Help - how did these shells get on this tree?

Brenda and I went to Aegena port today (not too impressed with the main drag, but that's another story). We took a wee stroll up to the 'Kolona' on the outskirts of town and were treated to great views as well as a very interesting archaeological site. But they also had lots of these wee trees covered in mostly tiny, tiny shells - with the occasional bigger one which you may see in the photo. We've not been able to find out how they get there - we've asked but no one seems able to explain. Google has been no help. Can anyone explain? They're located maybe half a kilometre inland on a hill so far enough away from the sea that it won't just be particularly rough bird guano (I don't think?)