My Mama Hen Died unexpectedly....

I have a coop with one mama hen and her 7 babies (all about 7 weeks old now) and on Sunday morning, mama hen was walking around and I left for about 5 hours, I come back and she's dropped dead in the middle of the coop, Babies all seem to be eating and drinking well, they have a cozy coop to go into for the cold thought, I still feel awful them loosing mama hen like that.

I hope you find out what happened, we looked all over her for any signs of fowl play (no pun intended) and nothing. I’ve never had a chicken mysteriously die.

I have 5 free range chicken who usually roost in that coop that the babies are in (but have been locked out since the babies were tiny) I want to open the coop back up as its getting cold at night, but i am worried about exposing the babies to the other birds, since they were in contact with mama hen and I have no idea what could have killed her. She wandered into my yard about 8 months ago and stayed, so no background.

Also how do I introduce the new babies to the older flock (3 hens, 1 rooster & one very large mildly aggressive hen, 1 Peking duck & 4(3male, (1 female) mallards). I know I have to do it slowly, but I have a sneaking suspicion that 4 are roosters and 3 are hens, and I don’t want a cock fight on my hands.