Ripe, cooked elderberries are considered safe and even nutritious, but raw elderberries, and the stems, leaves, and unripe fruit, contain cyanogenic compounds — so never feed them raw.
Elderberries are a real example of 'preparation changes everything'. The ripe berries cooked are antioxidant-rich and safe, which is why elderberry is a common ingredient in bird-safe herbal blends. But raw berries — and especially the stems, leaves, and unripe green berries — contain sambunigrin, a cyanogenic glycoside that can cause nausea and worse. This is one of the flagged disputed entries.