The chief architect of the selfish gene theory of family planning was the great ecologist David Lack. Each bird species tends to have a typical clutch size. Superficially it might seem that the gene for hating four eggs is bound to have an advantage over the genes for laying three or two. In other words it leads logically to an absurdity. Obviously there are costs as well as benefits in laying a large number of eggs.
Increased bearing is bound to be paid for in less efficient caring.
Lack would say each selfish individual chooses the clutch size that maximizes the number of children she rears.