James, Demetri and Alistair are three vampires each showing one of three closely related special abilities that can be generally called tracking sense. They can locate the position and movements of a target, generally of a living being, possibly intended as a prey, but on occasion even inanimate things. They therefore are called trackers.
- "He was a skilled tracker, able to sense in advance the most likely moves of his prey."
- ―Entry on James's gift[src]
After crossing the scent of a human or an animal, James could instinctively predict his prey's moves and use it as a means of tracking and then follow his target until he caught up. He could pick up a target's scent, then walk away and leave it alone for a while before beginning to hunt. The hunt may take time, but he never lost his prey. He could also predict his prey's moves in advance, and pick up an old, faint scent.
James's power of predicting his prey's moves wasn't perfect, meaning his target may change course of movement. Supposedly, he was only good at tracking living beings.
As a human, James was trained by his father to track and trap his prey, learning quickly and becoming the best tracker on the frontier.
As a vampire, James dedicated to hunting for sports. To challenge himself, he first picked a person in a crowd, picked up the scent with one whiff, then leave it alone for sometime (weeks or months) before he started hunting; when that became too easy, he started picking someone on a crowded dock, follow the ship a few months later, and search for his prey in another country. When he got bored of such quests, he started hunting vampires; he especially liked picking a coven member, as the other members would attempt revenge when he killed one of them. He even hunted a werewolf across Siberia once, though he didn't know what it was at that time.
While hunted Victoria, her ability to escape made it impossible for him to kill her, as she always sensed his dangerous presence and avoided him. Even after years of chasing, he never caught up to her. Eventually, James became so impressed with her evasion that he felt attracted to her. James never viewed letting Victoria survive as a failure, because he had found her and - in a way - claimed her life.
While pursuing a different prey, James came across Mary Alice Brandon's faint scent: she was what the Volturi called "a singer" for James. He followed her trail to an asylum. However, Alice's power of prediction alarmed his coming, she and confided in her only friend, an old vampire who worked nights at the asylum. After realizing he wouldn't be strong enough to stop James, he bit Alice and took her away. By the time James caught up to them, Alice was already in the final process of her transformation: there was no trail of blood left for him to savor.
- "He's a tracker, Alice, did you see that? He's a tracker!""
In Twilight, James and his coven met the Cullen family: another vampire coven, in Forks. James decided to hunt Bella Swan after seeing Cullens' protectiveness (especially Edward's) of the human girl and Alice's surprising presence, which made the most exciting game for James.
Laurent warned the Cullens that this power made him deadly, and they should be very careful around him, since hunting was James's obsession.
Edward attempted to mislead him with Bella's scent - it worked sporadically. Trusting his instinct, he followed Bella all the way to Phoenix where he set a trap for her by luring her to a ballet studio to separate her from the Cullens.
- "He is a skilled tracker. He catches the essence of a person's mind and then follows it like a scent over any distance."
- ―Entry on Demetri's gift[src]
By coming across the "essence" of an individual's mind and focusing on it, Demetri can find said individual no matter where he or she is. He is known as the greatest tracker in the world, far superior even to James. He doesn't need to have physically met the person to be able to track him down; by coming across someone else who had met the target, he can pick up the essence through him or her.
Seemingly limited to finding living beings only, Demetri can only link to his targets once he has physically met them or has picked up his or her trail from someone who has meet them in the past. It is believed that only a mental shield such as Bella Swan's can make this power useless. Supposedly, this power can also be thwarted by Fred's power of repulsive magnetism, which will render him and others invisible - since he will feel repulsed at the thought of Fred.
Demetri learned to improve his power over the centuries, thanks to his creator Amun. When his power drew the Volturi's notice, Aro replaced him with the previous tracker, who seemed to have a less potent power of tracking. No one had ever been able to hide from his power.
Whenever a coven had committed a crime that drew the Volturi's notice, Demetri was assigned to find them, since no one could ever escape his power.
- "He's a tracker - a tracker a thousand times more gifted than James was. His ability is loosely related to what I do, or what Aro does. He catches the... flavor? I don't know how to describe it... the tenor... of someone's mind, and then he follows that. It works over immense distances."
- ―Edward's description of Demetri's gift.[src]
Upon leaving Volterra by the end of New Moon, Edward tells Bella that he did not want to touch Aro's hand again, because he did not want him to find out he had discovered that Demetri's tracking ability is mental. Working from this statement from the book it is fair to assume that thoughts linger or leave behind some kind of trace Demetri is able to pick up and follow. The interesting part of how Demetri's ability works is that it would be ineffective on Bella, "her brain being wired wrong".
Demetri is assumed to have accompanied Jane, Felix and some of the other Volturi guards under Aro's orders to study and eradicate the Seattle newborn army. In the movie, Demetri accompanies Jane, Alec and Felix to Seattle and then Forks, hinting that he tracked the newborn army to their locations.
In Breaking Dawn it is also commented that, if Demetri's power did not make him the greatest tracker in the world, Aro would have replaced him by now, as Aro doesn't surround himself with "second best".
The Cullens believed that, because of his skills and capabilities, killing Demetri would be one of their main priorities if it came to open war with the Volturi in Breaking Dawn. Renesmee, Bella and Edward's daughter, was the primary target on the Cullens' side, and they knew that if somehow Renesmee did escape with her life, Demetri would be sent to find her and bring her in for "justice".
It is very likely that Demetri is the one who tracked down Toshiro in London.
- "Alistair can track both people and things. He can sense the general direction of whatever he is looking for, but it takes him a long time to narrow this feeling down to a specific location. If whatever he's seeking is on the move, he may never catch up."
- ―Entry on Alistair's gift[src]
Alistair is the only vampire known to be able to track inanimate objects in addition to living beings.
His power works by focusing on a target to find its general direction; he can also narrow down a location and the distance between them. It is described as a kind of a magnetic pull toward the target, though it is possible to avoid it.
Though useful, this ability is more limited than those of James's and Demetri's. Alistair mostly feels the general direction of the object he focuses on, and it takes some time to narrow his feeling down to a specific location. If whatever he is seeking is on the move, he may never catch up.
Alistair's power was manifested from his good hunting skills as a human, which converted into a power when he became a vampire.
- "Alistair was a tracker, though not nearly as precise and efficient as Demetri. Alistair just felt an elusive pull toward whatever he was seeking."
- ―Bella's description of Alistair's talent.[src]
- Alice Cullen has the power of foreseeing. Though this power is not the same as tracking, it can be used in that method to track a target's moves and the direction it is headed.