Commercial wren house design you see (pics later) tend to run against house design recommendations for wrens with respect to depth. However, as several bumblebees will use bird houses, I am going to use them all around the house this year in hopes of fostering just this occupancy. The holes are 1 inch..too small for unwanted occupants...and really nit recommended size for house wrens anyway. If house wrens move in, no problem! I have seen bird houses from 5.5 feet to 7 feet used by Bombus impatiens. I have yet too see any database based on actual occurences however. Like a lot of gardening /wildlife info, often seems like feelings are parroted in various print and online sources. Has anyone seen something a bit more rigorous?

Just caught part of a PBS TV program on farming in Maryland where the farmer being interviewed said since Honeybees were on the decline that he had begun using Bumblebees to pollinate his crops by installing Bumblebee hives and that home gardeners could use them, too. So I have been 'searching' for a source of Bumblebee hives. Maybe some of what I've found so far will be of use to you.

https://bumblebeeconservation....
A UK site that says most BBs nest in ground or low but mentions Tree bumblebees (B. hypnorum) as the ones who occassionally use bird houses.

Instructions for their DIY Bumblebee Nest.
https://bumblebeeconservation....

Sink the upturned flower pot into the ground and use the slate/ tile to cover any drainage holes to keep the rain out.
Run a hose or pipe underground to the pot, leaving a prominent entrance. Be sure to make drainage holes in the pipe.
Finally, fill with a generous handful of nesting material, such as old bedding from a pet mouse, guinnae pig, etc.


They say that the nesting material from mice attract the Queen to select a nest site. Wonder if used bird nest material would draw Queen of those BBs that use birdhouses?

And this site from Australia that sells Bumblebee Hives
http://zonda.net.nz/bh

Those wren houses could be rehabbed for Mason Bees?

Over the years, of all the bird houses I have seen which had been inhabited by bumblebees, each had previously been used by birds. I am not sure used nest material is the requirement however. I did purchase poplar shavings for my bird and bee houses. The thought was that cedar or pine shavings might not be attractive to bees due to . Additionally, I wanted to use new shavings so as to reduce the possibility of introducing something harmful or offputting to bees.

That said, the houses I am using are white cedar. They may need to weather a long time before being used. I hung 3 in late October and 6 more in the last month. I also installed the rest of my bird houses so they could not only weather but be seen by birds which already are pairing.

Bombus impatiens is the most common bumble bee species in my area. They routinely nest in bird houses and other non-ground nests. I do plan to make ground nests as well. I am hoping B. impatiens will take over the boxes and leave the ground nests for other species.

I have other structures I will be using for solitary bees.