My experience is limited but I have read of mommas killing chicks. I’ve also read of momma pecking eggs right before hatch. I’ve never experienced that. I did have one of my bantam chicks freak out and start attacking the nest with the chicks. I came I. In the morning to check things and found one chick in a different nest, a just hatched chick not under momma and other things. The other bantam was going at the nest with momma still in it. Momma was doing nothing. She had been fairly protective up until then. I wound up kicking everyone out of the coop except for momma. She was just in a trance and not responding to anything really. That’s the only reason I suggested it could be another chicken. If broody #1 is really protective then she may have prevented anyone from getting her babies. The other one might not be that good at mothering. Of course I don’t know. Just offering possibilities. Did any more hatch? And are they ok? I’m sorry you lost a chick.
