Perhaps this may be what is happening. We have been seeing a large gas sac appear near the base of the neck-chest on our juvenile and adult Coturnix quail. They can go a whole week with it before invariably dying. I did a post-mortem on one and discovered the sac had decomposing feed in it and the gas was from the decomposition. Going deeper, I saw that the oesophagus above the crop had ruptured and feed had been forced out through the chest wall under the skin. I cannot find anything about this online or from my network. My guess is that an ulcerative infection in the oesophagus causes this. There is only one stand-out potential source of infection... their sand tray. Quail LOVE their sand baths and clearly ingest sand as they 'bath'. It is quickly laden with their poop... The bird could be overfeeding, backing up the crop, sand bathing allowing infected sand to sit in the lower oesophagus...