Perhaps you have already come across such clay tubes that are fixed somewhere on or in the house. They are built by the Asian mud dauber (sceliphron curvatum). Why Asian, when it occurs in Switzerland? This is due to its introduction in 1979 from India and Nepal. This wasp species was first detected in Europe, in Graz, Austria. Since then it has spread everywhere.
The female wasp builds a structure of clay and loam about 2.5 to 3 cm long, into which it lays an egg each. At the same time a few captured spiders are added, which then serve as the first food for the hatching larvae. The resulting wasp then nibbles its way out of the protective clay tube and will then mate.
Based on AvS/Bert Inäbnit