Many people are confused by the different ratings associated with down lightweight sleeping bags. Down is rated in terms of fill power, which essentially represents the fluffiness of the feathers. The higher the fill power the more fluffiness or loft is available with the down. More loft allows a bag to trap more air inside the bag which results in greater warmth and a lower temperature rating.
Fill power typically starts at around 500 and is harvested from young ducks or geese used as food sources. Most of the down on the market today comes from these young birds. The higher levels of down (700 – 900+) are harvested from animals used as breeders and results from the natural molting process. The down harvested from the adult birds has more loft, is difficult to harvest, and consequently has a much higher cost associated with it.
Generally speaking the higher the fill power rating, the less number of feathers are required to achieve a specific temperature rating in a sleeping bag. In other words, it takes more feathers to achieve a 20 degree temperature rating at 500 fill power than a 20 degree bag using 800 fill power. The less feathers used in the manufacturing process means less volume and weight of the sleeping bag.
Duck down is able to achieve a maximum fill power of around 750 whereas goose down can reach levels exceeding 900. That being said if you compare a bag with duck down and a bag with goose down both will achieve the same temperature rating, volume and weight when using identical fill power. What you find in the market is a mixture of duck and goose down at the lower fill powers and it changes exclusively to goose with the higher fill power.
As a rule of thumb you will find that lightweight sleeping bags rated for very cold weather use the higher fill power down. The reason why is that more down and loft is required to adequately warm someone in these extreme temperatures. Consequently, bags rated for three-season weather typically use the lower fill power down.
Be sure to checkout our Lightweight Sleeping Bag Guide.