They do pretty good bug removal, but don't really till it up much, except in spots where you had juicy, garden-friendly earthworms, which they will eat.
A pig will do a lot better at tilling, and also remove all the roots and weeds. Chickens are great at picking out bugs before you plant, after you till, and cleaning out bugs post-garden season, after you're done picking. They will dig up plants, and eat your produce. Again though, the pig would do better.
You can put up a temporary fence every year, with inexpensive step-in posts, or hammer-in fiberglass poles. I want to use either welded wire or the plastic security fence like they use around construction sites. You know, the ugly orange stuff. Forget deer or bird netting, it's too light, tangles if you breathe on it, and they get under it too easily.
If it's at least 4 feet high, that'll keep most of them out, though you'll probably still have a few persistent ones that repeatedly fly over. One or two hens in the garden, after it's well established, don't do too much damage, if they aren't in there too long.