trait. 2. The heritability and underlying genetic, environmental and genotype-by-environment interaction components of variation in nestling body condition were estimated in the Collared Flycatcher, Ficedula albicollis. To distinguish between genetic and environmental causes of resemblance, a reciprocal split-brood design was applied where full-sibs were reared apart in two different experimentally created environments. 3. Nestlings reared in enlarged broods were in poorer condition than their full-sibs raised in reduced broods, indicating a large environmental component to phenotypic variation in body condition. 4. However, there was a significant additive genetic component to variation in body condition in all environments, but no evidence for genotype-environment interactions. As found earlier for life-history traits closely related with fitness, heritability estimates (in both experimental environments) were. generally low (h(2) approximate to 0.30) and sometimes not significantly different from zero. 5. These results suggest that environmental influences, e.g. amount and quality of received food, are major determinants of offspring condition. However, the small but significant additive genetic component suggests that there are also inherited differences in offspring viability and survival potential.