I have my first broody hens (3 at once) sitting on a total of 11 eggs some are EE some Australorp. and a few mixes 21 days will be up for 8 of them this thursday and the rest Saturday I'm guessing, because my Silkie took a few days to settle down after I moved her when I first got the fertile eggs. I have not candled at all but 1 egg cracked (crushed ) last week and the chick inside was nicely formed to that point. Sorry for her but happy they are fertile. Is 21 day pretty much a sure thing or does it vary?