I don't know of any way to prevent it. Even if you checked under her often, it only takes a minute or two to crush the chick. Like I said earlier, I have 2 otherwise excellent broodies who seem to be klutzes when it comes to crushing eggs and newly hatched chicks. But it hasnt happened every time with either of them, and the problem is luckily pretty rare. I'm sorry you have experienced it.Still so upsetting about 2 that got smothered. Any way to help others not face the same fate?