The feather shaft comes up through the skin on a bird. An ingrown feather shaft could conceivably cause a lump...like the same ones people get from ingrown hairs.
I haven't found anything yet except a couple of extremely scientific papers on 'Lymphoproliferative disease', which would have to be a glandular disorder, but now I'm trying to track down a description of it NOT written in *biobabble* .
Thanks. I've never run across anything about such things in the reading I've done, but birds do get infrected with things such as West Nile, from mosquito bites, so it makes sense that they might also be vulnerable to other small pests that can do big damage....mites, bees, fleas, ticks. But some of your other ideas make sense too. Maybe these things happen more frequently than we realize and we just notice them because we're watching the birds more closely.