To understand the reason for placing goose down feathers into a duvet it’s important to understand the anatomy of a bird and how the feathers it grows protect it from hot and cold temperatures. When you see a bird the outermost layer are its actual feathers, typically this is not what is found inside a goose down duvet. Under the birds feathers are the actual down feathers found inside a duvet. They’re much softer and are designed (by mother nature) specifically to keep warmth in during the winter and stay cool during the summer (although it’s not recommended to wear a down vest during the summer!). Goose down duvets stay warm because the feathers within them are able to actually trap small pockets of air (heat) and keep them around your body instead of allowing them to pass through as cotton would. Even with our technology today goose down feathers are considered the best insulation because of the combination of trapping heat within them and also being compressible to fit into almost any space. The “down” side of it would be the cost.