The bees have stopped taking the sugar syrup!
The stoppage was so abrupt that I actually popped the tops off the hives to make sure they were still in there. Yep. Their numbers are low because it takes about a month after a hive is established for new bees to start being born, but other than that, they look quite content.
I guess with all the blackberry blossoms and tulip poplars in bloom, they finally have more than they can eat.