Chickens will make a complete mess of your garden and eat anything green. Mine eat most insects regardless of friend or foe. They do not eat slugs. Where I live, there is a massive slug population. (perfect climate) My chickens just ignore them in favour of more tasty food.
As for my garden, well, I don't have a massive weed problem, they even take care of the bind weed. They don't eat brambles though.
Ah well. They do dig big holes everywhere that the soil is soft
enough, they have kicked the path shingle all over the lawned area. They have kicked the soil all over the
lawn as well. They empty out all my pots and tubs. The patio is covered in their produce! I'm in the process of fencing off the patio as I don't like the constant cleaning of it.
They have scratched around the base of my fruit
trees and damaged the
roots so that now I suffer from suckers coming up.
They don't appear to eat wind fall apples, that is, unless I'm eating it at the time. Then they will snatch it away from me.
As for fertility, I'm afraid I'm no judge, I have very fertile soil after years of mulching.