My holly bushes are from from the chicken run. But I have seen many wild birds eating the berries. And their instincts are usually pretty good about what is edible and not edible. This is what I got from the internet:
Holly is one of the 600 species of flowering plants from the Aquifoliaceae family of flora. The bright red colored berries, hard in nature, are mildly toxic. Consuming these berries can lead to severe nausea, vomiting, diarrhea and make the person feel drowsy. Though they are poisonous for human beings, various species of birds and wild animals feed on them.
Seems to say safe for chickens not people.