These are mason bees. They showed up in my bee yard because I had honey out. Spring was very long and cold and they were hungry. So I let them eat some honey too. Mason bees are solitary bees. They lay their eggs with a stash of pollen in tube-like structures. Then they take mud and seal off the egg. They're named after masonry. In spring the egg hatches into a larvae that will tunnel out and the process begins again. They have tiny stingers like a mosquito but they're quite gentle.