There are a total of 100 feathers in the game. You can find these feathers everywhere, on rooftops or overhangs and so on. Hit the menu button and select DNA and go to feathers, here you can see how many you have collected. Hit the button once over the feathers progress to see a breakdown of every location and inside the locations you see the breakdown for each district.
So as example if you miss a feather in Florence you can exactly see in which district.
For 50 feathers you can buy the Condottiero hammer and for 100 feathers you obtain the Auditore cape after you put the feathers in the chest in Ezio's mother's room in Monteriggioni. After the story you can also collect feathers.Florence
Romagna / Forli
These maps show you all feathers, all codex pages, all tombs, all glyphs and in Monteriggioni the location for the achievement with the 8 statues. The numbers on the maps match up with the counter in my video guides.
Romagna - Forli