We need to deal with this to prepare ourselves for studying the evolution of such traits. Each duck started with a clutch size of 10 eggs. We saw some genetically caused variability in our population. The magnitude of this variability was related to the allele frequencies and effects on the phenotype. For each non-face card allele, they subtracted 1 egg.
More genetic variability causes more phenotypic variability.
The variability we see in our 5 duck population is caused by both nature and nurture. We are stuck with figuring out, using a few simple tricks, what the heritability of a trait is. Therefore, I'd expect the heritability of this trait to be low. We use the time-honoured trick of exploiting the known proportion of the entire genome shared between relatives to figure out heritability. Look at two examples of estimating heritability from mz twins, Extrovertism and Geniality .