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Visiting flowers of Asclepias cordifolia. It's big enough & strong enough to work the milkweed flowers and not get a leg inextricably stuck in the stigmatic slits!

From the color, size, wing venation (with long , narrow, marginal cell; and 3 submarginal cells...the 2nd looking like a forward pointing foot with its posterior margin much longer than that of the 1st); shape of head and eyes.; and hind basitarsus longer than hind tibia...this can be seen to be Xylocopa. The lack of scopa on the hind basitarsus indicates a male (should have 11 flagellomeres, but they're hard to discern in the photos).

From the black w/ bluish-metallic tinges and short off-white pile on the thorax and abdomen...and the montane Sierran location/habitat...I'm guessing this is Xylocopa californica californica.