Hi there

The other hens are probably picking at her feathers because they can see she's under the weather and are using this opportunity to bully her-hens are like that!.

The first thing i'd advise doing, if you've not done it yet, is worming all the hens- when a worm infestation occurs (intestinal worms) you often notice it in one bird first, as all birds will have different degrees of worm burden, and some won't be clinically affected, intestinal worms are one of the most common issues in free range backyard poultry- so a 7 day worming course with in feed 'Flubendazole' (Flubenvet) will make sure worms are not an issue. You should be able to obtain that at a game bird supplier or agricultural merchants

If the worming doesn't help- then there are other infectious agents that could be the cause of the diarrhea, including coccidiosis and bacterial disease. So if the worming has no affect, then taking her to your vet for a faecal analysis, would be the best course of action.

Regards Andrea