I don't think it's a hugely common condition since birds can pass and even digest many of the non-food materials they may swallow. But it still occurs now and then. I wouldn't be terribly worried as along as Casper is eating and pooping normally... and as long as his crop empties normally each day. But if you ever feel a persistent lump in his crop area then I think it would be wise to take him in for a vet check.