Silverhawk down is the untradeable version of Silverhawk feathers that can be bought in sets of 10 for 35 thaler, or found on uncharted islands, used to charge Silverhawk boots. One down gives the boots one charge. The boots can have a maximum of 500 charges at any time.
The following calculator tells you how many feathers you need to get to a level from a given amount of experience. It can also factor in bonus experience.