This is a condition that often occurs in nestlings but can also be seen in fledgelings and adults.

The food in the crop does not continue its route through the digestive system and stagnates in the crop, causing a build up of bacteria.

In nestlings this can happen if they are fed when they are cold, if the formula itself is too cold or if the formula is too thick.It can also be caused by a canker nodule in the proventriculus or by a yeast infection, both of which would need treatment with prescription medicines.

It is often necessary to flush the crop, but this is a job for a veterinarian or for an experienced rehabber.  As an interim measure the pigeon can be given up to 10 cc (depending on the size and age of the bird affected) of warm water with a pinch of bicarbonate of soda in it.

Warmth can sometimes help, so place the pigeon on a heat pad. 

Metaclopramide, only available for the vet, increases crop motility and can be used for crop stasis (it is usally used to stop vomiting). 1 drop for every 100 grams bodyweight twice a day.