Hi Nilou, 

I think it's the black tailed bumble bee, Bombus melanopygus, which is one of our few species that nests above ground. Here is a nice key to the orange banded bumble bees of Washington as a resource:
https://washingtonbumblebees.org/bumbles-with-some-red-in-middle/

Note the black tail relative to Hunt's Bumble Bee, which has a yellow band to the rear of the the orange bands. Also, B. melanopygus has dirty yellow band (i.e., black mixed with yellow hairs) on the front end of the thorax. 

I ALWAYS get emails this time of the year about these bees in birds nests, and then the emails stop coming a month later. Here is why I think that is the case. These are among our earliest nesting bumble bees, with queens establishing nests in February. The colonies are starting to become visibly large. They will naturally collapse in a few weeks, with the nests producing new queens that will disperse across the landscape preparing for next year. 

Thanks, again, for taking the time to identify and enjoy these bees. 

Thank you for getting back to us!  This whole experience has been so fun… from finding them, learning more about bumblebees, to trying to identify our new guests.  They have been going crazy for our comfrey plants