There are 50 feathers scattered around the Frontier and you will be able to find and pick them up following the events of Animus Sequence 4B – ‘Feathers and Trees’. Collecting all of the feathers will unlock the Kanien’keha:ka outfit and will also contribute towards 100% sync.
Note that these will appear on your map after using view points, if you come into close proximity to one or, perhaps the most efficient way – by purchasing a map with all their locations from one of the general stores.
Here are the general locations of each feather, based upon hunting regions:
Note that the Black Creek feathers and one feather from Great Piece Hills and Monmouth are missing due to an accidental data loss on my end, but will be put back in as soon as I play through again!