I had a broody this summer, the eggs were bad and I slipped 4 day old chicks under her, all went well, but I lost 2 to pasty butt. I bought 2 more chicks, that were about a week old, 3 or 4 days older than the 2 under her, just enough age difference, at least a I think, that she wouldn't accept them. Wasn't totally mean to them, they were able to sleep under her for a couple of nights, but during the day, she would herd her 2 chicks away, and wouldn't let the 2 newcomers eat or drink water, so I had to remove them.