I don't know about what or who killed the chick, but I do know that a mouse can fit through any hole it can get it's head through! Sorry about your loss.
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Maybe instead of putting a non broody hen in with them, you can set a bucket trap outside the brooder. Fill a bucket 1/2 full with water and cover the surface of the eater with BOSS. Provide an access ramp. They jump in and don't come back out. I think if they have a "food source" (the bucket trap) they'll leave your chicks alone. This trap has worked well for me, with all of my chickens and cats I didn't want to use snap traps or poison. Just keep setting it until you don't catch anymore mice![]()
Nikki