I have a lady that drops boxes of cull fruit and veggies off a couple times a week. We at first went crazy, dumping in piles of grapes, cantelope, lettuce, strawberry, cukes and whatever else. Everybody was fine except Hennie, a very hungry hen. She eats and eats and eats, all day long. When all this treat stuff went in, I assume she stopped eating mash and had a lack of calcium because she layed 2 eggs that had no shell. or a very thin liquid shell..... I have backed off the treats ever since. And before I give treats I put a pie plate of layers mash down for a few min first. They get excited when it is in a pie plate for some reason.
I had a chicken drop dead earlier this year from too many treats (that is what the report said) so I am very careful now with the amount of treats I give. They get them once a day but not a lot at one time, just enough to be a treat. Mine love grapes too, a lot, and could eat them by the bucket full I am sure