Sour crop is actually "crop stasis," a condition in which the baby bird has a crop full of hand-feeding formula that has gone bad. An owner will approach the baby for a scheduled feeding and observe that most or all of the food from the previous feeding is still there. Never give additional food to a baby that still has a full crop. Crop stasis is an emergency situation and veterinary attention should be immediately sought.
For more information, please see the story How to Help a Baby Bird with Sour Crop.