Thanks, I'm really glad you liked the game! As @Pivony stated, the key is to build up speed and then let go off the controls -- the owl's collision box changes based on what kind of flight it's performing. I admit that this puzzle is maybe a little bit poorly explained in the game -- it is explained in the City level where it isn't required, and then the player has to remember that into the Cave level, where it actually is required. Nonetheless, I really hope you're able to enjoy the rest of the game now that you know how to pass through! :D