Gold (19.3 grams per cubic centimeter) is much denser than the keratin that makes up feathers (1.3g/cc). That means that 1 pound of gold displaces about 23.5 cc of air, while 1 pound of feathers displaces about 348 cc of air, not counting any air inside the feather shaft. The displaced air is pushing up on them with a force equal that the weight of that volume of air, so it takes a slightly greater mass of feathers to weigh a pound. Of course, if you weighed them both in a vacuum, their mass would be the same.