Turns out, it's a "cuckoo bee"

Went to town with the macro lens this morning to try and stalk this guy, who is smaller and different in coloration from other bumble bees (who were in profusion, too). Forty pictures later, this was the best I could do. They are a lot flightier than regular bumble bees. And I have just learned it is a Bombus perplexus, the cuckoo bee. Really!