Some internet research led me to an entomologist at Cornell University, Scott McArt, who was kind enough to reply:
"From what you describe, it’s very likely a colony that swarmed. During swarming, half of the bees will leave the hive with their old queen, while the other half of the bees will stay in the hive with a new queen. The exiting bees will typically leave the hive and congregate on a nearby branch for a few minutes or hours, then move on to the next branch (or their new home, once they’ve found it). It’s an impressive sight when thousands of bees assemble onto the branches and/or move en masse to the next location, hence why it’s called a “swarm”."
"Mid-April is a bit early for swarming, but not unheard of, especially since there’s been some warm weather and flowers popping up over the past few weeks. Aside from a bear getting into the hive, it’s really the only reason you’d see thousands of bees up in a tree right now. So if you didn’t see a bear, I’d say you saw a swarm :)"
Another bee sighting was much more subtle. Over the past month, the most numerous spring ephemeral wildflower has been the spring beauty. Again working on a trail, I happened one day to look down and see a tiny bee visiting one. I was crouching down to catch a photo when the bee dropped off the flower and stood motionless among the dead leaves--reminiscent of the freezing behavior rabbits use when approached. The same happened another day when I again sought a photo.
"Unlike honey bees, which congregate in hives, most of the forest bees are loners that spend the bulk of their lives in the ground. They emerge for just a few weeks in early spring to pollinate flowering plants, shrubs and trees before the forest leafs out and shades the understory from the sun.As a botanist, I had naturally assumed that all bees were designed by plants.
Many are "specialists," Droege said, focusing on a particular type of plant or flower.
For example, he said, there's a bee that specializes in collecting nectar and pollen from spring beauty, a ground-hugging pink or white wildflower that's one of the earliest harbingers of spring. One can't exist without the other, he said. In fact, many flowers have features that attract particular pollinators, while discouraging or excluding others.
"Flowers were all designed by bees," Droege said, over millennia of co-evolution."