You can get organic slug pellets, which according to the packet, don't harm wildlife. In my experience, they work well. The catch is that the birds love to eat them! The manufacturer claims they don't harm birds but it's probably better to keep them off if you can, or maybe your local birds will be getting enough to eat elsewhere and will leave them alone.
And if it's any consolation, we've had a mild winter so particularly bad for slugs this year. If the next one is colder, you might be fine with your eggshells next year!