@goats-n-oats has a good point. This could be a form of Mycoplasma that affects the feet. It's called Mycoplasma synoviae, Besides respiratory symptoms, the feet become swollen and painful. It can be difficult to treat. Have you noticed swelling in the lower legs and feet?
I'm with @WoDia. Try moistening the feed before you feed it. Or better still, ferment the feed. Chickens adore the sourdough taste and it's very nutritious.