I found several small clay pots of oval/cigar shape roughly 2cm long in a rarely used window in the house, because a wasp like insect was flying into a small cavity of the window.
I removed one and found dead spiders inside.
I let the rest of the nest alone and closed the window again. I guess I don't have to bother them if they aren't bothering me.
I'm wondering what type of insect is that building such nests? It looked like a cross between a wasp and a hornet. As far as I know, wasps and hornets build their nests from wood, which gives them a paper like material. This one however is made from soil/dirt.
I'm also not sure if spiders are the only food. But given the street lamps outside, I suspect that spiders are very available and easy to catch.
The region is central Europe.