My kids and I have been splitting clutches to rear some of the eggs in incubator and some for hen-hatching. Incubator start was so both treatments hatched on the same day, or at least very close. Each time the incubator and hen has eggs from more than one clutch. Genetic markers (down coloration) allowed consistent accurate ID of parentage. We may be seeing where the incubator-hatched chicks dry out a lot slower than the hen-hatched chicks.