Thanks Marches.
I didn't know that about their history. I checked crop this morning and there's no swelling. So I have:
1. Long term general being unwell
2. Slightly dirty vent
3. No obstruction in crop or hardness around vent.
4. Bright red comb.
5. Penguin stance, unsteady on feet, hunched constantly.
6. No rasping sound. She's normally very vocal but just the odd high pitched squeak when held.
7. No sign of louse or mite.
8. Slightly dirty nose (looks like compacted mud however and there's plenty of that!)
9. Not eating much - will eat treats such as sultanas, no mash, no corn, will peck at grass and ground every so often. Drinking ok.
10. Isolated behaviour - last in, last down, hunching away from the others constantly. Up and down into nest box but no eggs. No observations of 'straining'
11. Terrible feather condition - new growth appearing but falls out very easily. Breast exposed, red raw.
She doesn't fit into classic illnesses I've looked at and I did wonder about 'old age' She's not had any stresses or dramatic changes to her environment.
I think at this stage - having lost a whole load of ducks to DVE a few years ago I'm worried if it's anything contagious. I guess though the amount of time Penny's been ill counts in the others favour as all the others are thriving and laying.
Mx