The curious case of the bottles in the walls

As I cycle around Puglia in southern Italy I keep seeing them. At first I thought that they were just a drunken prank, litter, or even fly tipping. But no, someone has carefully placed plastic bottles tucked neatly into the insides of the stone walls.

On some walls, the concrete ones, there are regular circular holes and there I saw glass bottles perched upright in them. Here they are plastic bottles.

What can they possibly be for? I look inside one hoping for an answer. There’s something in there but I don’t know what it is? Are they wasp killers? To ward off evil spirits or just vases for summer cut flowers?

I have no idea.