You might also do a search for pendulous crop and sour crop. You may need to fast them until the crop is completely empty, then start them back out on small portions of soft foods until the crop is back to functioning properly. If the crop is not emptying completely overnight (you can check it first thing in the morning), I would fast them that day and check it again the next morning. Massaging the crop also seems to help. The unfortunate thing is that it can damage the muscles that support the crop and it may be an ongoing problem, so try to catch it early to minimize muscle damage.
I started seeing this with the extremely hot temps we had here in Texas. They were drinking and drinking until their crops looked like water balloons in the afternoon. Good luck with it and I hope you'll keep us posted on their progress.