Sour crop or thrush in chickens is caused by the Candida Albicans yeast in the crop. It can easily be identified by the putrid smell that comes from the bird's mouth.
Symptoms: Sour crop is very easy to diagnose. There is a terrible smell coming from the bird's mouth and once you have smelt it, you will never forget it! Birds can sometimes be lethargic and will gradually lose their condition.
Treatment: Nystatin or Ketoconazole are the treatments that vets will usually prescribe. They are POM-V (vet prescription only medications) that you will need to obtain via a prescription from your vet.
I have found many references to this condition in pigeons as well as in poultry. Some experienced pigeon fanciers have recommended using Garlic. They believe it works faster and better than Nystatin so might be worth a try if it's difficult to get to the vets. I haven't tried this myself before.
Similar crop conditions that you might encounter are: An Impacted Crop where there is a blockage in the crop and the crop is full and feels like dough but has not stretched to become pendulous. There will not be the smell that you get with sour crop. A Pendulous Crop is when the crop becomes blocked and the weight of the food and weak muscles stretch the crop so that it hangs down like a pendulum.