You walk in to them
a battle axe and will be available in the blacksmith
one is outside the palazzo della singhoria
After 50 feathers you receive a special weapon.
A huge hammer
It is the Auditore Cape and I believe it is unlocked by collecting all 100 Feathers.
you can get the aduitore cape after you find all 100 feathers and talk to ezio's mom.
For Assassins Creed 1 No, for Assassins Creed 2 yes.
No, but you can buy a map of all the feathers and once there arent any more feathers on the map you found them all in that city
Assassin's creed 2
You cant get assassins in assassins creed 2 but you can in brother hood and revelations
You unlock the Auditore Cape and the final warhammer becomes available for purchase.