Yew tree berries, on a yew tree (Taxus baccata). Yew trees are small to medium sized evergreen conifers that bear small red berries called arils. Arils are the only part of the yew tree that are not poisonous. The rest contains the heart-paralysing alkaloid taxine. Birds can eat the berries with impunity, as the poisonous seeds pass through them undigested. They therefore disperse the seeds over wide areas, fertilising them with their faeces. Yews grow wild in woodlands throughout Europe.