Cherry pits after the winter

I was pruning our little grove of cherry trees (thus the neatly cut
twig in the upper right), when I saw this cache of emptied, dried-out
cherry pits. I almost gathered them up to make a string of them. But
I left them to rot back into the soil. Each pit, about six mm across,
was just a hollow shell with a hole a couple of mm wide, and a groove
that looked like it was ground with a tiny file. I have no idea what
would have gathered them together like this and done such fine work.

Maybe it was a combination -- someone spilled a bowl of cherries and
left them, and worms in the cherry flesh ate into the shells in a
naturally soft spot. Nice effect after the winter snow melts back.