An avian was a type of lifeform.
While researching the Red Angel, Michael Burnham cross-referenced the entity with all known winged and avian lifeforms in the Federation, without success. (DIS: "An Obol for Charon") Similarly, after encountering the Red Angel on his homeworld of Kaminar, Commander Saru filed a report that included a section on "Avian/Winged Life Form Research". The report included the statement, "The correlation between any winged or avian lifeform has not been established," as well as a later-added section which regarded "New Findings" and implied that the entity was no longer considered possibly avian, since it was instead apparently humanoid, wearing a highly sophisticated winged suit. (DIS: "Project Daedalus")