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On of out chickens has lost all the feathers off her bottom which now looks very red and swolen and (sorry to be graphic) her poo is sticking to her - almost as if she's constipated. Does anyone know what could be wrong? He tail is all droopy and her comb is dark pink rather than red

When is the last time she laid - she could have a stuck egg, if she has caused the damage to her bottom herself and if she is wandering around with her tail down.