I ritually go around the 20 or so metal shutters of my house knocking these emergent wasp nests off during the summer time with a long-handled wooden spoon. Thinking this one was abandoned, I photographed it because it looked like a perfect little paper tree. On enlarging the photograph, I was so impressed with the threads of weaving (composed of wood pulp and wasp spittle, apparently) but then I spotted the eggs so this one was lopped off too, and suddenly I felt a bit awful.
I'm not overly enamoured of the wasp. During the hotter summers, here in Madrid at least, it flies through the air, dart-like, randomly stinging when you're just minding your own business with a watering can or admiring your fish pond. Today, however, I tried to find a way of appreciating the little tykes and discovered several plus points... 1) they are pollinators, 2) they have been used in pest control as some of them lay eggs inside naughty varieties of caterpillar, 3) if you leave out coloured paper, they weave amazing rainbow nests, see this website! http://www.thisiscolossal.com/2016/04/rainbow-wasps/
Other things I discovered about the wasp that impress me... 1) There are thousands of varieties worldwide, 2) Some varieties are solitary i.e. a queen builds her nest herself, pops in her eggs and single-handedly feeds the larva with munched up caterpillars, tending to their every need until they leave home and never see her again and 3) They know not to breed with their kin ensuring the health of their line.
So, after discovering all this, will I still be lopping off the nests with my long-handled spoon? This is the compromise I can offer the wasps - l will let the smaller nests stay, the ones that are a one-man [woman] band that swell to no bigger than a ping-pong ball, but if anything threatens to move toward Death Star size (see article posted in comments) then it can consider itself lopped.
If you'd like to know more, here are some pictures of all the different shapes and sizes of wasps' nests (some look more like the heads of mummies than trees, for example)...
And here's a short film showing a wasp creating its nest and feeding its young...