I have had them consume indigo buntings and 14" long snakes with no problems. Grapes, even with seeds are usually consumed intact.Oh. I see that grapes left in their intact round shape could be a choking hazard, but I still don't see how the seeds could choke a chicken. Seeds are one of the things they're engineered to eat, what with their marvelous little crops and all.
I have had a hen choke on a cherry tomato once, about the size of a grape, as a matter of fact, and she might have died if I hadn't been present when she fell over from lack of oxygen to her brain. Unless a seed is the same size as a chicken's airway, it shouldn't present a problem. Has anyone ever had a chicken choke on a seed?