Chicken sister being mean!

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Hello there! I have 3 chickens: Mardigan, Dorito and Chuck that have all grown up together (they are a year old). Mardigan and Chuck hang out while Dorito has been doing her own thing, no problem. But today my favorite chicken, Dorito, decided she utterly hates Chuck. She tries to attack her every chance she gets, for no reason, and now Chuck has resorted in hiding in the back pen part where its the hottest. Shes so afraid and Dorito doesnt stop as long as she sees her. My bf had to sit out there pretty much all day and tried separating for a few hrs but no luck. I dont know what to do or why its happening! Has this happened to you?

 

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Dorito also likes to try to go into the nesting box at night away from the other chickens. Weve been blocking it off but since she is being evil we didnt block it tonight and she's already in there