Chickens can fly high enough to get into the canopy of trees. Thats where they would normally roost. Their wings get clipped so they dont fly off. The modern chicken is a distant descendant of a bird from Asia but domesticated chickens can still fly high enough that coops would have to either have really tall fences or some sort of roof enclosure. I have seen domesticated chickens that didnt have their wings clipped and they could easily get up into tall oak, maple, and pine trees.