Blue jays (Cyanocitta cristata) historically eastern, but have expanded westward. Nevada City, CA (Sierra Nevada foothills, ~2,500 ft) offers suitable oak woodland habitat.
Bold blue crest/back, white face/throat, black necklace, loud "jay-jay" calls. Often at feeders (peanuts, sunflower).
Steller's jay (blue/black, no white) also occurs in Sierra. Confirm by crest/voice.