I have a broody hen. I also have 14 eggs in an incubator due in a week or so. Are there any downsides to sneaking those eggs under her in a separate pen? I have a tractor I can put a next box and feeder/water setup in.
I've never had to sneak eggs under a broody at all. They want eggs. I just put them in the nest, show them to the hen, she gets that ecstatic broody look on her face, (Oh joy! Eggs! MINE, ALL MINE!!!) clucks and climbs in, and sits down on them, arranges them to her liking, and growls at anybody who might bother them.
Your plan may work just fine, but be aware that some broodies will not accept chicks if they haven't been broody long enough. How long has she been broody? If It's been a week or so, her hormones could go ahead and shift from "brood" to "mother". She might accept them anyway, but just be aware, she may not realize that the eggs have hatched, if it's too early, and may attack them, thinking they're some invading creature, trying to take her eggs.
If she won't let you move her, are there a lot of other hens? Is she in the favorite nest? Or has she picked a spot where she's mostly left alone? It might be ok to let her have the eggs where she is.
Jenny's right by the way, I've never had to sneak the eggs in. Moving the hen if you need to and having her keep sitting is more of an issue. Usually showing a broody any eggs will make her cluck and shovel them under her like prized possessions.
Now you usually have to sneak chicks in if you are adding to a hatch or substituting for unhatched eggs. And that's best done at night so the hen doesn't really notice and gets used to them by morning.
I've seen my dark Cornishes scoop them against their necks tightly hold them under what would be a chin, if chickens had chins. Sort of like when they stretch out and tuck them underneath themselves, but they hold tighter so they can pick them up. I have found hens, when I picked them up, to be holding an egg up under a wing, or between their legs.
If my Cornies see eggs in the next nest when they're broody, it just drives them nuts that they don't have them. You can watch them plotting to take the eggs. It's pretty funny. They stretch waaay over, and pull the eggs as close as they can, then they start trying to get them from one nest to the other. I usually come and take them, before they get them moved, because I remove the new, unmarked eggs daily anyway. Otherwise they'd add new eggs everyday, and end up with a mess of partly developed chicks, when the originals hatch out.
Oh yeah, mark the eggs. Sharpie, pencil, or crayon. Sharpie stays best. Very important. So you know if there are new eggs, which ones to remove.
You're making me SO laugh here. They do have a gleam in their eyes when you give them more eggs or mine do. I just had a hen hatch eggs & there was 1 left after the hatch & the hen decided she was going to take her chicks out *she did not care what means I used to close her in* she DUG her way out of the coop. Anyway the one egg left I don't know if it's going to hatch or not but I had another hen that's broody on another egg that's to hatch in a day or so I think & so I slipped it under her. Of course getting your hand just NEAR a hen when she's sitting is a task & so I used an oven mitt & put the egg just along side her. I turned around not 3 min. later & the egg wahlah under hen. You have to love a broody Buff Orpington! Oh & I just posted the video of the hen with her chicks outside. TOO cute! under raising chicks...