From what I understand that is very much more rare than it is common. I would try to separate the other hens from the broodys for at least a day or two to let the littles get used to mamas instructions. I also had to separate my co-broodies after the chicks started hatching because one of the babies got pecked pretty bad in the head and almost didnt make it. I think they might start fighting over the chicks. I know that ItalyChickie had the same problem with her co-broodies at one time. Just try and watch things and see how it goes after that..Well, we're not sure what day we are on... The hens will randomly stay in a nesting box during the day so we didn't really notice at first. Also, the nesting box they chose was the most popular with all the hens. Yeah, I'm a bad chicken-mama. I'm mostly concerned that one of the hens has gone all homicidal or something since the last 3 chicks have been found dead (wet, no shell around, dead) and we've only had 3 live ones.
Marie