On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for The Sky Thief side quest.
Rewards: 1,200 XP.
In the place shown in the picture above, on a wooden platform on a tree, you will meet a girl. When you talk to her, you will find out that a bird has stolen her mom's ring.
The bird's nest is on the second tree, but you don't have to climb up there. Stand at the spot shown in the image above and aim for the jug of oil. After firing, you will cause an explosion and the ring will fall to the ground.
Pick up the ring, and after a while the girl's stepmother will appear on the scene. Once you tell her what you know about the ring, you can decide what will happen to the family heirloom. Regardless of your decision, speak to the girl and the mission will end.