The Xindi-Avian species was an extinct member of the Xindi race, wiped out during the Xindi Civil War which was fought between the six Xindi species and which destroyed their homeworld sometime in the 2030s. As far as known, none of the Avians fled before their homeworld was destroyed due to inadequate technology.
Gralik Durr's grandfather, who had resided on Xindus, remembered a time before the planet's destruction when Avians occasionally flocked overhead, filling the sky. (ENT: "The Shipment") A Xindi-Reptilian lieutenant once speculated that, if the Guardians had given the Reptilians dominance over the other Xindi species prior to the wars, their "Avian brothers would still fill the skies." (ENT: "Zero Hour")
The remains of an Avian settlement were found on a planet that dated back to the 2nd millennium BC. The ruins of the settlement were used for the meeting place of the Xindi Council. A skull of an Avian was kept at the Council's location. Commander Dolim commented that the Council chamber had "the stench" of "a race that failed to survive." (ENT: "The Council")
|Aquatic • Arboreal • Avian • Insectoid • Primate • Reptilian|
The Xindi-Avians were originally devised as a sixth member of the Xindi Council, though this idea was discarded due to uncertainties concerning how to actualize them as physical designs. Executive Producer Brannon Braga explained, "We did [consider avians, or bird people, among the original five], but we thought two CGIs [the Xindi-Aquatics and Insectoids] were enough – and you don't want to do actors as bird people." (Star Trek: Communicator issue 149, p. 50) To represent a Xindi-Avian skull in "The Council" (described in the episode's final draft script as "the petrified skull of an Avian"), a model replica of a giraffe skull was used. 
According to the reference book Star Trek: Federation - The First 150 Years (pp. 57, 56, 53 & 52), the Xindi-Avians were the only Xindi species that did not trust nor believe stories from the duplicitous Sphere-Builders. The book contains an illustration of an Avian, which the publication describes as "the only known image" of a member of the species, and an historical writing involving an encounter between an Arboreal child named J'Acov and an Avian, on the final day of the Xindi homeworld. Apparently written by J'Acov, the account details how, having climbed up a remarkably thin stalk to reach the doorway of an Avian home, J'Acov saw the Avian fly directly at him, land on one of several thick vines surrounding the dwelling, then ask, talking in a harsh rasp, "Why have thee invaded my home?" The document also mentions many nightmare tales, all of which had been heard by J'Acov, of angry Avians plucking Arboreal children from their homes.