Warning - only vaguely gardening-related.

Just wondering - the Garden Bird Catalogue sells oyster shell grit for adding to bird mixes in spring supposedly to boost calcium levels when the birds are needing to produce eggs.

I've been making my own bird mixes this winter, and have been wondering if the same effect could be acheived with ground eggshell. I've been collecting (chicken) eggshells for a couple of months to experiment in springtime to see for myself once and for all if broken eggshells deter slugs (yes I know, I'm a saddo. It keeps me out of trouble though).

I put some (washed and dried) eggshells in my grinder and they produced an exceptionally fine powder.

Does anyone think this idea has merit? Would the birds be able to absorb the calcuim from the eggshell? Or is this a terrible idea which will lead only to a mad cow scenario of avian semi-cannibalism?

Any advice from your vast reserves of various brains much appreciated!

Thanks, Bob