I have Turkey Vultures soar frequently over the yard and once, I had one soar extremely close to the ground. The chickens were somewhat freaked out, but I think the vulture was just trying to catch a thermal. My hens can tell the difference between a a vulture and a hawk soaring overhead and when they see a vulture, they barely glance up because it is hardly a danger to them. If you have plenty of trees where the chickens roam, the vultures tend to stay away because its hard for them to land in such dense stuff. I wouldn't worry about your chickens otherwise though.