Right now your chickens seem to think that outside is "home" and the coop is just where the food and water are. It's like your concept of your house vs. a restaurant. You can eat at a restaurant every day, but you won't see it as home. But if you actually LIVED in that restaurant 24/7 for a week, it would start to feel like home, wouldn't it?
Mine would love to roost in the trees and on the top of the fence. I trained them out of it by locking them in the coop for a week straight. They learned that the coop was home, and after that they started going in on their own at night. I think it may have taken some encouragement on the first night, but very little. Now I can leave them to their own devices. My dog keeps the predators at bay, so I've been known to just leave the coop door open and let the girls go in and out on their own. They get up at dawn and go foraging, then put themselves to bed at sunset.
I live in suburbia, by the way, so keeping my birds out of the trees (and the neighbors' yards) is very important to me! In 4 months, the only escape (or incidence of roosting in the trees) happened when I was unloading a new bird that I had just brought home.
Best of luck!
