Agree with you ninepointer. My thoughts are that the avian predators are a big part of the problem where there isn't good brood cover for grouse.
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There was an article in OOD about a year or two ago supporting your theory. Back in the day most predators were shot by farmers/hunters including all the predators you mentioned. It can't be easy being one of the tastiest birds in the bush. It'll be interesting to see what impact the new grouse regs will have.