Sour crop in pigeons can be quite common and is usually linked with the poor grit or the consumption of bad grains.
The term “sour crop” generally refers to an infection in the crop or digestive tract. The infection causes the crop to shut-down and any food content in the crop to turn “sour”.
Additionally, the hand feeding of pet birds such as parrots, especially young birds has been known to increase the risk of infection.
Contaminated birds often show the following signs:
Plenty of good quality grit at all times, a good diet and corn that isn’t mouldy.