Assassin's Creed II Guide
There are 100 Feathers to collect in Assassin's Creed II. Twenty-seven of the Feathers are in Florence, four are in Monteriggioni, 11 are in Tuscany, 12 are in Romagna, and 46 are in Venice.
You'll have to progress through the story to collect all of a city's Feathers, since not every district is available from the get-go. The earliest you can collect all 100 Feathers is Sequence 11.
Deliver collected Feathers to the chest in Maria's room on the second floor of Villa Auditore in Monteriggioni. Collect and deliver 50 Feathers to unlock the Condottiero War Hammer, which will then be available for purchase at the Blacksmith in Monteriggioni.
Collect 50 Feathers to unlock the Condottiero War Hammer for purchase.
Collect and deliver all 100 Feathers to unlock the Auditore Cape. Ezio's Notoriety will remains at 100% in every city but Monteriggioni while the Auditore Cape is worn. Posters, Heralds, and corrupt officials will not appear.
Collect all 100 Feathers to unlock the Auditore Cape.
Collecting all 100 Feathers also unlocks the “In Memory of Petruccio” Xbox 360 achievement or PlayStation 3 trophy.
Refer to the maps in the following sections for the locations of every feather. If you're having difficulty finding a particular one, match the number next to the marker on the map with the corresponding image to pinpoint the location.
With that said, the markers are numbered based on when each district becomes available. If you were to collect the Feathers district by district, it would be efficient to collect them in the order the markers are numbered. If you're collecting the Feathers at the end of the game when all of the cities and districts are available, it would probably quicker to not follow the order the markers are numbered, though this might make it harder to keep track.