Sounds like capillary worms.
Scroll down to Capillary worms and read about the two types that infect the crop/esophagus: Dose with safeguard liquid goat wormer orally undiluted, 1/2cc once a day for 5 days straight. I recommend that you dose all your birds.
http://msucares.com/poultry/diseases/disparas.htm
X2. Vermisol, which contains Levimisole, is also very effective against crop worms: http://www.thepoultrysite.com/diseaseinfo/42/cropworms
Vermisol: http://www.jedds.com/-strse-1524/Vermisol/Detail.bok