The insulation values of duck and goose down, known as its fill power, are measured using an identical standard technique. This is why high fill power duck down can provide more insulation value (warmth retention) than lower fill power goose down.
Fill power measures the lofting power of down, which is its ability to trap air. To measure fill power, one ounce of down is compressed in a small glass cylinder. When the weight is removed, the down’s ability to spring back can be measured. Down with a higher fill power rating is more resilient to compression, lofts better, and can trap more air. Besides being warmer, this also means that sleeping bags or parkas with a higher fill ratings require less insulation by weight to provide the same level of warmth retention than an item made with lower quality down.
Why is duck down perceived by people as being lower quality than goose down? Advertising and history, mainly. Bedding and clothing manufacturers have spent decades promoting the virtues of goose down to get you to buy products made with it. Duck down has only been recently introduced as a lower cost alternative (because ducks are easier to farm and managed than geese) and doesn’t have the same track record as goose down as a luxury product. In reality, consumers can’t tell the difference.
When comparing duck down and goose down, compare the fill power ratings of the two. The fill power ratings consumer products generally range from 550 fill power to 950 fill power. The higher the fill power, the less down (by weight) it takes to provide the same warmth retention. In other words, a jacket filled with 650 duck down will insulate better than the jacket filled with 600 fill power goose down.