I often find wild birds inside my coop, probably attracted to the chicken feed in there. I even had a mockingbird try to build a nest up high in the coop. Wild birds regularly visit my outside feeder. Don’t worry about attracting birds with what you plant in your yard, they will be attracted anyway.
Lots of things can happen. Wild birds can bring diseases into your flock. Your chickens can get parasites by eating certain creepy crawlies, the creepy crawlies act as hosts for part of the parasite’s live cycle. A piece of space junk can fall out of the sky and hit your house. I think that actually happened in Australia a few years back. You could have a fender bender the next time you go to the store.
It’s a good question but when I put in in perspective to me, it’s not something I worry about. By exposing mine to their environment I believe I’m strengthening their immune system so they will be healthier. I don’t try to keep them in a sterile vacuum because I can’t anyway.