Stay at the Mount View Hotel in Nethy Bridge. Near both Atherton Forest and the Cairngorms. Heatherlea (dot co dot uk) birding tours is based there with great local guides, frequent tours, and day trip access to all the sites you want to go with a guide or on your own. I stayed there last spring and with information from the guides easily found all three targets, black grouse leks, Scottish crossbills, and five capercaillie (which are becoming quite rare). Can't go wrong there.