Yes, I have heard of a gang attack by crows on a single hen, IIRC, the hen was killed. It's not common for them to attack full grown birds, but they will pick off chicks. And I had 3 nests of robins fledge last summer. Every single one was picked off by crows within 50' of the nest. They sat vigil when they knew the babies were ready to fledge.
I did battle with a red tail haek trying to eat baby robins in a low bush five feet from the door. That hawk didn't back down. The mom kept flying and trying to attack the hawk. I put an umbrella over the bush, the gawk moved it then I finally got the umbrella where it couldn't move it.