According to the statistics, grouse are most vulnerable to predation during March and April, but I find that there can also be a burst of very destructive predation pressure during the back-end and early winter. Young grouse hens are dispersing into new and unfamiliar places, old cocks become distracted by their obsession with territories again and the predators of the year are always on hand to gather up any bird that is not 100% capable of looking after itself.
A friend on some neighbouring ground showed me this photograph (above) of a grouse hen which he saw being devoured by a cock goshawk, and I came across a puff of hen grouse feathers up on the Chayne this evening while walking the dog. There was a clump of tail feathers and the rest were from the flank and undercarriage, suggesting that the fox that killed it had paused for a moment to adjust its grip on the way back to cover.
I am quite sure that predation during the breeding season is a major pressure on all grouse populations, but these overwinter losses are what will set the tone for the Spring. On one hand, a bird which loses its partner now will have found another by April, but on the other hand these losses can only happen so many times before all the gains of a good year are set back to zero again.