black-butt Mining bee (Andrena subtilis)
Found in a swampy area near the Bay.
I think I disturbed their nest landmarks when I sat down in the grass; they kept circling around, landing on the grass then taking off again, then finally landed on the grass (after an hour or so), climbed down into it and vanished...
Note: this bee is somewhat larger than a typical honey bee (which we also have in the area); roughly 1" in length.
"Likely species are A. subtilis, subaustralis, or sola. I suspect it is the former or at least one of the first two listed, which are both in subgenus Tylandrena"