If the eggs are serama be prepared for them hatching sooner than the 21 days; I would suggest that you might want to do lockdown a bit sooner than with other chicken breeds. The serama I hatched this spring were in a mixed clutch with silkie eggs. The serama hatched on the 19th day and the silkies hatched on the 21st day. I do know that serama vary as to how many days it takes to hatch. You might want to question the source on this; again, providing the eggs are serama.