You will find a man named Charles Kinnear southwest of Casa Madrugada (Point 1). Kinnear is trying to fly across the valley with a primitive glider. Unfortunately he's out of supplies and needs you to help collect them. Know that unlike the challenges, items that you already have can be used to complete this task. However, it is unlikely that you'd have his requirements on you. Travel back to Rio Bravo and pick up 10 Red Sage plants. Now make your way northeast to West Elizabeth. Along the northern river bend between Tall Trees and the Great Plains you'll find the 5 Beavers to kill and skin. When traveling to both of these places, be sure to shoot birds out of the sky and collect their feathers from their corpses, as you'll need a total of 20 feathers (mixing feather types is okay). An extremely easy way to net 20 bird feathers in a matter of minutes is to complete the
Pestilence home mission, where you shoot the crows to stop them from eating all your corn. Just shoot all the crows to complete the mission and after the mission is over the crow corpses will still be around, so you can use your map to find the dead body indicators to pick up their feathers.
Once you've collected all this, make your way back to Charles and give him the materials. He needs some time to finish his glider, so head to Casa Madrugada and sleep through a couple of days, then return to Kinnear's cliffside location. This will trigger one final bit of dialogue before Kinnear takes his maiden voyage. Witnessing this first take off will conclude the Stranger mission.