While back Buff got broody. Sat for a week. I was all excited. Went out a week later when I caught her eating and as you can guess. No eggs. But she went right back in to sitting. So I got my other buff egg that day and put it under her. Next day same thing. Well the first egg I found broken at about 2 weeks. But yesterday at 21 days baby chick. It was healthy and under mom. Well tonight I couldn't hear the chirping and hubby and I checked and it was dead under mom. Important note. We have a pair of bantams. And for some reason the bantam started laying eggs under buff last week. I did not know this and was on vacation. Yesterday when I checked for the chick mom had bantam eggs under her. She is still sitting. I am wondering if the sitting on the eggs is different than the way she sits on chicks (I would assume so) and if that smothered the chick. I know they keep the chicks under them but would assume they would have to let more air in for the chick than an egg. I am checking for new eggs so that no more are laid under her. We are very upset. And have to give my 8 year old an answer in the morning. And would like to have a real answer. Welcome to the world of chickens.
I am sorry that this happened. Is she a first time mom? I think they have to "learn" how to properly take care of the chicks, at least somewhat. So like you said, she probably smothered it because she didn't know any better. Was she attentive to the chick? If so, then she was being a good ma, she just didn't realize she had to let it breathe. (I have that problem with my hubby! ) Good luck and I would just explain everything to your son. Thats a good age for him to learn about the facts of life.