Each year, several migratory species return to this region. The giant white storks are hard to miss, and the air is quickly filled with thousands of House Martins (Delichon urbicum). The Martens are like mini-swallows and have the extremely useful function of keeping down the insect population. However, there is a caveat to their arrival and that is that they nest in windows.
Every window has a Martin nest made of mud and saliva and bits they’ve found, and every window sill quickly mounds up with guano. Most people tie ripped plastic bags in the corners of windows to frighten them away. We don’t bother. In the window where this Martin is building a nest, there is another Martin doing likewise in the other corner.
It’s ironic because I’ve got House Martens nesting in my windows and Pine Martens nesting in my house….
Carpathian Adventure 
Must be nice to be able to peek at them through your window