Not yet sure what to do with it. It’s unfortunately right behind the window, and I don’t really want the waspy visitors inside. I hope they don’t plan on building too much more either.
Looks like carpenter bee to me, not wasp.
Hm, you’re right, doesn’t look like a wasp. The color of the nest is not gray either but rather seems it has a yellowish brown. (It’s all sealed up now. Quite amazing actually.)
[After looking at pictures, I don’t think it’s a carpenter bee though. The tail is fuzzy and striped in black and muted yellow.]
It some variant of wild bee, living solitary (no swarm, no queen). It‘s building several of these „cocoons“ each with a single egg and some food (pollen). After some time each will hatch and fly away.
Yep, it’s the kind of bee that insect hotels are for.