It seems like they are simply underfed. It is never too late to quarantine birds though. It is probably just that they have no meat on their bones.
Also, ducks don't have crops so it would not be related to having a full crop. Sometimes, the term "crop binding" is used for ducks, but it is usually in reference to an impacted proventriculus (and it doesn't look like the pic). Ducks should be given a little grit to avoid that, but again, I don't think it is that.
I would just follow the other suggestions on feed and give them lots of greens and other foods to chose from.