Natural goose or duck down is three times warmer than any synthetic one. Synthetic materials may stray into lumps. Over time this can lead to the creation of cold cavities in sleeping bags or coats.
Feather and especially down have excellent insulating and hydro absorbing qualities, which make them the best fillings for bedding and clothes.
At night people can lose between 0,2 to 0,5 liters of water from the surface of the skin. Natural down unlike polyester fibers absorb moisture 14 times more.
This is the main reason why people who use synthetic duvets are getting sweat during night.
Feather and down absorb evaporation letting them out through the cover and linen in the air. Thus the bed and the bedding remain dry.
Synthetic down is much heavier than natural one with the same warmth ratio.