Very large lump on chicks
Why are you doubtful it is the crop? Just from the description and the fact that multiple birds have it it sounds like the crop is the most liekly answer here. Have you checked them first thing in the morning to see if the lump is decreased? How does the lump feel when palpated? Which side of the chest is the lump most on? Can you take, and post, a photo of the birds so that we can see the lump?