I am posting this here in the hopes that an ornithologist might be able to shed some light on this unusual finding. Locality is Boise, Idaho.
This appears to be a roughly spherical mass approximately 6 cm diameter, most of which comprises tightly packed feathers with a central core of dermal tissue. The mass has been parasitized by maggots.
I speculate that this is some kind of tumor, which detached from the afflicted organism, and the decaying tissue attracted flies after detachment. But that's just a guess. If anyone has seen such a phenomenon before, please feel free to enlighten me!
Note, I am not the individual who found this object. I am merely relaying the question to this forum.