Last night, after sundown, I took our dog Lucy out in the yard a last time before bed. As we walked under one of our conifer trees, I heard a sound above me, a buzzing like an aging electrical box. I looked up and saw dozens of big, fat insects, like bumble bees, flying around the crown of the tree.
They weren't building a nest or a hive as far as I could see, mostly buzzing around the outside. Lucy and I moved around the yard a little, and a few minutes later I saw (and heard) that the bugs had moved to the crown of our chestnut tree a few metres south. They were gone from the cedar.
While it was too dark to see their coloration, I could see in silhouette how they were clumsily bumping into the big chestnut leaves. I could hear it too: thwack, thwack. They were all gone by morning.
Inside, my wife Air pointed out they were unlikely to be bees, which aren't nocturnal and generally fly more accurately. She noted they were probably beetles, likely June bugs or maybe the adults of the European chafer beetle grubs that have become a lawn pest across temperate North America.
By the morning, they were gone, and I don't think they were there tonight. The beetles have moved on.