For this achievement, you’ll have to glide with a Monstie’s flight ability for a total of 1500 (unnamed units - metres presumably as this game is Japanese and I’m British and also controlling this narrative!)
To do this, you must use a Monstie with the Fly Riding Action, such as the Ratha everyone starts with, or a sweet Dreadqueen Rathian you got by doing your part in raising the ecosystem, and fly a total distance of 1500 or more.
This may seem like a lot, particularly given that flight isn’t as free as in previous games. However, it is doable, and very early on, thanks to the wind columns present on Mirror Lake, just outside Azuria Castle.
Simply fly towards one wind column from the Azuria Castle runway, and once you’re in the wind column, you’ll float to the top. Then, turn to face a different wind column and glide towards it. Float to the top, turn around, and glide towards your original column. Rinse and repeat, and once you’ve flown a total of 1500 without landing, the achievement will be yours!
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right). The Windblown Feather medal is the first medal on the fourth row.