Bumble bees in your birdhouse? Or: do you need to relocate a bumble bee nest?

Black-tailed bumble bees, Bombus melanopygus, nesting in a birdhouse over Kim's mailbox in Kitsap County.

We’ve received several requests over the last few years for advice about bumble bee nests in inconvenient locations, particularly in birdhouses. While in natural settings bumble bees often choose old woodpecker nests or rodent dens as nest sites, birdhouses seem to be an attractive substitute. 

Are bees in a birdhouse a problem? It depends. A bumble bee colony only lasts for one season; if you find one on your property, giving the nest space and leaving the bees to finish their life cycle is the best choice whenever possible. If the nest is not in a busy area, it can usually be safely ignored. If it’s located where excited pets or running children might get too close and risk being stung, you might be able to install a barrier to direct people away from the nest. If this isn't possible, you can attempt to have the nest moved, though it requires skilled effort by the humans and disturbs the bees, potentially risking the colony’s survival. We are aware of a few people who are willing to try moving nests, and if you send us a message with your location, we can let you know whether there are any in your area. We recommend cleaning out birdhouses each year to avoid attracting bumble bees. 

Our first call about bumble bees in a birdhouse came from Angel, who had bees nesting in the top story and birds in the bottom story of their triple-decker birdhouse. Angel could tell that the bees had red on their back ends, but was stung attempting to get a photo. We’d heard that Bombus melanopygus (whose most noticeable feature in WA is the orange/red section on their abdomens) have a habit of nesting in birdhouses, so I suspected them right away. Bumble bees sometimes react protectively if they feel their nest is threatened. WaNBS board member Will Peterman has helped move a few nests and has noted that B. melanopygus have a tendency to be more defensive than most bumble bees in their guard duty. Angel decided to give the birds and the bees space and let them be. After the residents moved on, Angel moved the birdhouse further away from their pool.

Bombus melanopygus have cloudy shoulders (mixed yellow and black hairs) with a yellow first segment followed by two red segments on their striped abdomens. (Count the segments from the ‘waist’ down 1-6.) Notice the narrow yellow fringe in the black band on the 4th segment of the abdomen of the bee on the right. Though in Washington this species has a wide red band across their abdomens, their common name is the Black-tailed bumble bee because these segments are black in southern Oregon and California.

Our second message came from Christine in Shoreline, who wrote, “Hello–Have an old birdhouse that looks now to have maybe bumblebees.” She had stopped in a local garden store for help, and they had mistaken the bumble bee for a pest insect. I could see from her photo that she also had Bombus melanopygus. (See here for more on bumble bee ID.) Christine was glad to get a correct ID from us, and was reassured to hear that it wasn’t wasps or hornets. (We haven’t heard of any wasps living in birdhouses.) 

Birdhouse with Bombus melanopygus living on an old chickadee nest in Christine's birdhouse.

Christine told us “this birdhouse was used last spring by chickadees, but it looks like the bees beat them to the space. We had a new fence and garden work done this month - they chased the garden worker who was working next to the birdhouse. After that happened, we stayed clear of that area.” She asked, “Will the queen stay in the house and overwinter?” We let her know that bumble bee colonies only live for one season; once they send out their reproductives for the next year, the rest of the colony dies naturally at the end of that season. The mated gynes (future queens) are unlikely to reuse the same nest the next year, which is a good strategy to avoid buildup of parasites or other disease-causing organisms. 

Bombus melanopygus seasonality graph from iNaturalist. Notice that these bees have an early season and will dwindle naturally by midsummer. Bees spotted from October through January are nectaring gynes topping up on calories then going back to hibernate until spring.

Christine mentioned that she was “planning to remove and replace the birdhouse - you can’t open it up to clean it out. We got two new birdhouses that you can open and we prefer that style. Would it be OK to in the fall to just compost the birdhouse, knowing that the bees are dead already?” We replied that this would be fine.

She also asked “What is that yellowish material around the outside of the entrance to the house? Food? Smell or something to alert or warn other bees not associated with that house?” Once the bees left the nest, Will Peterman was able to stop by and dismantle the birdhouse to do an inspection.

Will found that “the previous generations of birds had left the birdhouse almost completely full of material. It was so packed that I was unable to completely empty it. The bees moved in because there was no room for birds. The structure of the [bumble bee] nest was mostly intact. I found a couple of larval wax moths [a parasite of bee nests] but they hadn’t really started to chew things up. There was still nectar in some of the nectar cups. The nest didn’t starve. I found a few dead workers, but no brood. The queen quit producing before the nest was abandoned. The workers were not quite intact, but I’m pretty sure they were melanopygus. When I got there, the entrance was relatively clean. Looking again at the photos, that’s a lot of poop. That’s consistent with infection by Nosema bombi [a fungal parasite], which is (as far as I know) still the prime suspect in the decline of Bombus occidentalis (the Western bumble bee).” Because it seemed likely that the birdhouse was contaminated with parasites, we recommended that Christine go forward with her plan to compost it.

Christine also asked, “How can we prevent bees from taking over a birdhouse? Can we offer them something else?” She had already found the right answer: replacing this style of birdhouse with one that can be cleaned out each year. Christine purchased cleanable birdhouses at Wild Birds Unlimited. 

Alternatively, you might want to encourage bumble bees to nest on your property (in a safely out-of-the-way location.) Leaving a nest inside a birdhouse over the winter might invite a bumble bee gyne to start a colony there. Another way to attract bumble bees is to grow a variety of plants with flowers that bloom in every season. Some bumble bees, such as B. melanopygus and B. vosnesenskii (the Yellow-faced bumble bee), have life cycles that start early in the year and are often observed in our iNaturalist project if the weather is warm on flowers blooming in early spring or even late winter. To offer resources during this time, you can plant heather, Oregon grape, and currants. 

Bombus vosnesenskii gyne feeding in February. Bumble bee gynes nectaring might be searching for a place to call home. Once they begin collecting pollen they are likely nesting, and they are called queens.