I kept call ducks a few years ago, with the aim that they would eat slugs and snails. They definitely eat slugs (we used to feed them to the ducks), but only quite small ones (unless we cut the big ones up first, which was a bit grim), and possibly more importantly, they also eat slugs' eggs (you can see them sifting through the soil with their beaks). But whether they have any positive impact on the number of slugs in your garden, I'm not sure. There were still loads of slugs, whatever we did - beer traps, nematodes, you name it, we tried it.