The following items can be unlocked by performing the actions below:

* Condottiero War Hammer - Collect 50 hidden feathers and the Condottiero War 
  Hammer will become available in the blacksmith's shop in the Villa Monteriggioni.

* Auditore Cape - Collect 100 hidden feathers and then talk to Mario in the Villa 
  Monteriggioni. He'll award you the Auditore Cape.