I'm very excited to have observed for the first time in the garden a masked bee Hylaeus rudbeckiae. This one is a female on the blossom of a bee plant Scrophularia californica.
These bees may have been visiting the garden for the past couple of years that we have lived here, but they are so tiny that I haven't noticed them.
They are called masked bees because of the distinctive yellow markings on their faces. I only have a photo of this bee from the side, so you can't see the vertical yellow marks on its face. In addition to the yellow markings, they sometimes also have a small white stripe on the side of their head. You can see that white stripe on this bee.
They also have yellow markings on their legs, which you can see as small yellow dots here.
Females carry pollen internally in their digestive system, regurgitating it when they return to their nest.
I'm probably going to notice these tiny bees more often, now that I've seen this one.