Bee Swarm on Ground

While cutting down buckthorn in my yard I inadvertently disturbed a bee (honeybee?) swarm. I abandoned the area quickly without getting stung. When I returned in the cool evening after sunset to retrieve my tools the bees were clustered in an approximately pizza-size mass on the ground. I doubt that there are any managed hives in this suburban residential neighborhood, thus it's a bit of a mystery where they came from.

 

Woodridge (DuPage Co.), Illinois

Done