Drooping chicken

We bought 3 rhode island reds from auction about a year ago to add to our flock. A few days ago 2 became lethargic, typical I'll chicken symptoms.

They both have dirty bums and now 1 has started to lean its head to one side and twists it's one leg when walking making it unstable. It is still happy to chase treats however. We have no specialist vets near us so we will probably have to cull them however does anyone have any idea what could be affecting them so we may be able to treat them.