Does anyone keep chickens?

butterandjam

Egg drop syndrome?

I don't think so. They haven't been near anything that they could have caught it from.
They are permanently in a run rather than free range. There are foxes around but no sign of them trying to access the run
Other than the egg problem they appear to be in excellent health

Egg drop syndrome?

Assuming it's not a calcium deficiency, are they getting stressed for any reason? Do you have predatory foxes around or even a rat?

Crushed calcium is probably best, though. I remember my mother saying "they need grit" but I never asked what 'grit" was as a child.
I found this online:
Grit: Ensure they have access to grit to digest food properly
At 2 years old they should still be good layers.

I don’t keep chickens now, but I grew up around chickens kept by neighbours, relatives etc. I do know that if you give them too many scraps the shells get soft, and if the chickens are very young or elderly the same happens.

Assuming that they are in good health perhaps the calcium balance is wrong?