Jane,

I've seen blackbirds in my parents garden eating both slugs and using anvil
stones to each snails. My parents have very sandy soil and worms are often
deep down so perhaps it might be more correct to say that blackbirds prefer
not to eat slugs but will if that's all that's around.

My own garden is clay, normally quite damp and with worms near the surface.
I see blackbirds pulling worms all the time but they've never gone for
either the slugs all snails as far as I can see. My wife wishes they would
because she likes hostas!

Paul DS.