Jason, the holes are in both newish and old shells, so I don't think that it is due to weathering.
There is a remarkable consistency to the size and shape of the holes. That more than anything else makes me think that birds and rodents are not the culprits. The holes are at most around
3 - 4 mm across, and no more. Most of them are roundish, although one shell has 2 holes, one round and one thin and long which may have been due to a failed attempt to breach the shell. If I get a chance I will find a lot of them and line them up for more photographs. I have no shortage of specimens!
In the mean time, there is a lot to look at and some reading to get to grips with. Thanks for digging out that article in Psyche Martin. In my google search I found a lot of links to articles that I could not open because I did not subscribe to the journals, the rest were tantalising parts of synopses and so forth, but no pictures to compare with and make a positive ID.
If it is a rove beetle making the holes, I presume that it would be with it's powerful mandibles, just biting through the shell. That would explain why the holes are not perfectly round. Marine shell boring gastropods dissolve their way through using dilute acids, and that is why they are so uniform and round. Anyhow, there's a good photo of Ocypus olens showing off it's formidable jaws in a defence pose in the Wikipedia article here -
Devil's coach horse beetle - Wikipedia, the free encyclopedia . Frustratingly the article says nothing about snails... Natch
Please keep your ideas coming
Jo