The sixth Xindi species, capable of flight, which died out when the Xindi home world was destroyed at the end of a long war among its peoples.
The Xindi-Avians were one of the six intelligent species that evolved on the planet Xindus, as described in the third season of Enterprise. They could fly, and it is said that they once filled the skies of their world.
About a hundred and twenty years before Enterprise entered the Delphic Expanse, a century of warfare among the Xindi ended when Insectoids and Reptilians set off massive explosions beneath the planet's most unstable seismic fissures. Xindus broke apart. The other five species had ships or colonies to flee to, but the Avians did not escape, and none survived.
No living Avian is seen. They are spoken of as a loss the other Xindi still feel, and in The Council the leaders meet near a ruined fortress the Avians built before their extinction, where an Avian skull is on display.