The metal feather is a quest item obtained during the Eagles' Peak quest, and is used to open the door leading into the dungeon atop Eagles' Peak.
Players can find the metal feather inside the bird book the first time and in the stack of books after. Both the bird book (item) and the stack of books can be found inside a tent at the northern base of Eagles' Peak.
Once the quest is complete, the metal feather is rendered useless and can be destroyed. However, it is unobtainable for those who wish to keep the item as a souvenir.