curious about it

Today, while washing the dishes, I noticed some unusual activity on our kitchen window. Small black bees were on the window frame, producing a kind of brown-black sap or oil. I don't want to use spray to kill them unless they become a nuisance. I’ve checked various sources, and some suggest they are small black bees.

Are they indeed black bees? And why are they on the window instead of forming a colony on one of the many plants or trees around our house?
They are very tiny, around 0.5cm, look like flies, but when they are flying, they look like honey bees.