Group size preference.
 
Charles and Mary Brown studied heritability in the cliff swallow, which build nests in colonies of various sizes.
 
Group size in which individual swallows lived was statistically similar to the group size in which their parents lived, even for offspring who bred away from their natal site, indicating strong heritability of this trait.
 
The expression of a complex behavioral trait such as group size may involve may genes located in quantitative trait loci (QTL).