I would also say slugs or snails - my hostas get the same thing - if you go out early in the morning, or late at night, you might actually be able to see them and squash them
The beer seems to work pretty well, but when I had a serous infestation, I did resort to using Ortho slug bait. I'd start with beer first, and also the seek-and-squash methods before resorting to chemicals though - much better for your garden and the environment in the long run.