Feather

"They're eagle's feathers. There's an old nest up there. [...] Ezio, would you mind getting a few more for me?"

- Petruccio to his brother Ezio.

Feathers were markers used by the Assassins to prove that they had successfully assassinated their targets. However, after the fall of the Order, the feather lost its importance.

In the Renaissance, Ezio Auditore da Firenze collected feathers for his younger brother, Petruccio; first as a request from him, and later, in his memory.

High Middle Ages
In Altaïr Ibn-La'Ahad's time period, feathers were used to both grant an Assassin permission to assassinate a target, as well as for them to prove that the assignment had been completed.

After investigations on the target had been completed, Assassins' Bureau leaders would give the chosen Assassin a feather, allowing them to proceed with the actual mission. Upon killing his target, the Assassin would stain the feather with the blood of the slain, and present it to the Bureau leader.

Among Assassins, the phrase having a "feather on one's head" meant a mark of death. For example, an Informant in the Rich District of Damascus once commented that "there is a feather on Abul Nuqoud's head," implying that the Merchant King was marked for assassination.

Renaissance
As he was sickly and often confined to bed, Petruccio Auditore once asked his brother Ezio to collect feathers for him. Though Ezio asked him what they were for, he only said that it was a secret, which he would reveal in time.

After the murder of Ezio's father and two brothers (Federico and Petruccio), his mother, Maria, was struck to silence for years, and only prayed over Petruccio's feathers, in mourning of her lost child.

Ezio thus sought to collect feathers, both in memory of his brother, and to coax his mother into speaking again. As he traveled across Italy, he gathered and placed any feathers he found into a box in Maria's room in the Villa Auditore.

After Ezio had collected fifty feathers, Mario Auditore spoke with him about what he was doing, suggesting that he give up what he believed to be a wasted effort. In an attempt to have him turn his attention to other matters, Mario also let him know that a new weapon, the Condottiero War Hammer, was waiting for him at the Monteriggioni blacksmith.

After collecting one hundred feathers, Ezio's efforts were rewarded. Maria spoke again for the first time in years, thanking Ezio for not giving up on her, and presenting him with the Auditore Cape.

In later years, even after the Fall of Monteriggioni, Ezio would continue to collect feathers in Rome; particularly those found on landmarks throughout the city. He placed these feathers in a chest at the Tiber Island headquarters, which resembled the feather box that had once been kept in Maria's room.

Trivia

 * Due to his death, it is never revealed why Petruccio was collecting the feathers in the first place; although a line from the novelization suggests that they were meant to be a present for his mother (which would explain why Ezio would want to give them to her).
 * Collecting all 100 feathers in Assassin's Creed II gives the achievement "In Memory of Petruccio."
 * Collecting the 10 feathers in Assassin's Creed: Brotherhood unlocks the achievement "In Memoriam."
 * After Sequence 3/4 in Brotherhood, you can buy a map detailing the locations of all the remaining feathers from any art merchant.
 * Maria's feather box is an artifact that can be found in modern Monteriggioni by Desmond Miles.