They are really, really good at hiding the symptoms, it's often missed until it's pretty advanced. A very firm, bloated abdomen is usually a sign as it progresses. Sometimes as it gets worse crops will back up. I have one right now that is very, very advanced, she's in her own enclosure with a couple of other birds she gets along with so she can have peace and not compete for food and water, she doesn't get around real well anymore due to the abdomen, hers is quite bloated. But she still eats and drinks and dustbathes, and seems pretty happy so I've left her be. Every morning I expect to find her gone, but she just keeps going. There is little information on possible causes, and I've looked, so I think they are probably many and varied, from bacteria getting in the oviduct, inflammation, etc. I've never seen an obvious causal link between birds of mine that have had it. I recall one post where someone thought it was getting spread by their roo, but I don't think that's likely. Hens in rooster free flocks still can and do get it, and if it was a roo it would seem likely that a lot more in a flock would get it, and it's generally sporadic.
