How long do I have to sneak fertile eggs under my broody hen? She has been broody for about 3 weeks. I'm afraid if I get eggs to sneak under her she will stop being broody before they hatch. I mean, she can't possibly know the difference between fertile and non fertile eggs so what if she is no longer broody a week into the process?
Do you have an incubator? I've heard of broodies being desperate and stay broody until they die or have chicks.

she has been setting for about 18 days. If i take the eggs away can i put new eggs under her? Will she keep sitting for that long?