There are several possibilities. One that might be easy to sort out is whether they have access to something poisonous. If they are in an earth-floored run, possible poisons include spilled lead shot (eg from buried shot-gun cartridges). There are other 'poison' options, but try that one for checking as a start. Birds need grit to grind their food. Lead shot (if available) is about the right size for chickens, and it gets worn down releasing toxic lead into the digestion. There have been cases of such deaths in swans when the water level on their lake revealed the place someone had buried cartridges that would have got them into trouble if found.

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