Alethea, I am not sure what part of the country you are in. I read somewhere, years ago, that snowberries of the PNW would be poured into streams by native people to stun or kill fish. Something in the berries knocked the fish out. I not know if this is true or not. My yard has dense patches of snowberries, I have never seen my chickens or any other bird or mammal, not even Labradors, eat them. Hope this is helpful.
Common Snowberry (S. albus) is an important winter food source for quail, pheasant, and grouse, but is considered poisonous to humans. The berries contain the isoquinoline alkaloid chelidonine, as well as other alkaloids. Ingesting the berries causes mild symptoms of vomiting, dizziness, and slight sedation in children.
Thank you. We are in Oregon and I'll go with not giving the snowberries to the chickens. If grouse (I haven't seen any here, but we have wild turkeys) or anyone else wants to eat them, they are available. If not, we'll just enjoy how pretty they are.