Avian Leukosis. It is a vertically transmitted virus, from infected mother to embryo chick. Miss Cecily's body was riddled with these tumors. All her major organs were affected (especially her liver), and her skeletal muscles as well. There is no cure. It was a minor miracle we kept her alive for this long.
With two separate birds/breeds post-mortem diagnosed with Avian Leukosis, this means the poultry breeder from whom we bought these girls has some problems. To be fair, however, no breeder is completely free from this issue. There's no way to tell if a young bird is infected other than biopsy. Regardless, because of other problems we have encountered with birds from these folks, we have already started buying from another breeder.
Photo: a retrovirus, courtesy of The Full Wiki.