At least one of our Scots Grey hens has been taken by a buzzard - OH saw it in the act. The rest were taken, we assume, by our family of stoats, although the buzzards may have had them too. So far the larger hens have been alright, and the SG cockerel.
The only real way to prevent these losses is probably to keep the hens in a run with fruit cage mesh over the top. It's a quandary for us as we like our hens, and the SG in particular, to be pretty much independent and free range. They sleep inside, but from soon after dawn until dusk they are outside.