Rats. I'm hoping you just have abnormal chickens
I guess I wouldn't assume they'd want to eat slugs, but I've never heard of a chicken not eating earthworms. It's actually unfortunate, as the worms are great for the soil......usually when I'm weeding I'll carefully tuck them back in but when the chickens arrive I'll be torn.
Soil amender or chicken treat? Hmmm.......

Soil amender or chicken treat? Hmmm.......