You are using a broody hen. I've had eggs hatch a full two days early under broody hens. Others I trust on here have had eggs hatch two or even three days late under a broody. There are different reasons for that, it's not only incubating temperature. I've had hatches where one chick was a full day earlier than the rest, some where they all hatched within 16 hours of each other and were early. I've had broody hens hat hatched when they were supposed to. That 21 day thing is a target, an approximation, not a sure thing.
Are you counting the days correctly? That's a pretty common mistake on here. It takes 24 hours for an egg too have a day's worth of development so you say "one" the day after you start them. A quick way to check your counting is that the day of the week you set them is the day of the week the 21 days are up. If you set them on a Tuesday then the 21 days are up on a Tuesday.
The younger the chicks are the better chance the hen will accept them and that they will accept the hen. What day of the week does that feed store get their chicks in? I'd ask. They will already probably be 2 days old when they come through their door.
If Tuesday is really your day and the farm store gets the chicks on a Monday or Tuesday I'd get the chicks Thursday and put them under her Thursday night. If the chicks come in Thursday or Friday I'd go for Friday night. Wednesday could go either way.
Good luck with the hatch. Hopefully this is all academic and you'll have your own chicks in a couple of days.
Are you counting the days correctly? That's a pretty common mistake on here. It takes 24 hours for an egg too have a day's worth of development so you say "one" the day after you start them. A quick way to check your counting is that the day of the week you set them is the day of the week the 21 days are up. If you set them on a Tuesday then the 21 days are up on a Tuesday.
The younger the chicks are the better chance the hen will accept them and that they will accept the hen. What day of the week does that feed store get their chicks in? I'd ask. They will already probably be 2 days old when they come through their door.
If Tuesday is really your day and the farm store gets the chicks on a Monday or Tuesday I'd get the chicks Thursday and put them under her Thursday night. If the chicks come in Thursday or Friday I'd go for Friday night. Wednesday could go either way.
Good luck with the hatch. Hopefully this is all academic and you'll have your own chicks in a couple of days.