It happened this morning. She basically got up when I came into her isolation coop, stretched, arched her back ... and fell over dead. Upon getting a closer chance to examine her, I determined that what I thought were roundworms were, most likely, some kind of early-stage maggot, starting to breed around her back end due to her diarrhea, which she's had for a week or so, ever since she became ill.
So while I now know it's not internal parasites that killed, her, I'm not exactly sure what did. She was on the older side, and so it's possible some opportunistic disease got to her. This is the only explanation that I can come up with, since the rest of the flock is parasite-free and healthy. Often worms, maggots or other opportunistic parasites will flourish in a sick bird, which is what I think happened here.
One thing I have realized though, is that when you have a hen who goes into decline this way, the kindest thing to do it cull them, rather than let whatever it is that's ailing them take them to their grave. If you keep chickens as livestock as we do (not pets), a vet call is just not among the treatment options. Portia looked a bit bedraggled at the weekend but I elected to take a wait-and-see attitude. Taking into account her age and general state of heath, Big Ag and I should have dispatched her quickly and ended her suffering...and by doing so, also removed a potentially contagious animal from the property. An ailing animal attracts parasites, including those mentioned above, as well as mites, and so keeping a sick animal around helps no one -- not even the animal itself.
Lesson learned. And now, we have five hens who, God willing, will remain healthy and untouched by whatever got old Portia. Rest in peace, old girl. You will be missed.