I have three grape vines, and when the grapes start to ripen, I have hopping chickens who strip the vine of grapes from the bottom 4 feet. I've never had any chicken act like they had botulism. I think the chickens and wild birds seem to eat all accessible grapes long before they would be fermenting. Granted, this is desert here and grapes dry rather than rot, for the most part. I'm glad this thread was here, because we'll all remember that the next time we see a chicken looking like that in our own yard.