From my previous post on Care of Feathers, I received an informative comment from Amy Reineri: Be careful of storing feathers or other natural materials in sealed containers if you live in a more humid environment. Trapped air that has any moisture, if subjected to temperature changes that take it through it's dew point, will deposit and withdraw moisture from the contents of the bag and this can fatigue the materials to the point they decompose. Feathers, leather, dried insects - all of these will decompose due to the humidity cycle if kept in a sealed container with variable temperatures. If you choose to store something in a sealed bag, you can put hygroscopic crystals in with them or you should keep them in the same temperature environment you bagged them in.
That said, if you vacuum-seal your bagged feathers thereby taking the air out, water absorption would not be a problem. I usually just squeeze the air out by hand and call it good. Feathers under 35% moisture can get so dry that they become brittle. A sealed shadowbox using acid-neutral components with UV protected acrylic kept in a stable environment is a better long-term storage container than plastic bags.
Here is a link to a museum’s more thorough explanation of how to keep feathers.