Must be Hobo Marmots . . . (focus isn't real sharp - took these at 1600mm and 140 yards or so. Sometimes . . . "You gotta pick the fruit where you find the tree." Sony Clear Image Zoom is a useful tool!).
Are there dead limbs/perches near to the nest box? Is there water close by? Since they are natural-cavity nesters, I wonder if a round or oval entry hole would be more appealing to them?
Worst I ever saw them was at Hutchinson Lake and south of Sprague Lake, but ticks are pretty much everywhere now. Birds and other critters have helped spread them.