Flower bees are solitary (though nests may be aggregated) and in my experience the males emerge on average around 7 – 14 days earlier than the females, perhaps to give them time to scout likely nectar sources to which females will be attracted in the coming days.
Males act in a territorial fashion and I’ve witnessed several males at a time acrobatically battling in an engrossing ‘bee-fight’ before the vanquished male gives up and tries his luck elsewhere.
Once mated, females find nest sites in walls, holes in wood, cliffs and sometimes in the ground, where soils are suitable. The males disappear in late April and May whilst females remain busy for several weeks provisioning their nests. They are usually gone by early June.