If you fly a unit somewhere, treat the unit as if it had Airborne while crossing borders, e.g. Avatar of Air. Fly is synonymous with 'throw through the air,' e.g. Siege Giant. For example, a unit being moved by Avatar of Air's ability will be able to cross the border of a Perilous Bridge or Great Wall, and will not take damage from a Wall of Brambles, even if it's disabled, silenced, or Sky Baron is in the realm. Note that this effect is specific to crossing borders (and not entering sites), and therefore Bottomless Pit will still trigger for non-Airborne minions moved via Avatar of Air's ability. If you fly a site somewhere, e.g. Cloud City, treat all units occupying the site as if they had Airborne while the site moves.