6 июнь 2021, 15:4706.06.21
It is part of the "upgrade rank" function. A chicken serves as like a joker to substitute for a real champ. So to upgrade a 3 star champ you need a total of 3 other champs that are 3* or a chicken 3*. And the one you are upgrading needs to be at full level, 30 in this case.
So if you press the upgrade rank button you should see the chicken in there.