http://www.lyon-court.com/lordlyon/242.html
This link from the Lord Lyon's website spells it all out. Although eagle feathers are traditional, I would hazard a guess that any feather, seen from a distance, would tend to indicate a presumed status as a Chief, chieftan or armiger and it would be best to avoid any feathers unless you have a right to the above mentioned designations. It would invite a challenge from the "eagle eyed" among us. ;)