Thank you for your reply Patricia.
Our problem is that we live on a remote island with no poultry vets available. Our vet recommended that if another dies, we get a post mortem done, but perhaps we will see if we can persuade him to send samples to either the labs in Aberdeen or the vet school in Edinburgh. We checked about using eggs meantime and the vets said we can carry on using them, though obviously this might change if they need antibiotics or whatever.
In the winter they were free-ranging, but to avoid damage to our (and our neighbours') gardens in spring/summer, they are in a good size run which we move every second day. Our hope is to get the garden area they are in fenced with a higher fence before next spring so they can continue to free range. As our ducks and chickens are on there, we have not used fertiliser or weedkiller of any sort there. As a result there are lots of docks which they love and we give them extra greens from our vegetable garden/weekly veggie box. They are on a good quality layers pellet and mixed grain mix. Food and water is fresh every day.
On our vet's advice when they started losing feathers, we changed the bedding from woodshavings to straw in the nesting boxes. We could try hay now and see if that helps.
When the problem started, there were only 2 chickens in the shed. Now there are 3, but the house is designed for 4-6.
We used to worm them with a wormer available from our local agricultural supplier which is mixed with their food. However this relies on them eating the food in proportion to their size, so when last wormed we used syringes of worming paste from the vet and did them individually. Our vet did a thorough examination and could find no sign of any mites/fleas etc. He also found no sign of being egg-bound or any other abdominal abnormality.
We'll see if we can get the vets to send off samples and maybe get a better idea what is going on. If so, we can put them in a separate house one-by-one to overnight to check which sample comes from which hen.
Thanks again for your time and trouble.
Thank you again for your time.