Thanks for keeping your eye out for bumble bees! We have over 25 species in Oregon, making the state a real hotspot for bumble bee diversity.
Late summer is a special time of year for bumble bees as the colonies are beginning to disband as they produce queens for next year and the males to fertilize them. The males leave the nest soon after they are born and spend the nights on plants. In the morning and evening you might see these male bumble bees and they might be a little lethargic. They typically pop back up when temperatures warm and proceed with their hunt to find females to mate with.
Bumble bee identification can be somewhat tricky as bumble bees evolve to look alike. You can hear about this process on a podcast we did with a bumble bee evolution expert:
The Xerces Society has a really nice guide to the bumble bee queens of the PNW:
The problem is that male bumble bees can have quite a variable appearance relative to the queens. You often need an expert to definitively to take many male bumble bees to species.
That being said, we have two relatively common bumble bees whose queens have bright red belts, the black tailed bumble bee (Bombus melanopygus) and Hunt's bumble bee (B. huntii). It might appear that these are not common, because we have one species that is super super common (and lacks a red band), namely the yellow-headed bumble bee (B. vosenensenskii).
Keep up the good work.