No, that wouldn't have an impact on birth size (they can only become infected with it after they are hatched. Three eggs from one pair is extremely rare, so the most likely scenario is another hen laid an egg in their nest. It would come down to the genetic makeup of whoever laid the egg and whoever sired that egg. It could be some genetic defect, it could be a genetic throwback to some smaller breed that was a part of those ferals... whatever it is, the baby is certainly not normal in it's development.Would that impact size from birth? It hatched out much smaller than the other 2.