Feathers are placed along the main quest areas of the game. This is unlike 
the
flags in the original Assassin's Creed, which appeared seemingly at random
throughout the world. Finding hidden feathers is best done by running across
rooftops, since naturally that is where birds spend the most time losing 
them.
Walk around the rooftops with Eagle Vision turned on to see feathers in a 
white
glitch type of graphic.