Findings in a study done about 25 years ago in Merriams habitat in the mountains of southern NM showed that our hens preferred to nest on slopes rather than flat areas, nested at the base of a tree or other protection like a stump, and directly under overhanging vegetation, and preferred areas with good cover in the form of slash, shrubs, downed trees, or protective ground contour effects. The study was done in a mixed-conifer forest type typified by moderate to thick stands of large conifer trees of various types (pine, fir, spruce), aspen, oak, and a variety of shrub and grass species.
I suspect Mark's comment about having good cover in the vicinity of the nest is probably right on for just about all subspecies. Also, the availability of good brood-rearing habitat close by, including surface water, is probably a critical factor. Good escape cover for broods during that critical two-week period when they are confined to the ground is, no doubt, a very important factor, as well.