It's my understanding that sour crop has to do with fermentation of grain or vegetation left in the crop and the crop having the wrong pH for proper digestion. The bird ends up burping gas given off by the fermenting goo in her crop, the squishy enlargement comes from the gastric juices flooding the area to try to digest the food, etc. Giving ACV (or sour wine) re-acidifies the crop to create the correct pH for enzymes to work in breaking down the vegetable matter in the crop. So, while experiencing sour crop, the bird should not be given any grain or tough vegetation until the crop is normal because it will only feed the bacteria causing the fermentation in the crop. No bread, mash, grain, greens, cabbage, etc. Also nothing sweet or with sugar, which also feeds fermentation. But soft protein like yogurt, scrambled egg, and such can be fed in small quantities to keep up strength. Bread with oil helps with an IMPACTED crop, which is a different problem (but can lead to sour crop.) An impacted crop is a buildup of undigestible matter like stringy hay or wood shavings that hardens the crop. The oil and massaging can help loosen the mass and get it to pass. Hens with sour crop usually still poop because food moves sluggishly in the enlarged crop, those with an impaction stop pooping because food isn't moving through the crop at all. An impaction sometimes requires surgery to clean out the crop. (Do it yourself is possible.)