Floating goose body feather
Most feathers have some curve to their shafts. One of the reasons is for warmth. A front to back bend on a body feather serves to control a bird's temperature.
Feather curves are the main reason I use shadowboxes rather than trying to make the feathers lie flat. I want to honor their natural shapes.
The most consistently and intensely curved feathers I know of are the body feathers from waterfowl. Many of the body feathers on a swan are so inwardly curved that it takes only two to make a complete circle. On most birds, the curve allows a larger air space between the body and the elements. It’s like the wider the insulation is in your house or the thicker the layers of your clothing is, the warmer you are. A neat thing about birds though is they can control how thick the air layer is. Through muscle-feather control they can flatten the flexible shafts, pushing them next to their body. Or they can fluff themselves by letting their feathers naturally curl to their max like you see songbirds do on a cold winter day. I wish I could do that with my clothes.