A bee aggregation can be an impressive sight. This one has a couple hundred mining bees (Andrena spp.) flying back and forth between the nesting grounds and a Camellia tree that grows overhead. It’s a high energy occupation: as far as I can tell, they’re spending about 90% of their time in the air, all day, with a noticeable drop around noon. Camellia nectar must be potent stuff.
Of course, they’re not the only ones waiting for the females to show up. The Nomada cuckoo bee looks a bit like a small red wasp, but she’s actually a distant relative of the mining bees whose nests she raids. Her plan is to find a nest where an Andrena is actively laying and provisioning eggs, then sneak in and lay an egg or two of her own. The Nomada egg will hatch before the Andrena egg does… and the Andrena egg will never hatch.
The Nomada shows up early, so that she will be ready to go once the Andrena females start working. However, that means that she has to put up with the Andrena males. It would be satisfying to imagine that the encounter on the right is a case of nest defense, with the Andrena chasing off the Nomada on behalf of his fellow mining bees, but I think it’s more the case that to a male Andrena, anything that is more or less bee sized looks like a potential date. He’s certainly no threat to the Nomada: she is well armored, and like all male bees, he has no stinger and is almost completely harmless.
Finally, I would be remiss if I didn’t make a pitch for more backyard bee habitat. Mining bees are excellent pollinators, but they require patches of reasonably dry, undisturbed earth in order to make their nests. The patches don’t need to be huge: the one here is about four square meters, and it’s mostly rock.
(A note on identification: I don't know Nomada very well, so I am unlikely to ID the cuckoo to species. I may be able to ID the mining bees, but not until the females show up. The weather doesn't look like it will hold for very long, so that may take a few days.)