Towards the end of summer each year a roost of tiny bees builds up in our garden. They are male common furrow bees Lasioglossum calceatum. The females live underground in burrows but the males, with no home to go to, gather to roost together, usually on a seedhead. During August this year I noticed the roost and this built up to a peak of 14 in mid September with just a few remaining until the end of the month.
These bees spend the day patrolling flowers in the garden hoping to intercept newly emerged females to mate with. They often gather around the fleabane which is magnet for bees and other insects at this time of year. About 5.30 pm they begin to return to the roost site. This was almost always on the same Deptford pink seed head. I am not sure how they find the same one each night with dozens of other seed heads to choose from but suspect that they mark it with a pheromone scent.
Whilst settling down to roost they often clean their wings and body with their legs and extend their tongues whilst exuding a drop of liquid from their mouth parts. I am not sure exactly what they are doing here, possibly evaporating excess water from the nectar they have gathered during the day? Watch video here.