Hens have been hatching and raising chicks with the flock for thousands of years. They are not extinct yet.
You are talking about living animals. About anything can happen but there is a lot of difference in what can possibly happen and what absolutely will happen. It is possible that another hen will try to kill a chick, especially if it as alone and unprotected. It is possible and it does happen. But what happens with my flock is that the mother hen protects her chicks. In the first place, most of the other grown chickens pretty much ignore the chicks anyway unless that chick goes into the hen's private space. Occasionally one will show an interest in the chicks, but Mama is practically always very protective and will get really violent in protecting her babies. Some broodies are not as protective as others, but most have such a bad attitude that the other hens quickly learn to give her room.
Yes, bad things can happen, but I let my broodies hatch chicks with the flock. I let my broodies raise their chicks with the flock. My last broody hatched 11 chicks with the flock. She is raising them with the flock. The will be three weeks old tomorrow and all 11 are still alive and doing well.
A couple of years ago I lost a chick to other chickens. A chick about a week old got separated from Mama and got into a pen of 8 week olds. Mama could not get to the chick to protect it and the 8 week olds killed it. I'm fully aware that bad things can happen.
But I don't want people to think that bad things will happen each and every time without fail in every flock in the world if you let Mama rasie them with the flock because that simply is not the case. Something bad can possibly happen, but it usually doesn't.
Maybe this will help explain it. Several times I have seen chicks about 2 weeks old leave Mama's protection and stand next to the big hens with all of them eating out of the feeder. Occasionally the older hens will ignore the chick, at least for a while. But usually one of the adult hens will peck that chick to remind it that it is bad flock mannerts for a socially inferior to eat with its better. That chick runs back to Mama as fast as its little legs can carry it, flapping its wings and peeping indignantly. Mama generally ignores all this. But once I saw a hen take off after that chick. In that case Mama got very upset and taught that hen to not bother her baby.
Chickens are flock animals. They have developed ways to live together and raise their families together. Bad things can happen, but in my experience they usually don't.
