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.

To find the final few feathers, pause the game, and go to the DNA
strand. There is a section at the end of the strand that shows you where
the feathers are located. Then, go to the feather section to find out
what districts the missing feathers are located in.