Squirrels as wild animals (that have lived alongside oaks and other tannin-containing plants for millennia) have the natural ability to metabolize the tannic acid in acorns, as well as the toxic elements in mushrooms that are poisonous to humans and many other animals. Not sure about chickens though. Leaching out the toxins is kind of a pain in the rear, so might not be worth the work.
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And they sting like heck when your brother chucks one and it hits you. As far as being edible--they can be eaten if you carefully prepare them by leaching out the tanic acid--like treating acorns--but I don't know why you'd want to. If the chicken eat them and don't die from it, then I guess it's ok--squirrels do and it doesn't have an adverse effect on their population.