Settling scuffles versus real bullying
Female mice are naturally social and a well-matched group of sisters or long-settled females usually runs peacefully, with brief chases and squeaks while the hierarchy is confirmed. Bullying is different: one mouse is chased relentlessly, cornered, kept away from food, or bitten. Persistent over-grooming that strips fur from a cage-mate's face and back, called barbering, is another sign the balance is wrong. Male mice are the exception to group living — adult males usually fight seriously and are normally kept alone, with extra human company making up the difference.
Give the group room to get away from each other
Most bullying starts with crowding. A group needs enough floor space that a lower-ranking mouse can break line of sight and be somewhere else entirely, plus deep bedding for burrowing and enough height clutter — ropes, branches and shelves — to break up chases. Every hide needs two entrances so nobody can be trapped in a dead end, and one hide per mouse plus a spare means a shy mouse always has somewhere to sleep undisturbed. A barred cage with several levels suits mice well, since the bars themselves become climbing routes that use the space three-dimensionally.
Remove the things worth guarding
A single food bowl and one water bottle create flashpoints a dominant mouse can own. Scatter the day's food through the bedding instead, so the whole group forages and no mouse can guard the supply, and hang two water bottles at different ends of the cage. Wheels are guarded just as fiercely, so provide two if space allows. Tunnels and cardboard that change every few days also help, because a bored dominant mouse invents her own entertainment, and it is usually a cage-mate. When you clean the cage, keep some of the old bedding to mix back in — a fully wiped cage smells like nobody's territory and can restart the fighting you had just settled.
Keep the group stable, and add mice carefully
Every change of membership restarts the hierarchy contest. Avoid adding a single new mouse to an established group, because she becomes the instant target; if you must introduce mice, do it on neutral ground like a clean playpen with fresh bedding, let them meet there first, and only move them into a freshly cleaned and rearranged cage once they are ignoring each other and sleeping near one another. Groups of young females raised together are the calmest of all, which is why rescues usually rehome females in their existing pairs and trios.
When to step in and separate
Separate at once if you see blood, bite wounds around the tail base and rump, or one mouse growing thinner because she cannot reach food. Check injuries with a vet, since mouse bites infect easily, and rehouse the aggressor or the victim permanently rather than re-trying the same mix. Keep a simple daily note of who ate, who hid and who was chased, and KinsLe's Care Teams turns that watch into a shared routine — each family member can be given the evening group check, with completed checks timestamped so a pattern of worsening bullying is impossible to miss.