Firstly we need to understand what exactly goose down is, what it offers and what it actually does with regards to insulation and heat retention. Goose down is the soft feather like material under the more prominent feathers of geese. The down has amazing thermal properties which trap small pockets of air which ultimately create a barrier between the duvet and the atmosphere. This barrier acts as an insulation mechanism which retains the heat and ultimately keeps you warm in bed at night.
While basic Goose down is one of the more popular elements used in the duvet mass-market, Siberian white goose down and Hungarian white goose down are two of the more prominent elements of the luxury duvet market. These two particular birds have above-average insulation qualities offering excellent heat retention, softness and a feel which cannot be matched. So ultimately this is why goose down is used in the duvet market and why Siberian white goose down and Hungarian white goose down have literally taken over the luxury end of the market.