Australian native Nomia bees, settling down to sleep for the night on a flower stem of Gaura plant. These bees are all males, they congregate like this at the end of the day, to sleep in a group. They like thin blades of grass or similar strands and cluster like this on the end of the blade of grass, probably for protection against predators.
The females live in solitary nests or in groups of nests, while the males sleep in the open.

The picture was taken in my garden where I observe them settling down each evening at sundown. If they like a roosting site they will come back each afternoon for several weeks.