When you think of buildings, you probably picture a structure with walls, floors, and a roof, right? Imagine this OP’s surprise when they kept spotting these buildings without any walls in many fields all over. What purpose do they serve? According to enthusiastic ornithophiles on the internet, these are nesting barns for barn swallows.
Apparently, they are the most widespread bird species in the world, and back in the day, they used to nest in caves. These days, they prefer to build their nests in such barns or similar structures, although North American swallows in the Channel Islands still do it the old way.