Early on in the game, flight is an extremely powerful skill. While Rogues and Monks can move up to 120 feet every round near the end of the game, Aarakocra have a 50-foot movement speed from the start. Aarakocra can have a 100-foot flight speed by level five when combined with Haste, a spell that can also be obtained early in the game. This effectively eliminates all terrain for our Birdfolk. Regardless, there are solutions that all of you Dungeon Masters can employ rather than outright prohibiting Aarakocras from your players! Don't remove the ability totally, but put your players in circumstances where they must discover workarounds rather than relying solely on their rapid flying speed! Consider having certain bouts take place in caves, where flying isn't a possibility, or beneath extensive forest cover.