Hi Grannie Annie,
So sorry to hear about your dilemma and difficulties with Mabel. I was once asked to look after a friend's chickens whilst they were away. On the first day I noticed one of them waddled towards me, rather than walked. When I picked her up she was very heavy and her abdomen appeared very swollen. I took her straight to the vets. The vet I saw was very fascinated as he had never seen anything like her before. The vet said that he thought that she either had a form of peritonitis, perhaps with the egg causing a problem deep inside, hence the not being able to feel a problem through the vent, or a tumour.
He suggested either an ultra sound or an x-ray, both at great expense, or to put her out of her misery. Knowing the owners and their financial situation I took the difficult decision to have her put to sleep.
When I spoke to the owners they were glad of what I had done, as they couldn't have afforded to do anything as expensive as a scan. They also said that they had not noticed anything untoward, just that she was a little slower than the others but as they didn't handle the hens regularly they wouldn't have noticed how heavy she was. They had kept chickens for a number of years but I guess they weren't as obsessed by them as we perhaps are...!!! The vet also said that it may have come on quite quickly. I really wish that I had done a PM, to see if it was anything obvious but I felt a bit bad about cutting someone else's chicken to bits and so she was buried and I guess we'll never know the cause.
I really hope you have a happier outcome for poor Mabel.