Like lambs to a slaughter suggests that the slugs (much like the sheep) have no idea that death comes soon, for the slugs either the birds will get them, they get dried out(?), or they risk being found by a human. For the sheep, they’re happy because they have an unlimited supply of food and they’re happy, but they’re being led to their deaths.
Linking verbs don't show actions but action does hence the name action verb
