Yew tree berries are very toxic to humans because our digestive system dissolves the hard coating that surrounds the seed inside the berry. It can cause sudden death.
Birds appear to be able to eat this sweet berry while exhibiting no ill effects because the seeds don't get broken down in their digestive track.
However, if I had such a plant where my chickens could access it, I think I'd be inclined not to take any chances on the berries being safe. The rest of the plant is also poisonous, so the leaves could cause problems. I wouldn't chance it, and I'd get rid of the plant.