A Frenzy Of Bumblers. Lumix M4/3 20mm

At lunchtime today, I noticed that one of the trees outside had come into flower, and that the pollen had attracted a literal swarm of bumble bees, hundreds of them.  They hovered and alighted on the blossoms, and when a strong gust of wind blew, they all took flight again, suspended in the air.  I've never seen anything like it.  Since I only had my pancake lens to hand - not much use for this assignment -, I enjoyed the spectacle, and then forgot about it.  
But when I came out the door at the end of the day, the gale had blown a carpet of the flowers to the ground, and the bumblers were in amongst them, behaving like cats do with catnip, completely dilly.  This time, I could fudge it with the pancake lens which will focus to about 6 inches.  Here's the result: not quite what I would have liked, but you get the idea.  My next task is to find out what tree it is, and an explanation for this phenomenon.