Mine love grapes, but they love them so much that they WILL swallow them whole in a feeding frenzy, or to keep the others from getting them, so they can be a choking hazard.
I always cut my grapes up, as I do have a piggy hen named Lily that actually did choke and would have actually choked to death on it had I not been there to smack her and get it down. That is why I always cut mine, cuz of my silly Lily. Before that, never realized a bird COULD choke.
My ladies' fave treat. I don't cut, but there's a grapevine that drops ripe grapes into the yard all fall, so they seem to know to pierce them before swallowing.
i take the red grapes off my FIL's vine so they wouldn't attract rats, the only thing i can poss think is that if the grapes are on the vine too long they can ferment, so you could end up with a drunk flock!
A relative gave me a big bucket of small grapes last summer. The chickens acted like it was the greatest treat they ever had. The grapes were very small and so I did not cut them in half.