Answer:

If no other symptoms of disease are apparent, it is very probable that something too large for the bird to swallow has become lodged in the throat.

This sometimes happens and the remedy is to remove the obstruction by gentle pressure and rubbing, forcing it either out of the mouth or into the crop.

The latter if there is reason to believe that the object is in the nature of food that can be properly taken care of by the crop, and the former if the obstruction appears to be bones or other hard and indigestible matter of extraordinary size.