Digger bees, although solitary, nest in large aggregations. Each female digs her own tunnel, which can be up to a foot deep and have several branches. Each branch terminates in a chamber where the female lays a single egg, providing it with pollen and nectar collected from flowers. The larvae hatch and consume the stored food, then grow into pupae and then into adult bees, all while underground.
Next spring or early summer the adults will emerge, mate, and do it all over again … Right now we‘re at the mating stage, by the looks of things! I don’t know if they will re-use the existing tunnels or dig new ones. I‘ll keep checking on them.
There are also a few bee flies (possibly tribe Villini) hovering about at ground level. The larval stages of bee flies are predators or parasitoids of the eggs and larvae of other insects. The adult females usually deposit eggs in the vicinity of possible hosts, quite often in the burrows of beetles or wasps/solitary bees. So I’m pretty sure that’s what they’re looking to do! It’s a fly-eat-bee world.