An avian species is a race that is reminiscent of Earth birds. This generally means that avian species have feathers, and sometimes a beak and claws. Avians usually reproduce by laying eggs.
The study of birds is called ornithology.
Chickens and ducks were among the animals the Bringloidi took with them when they were evacuated from their planet by the USS Enterprise-D. (TNG: "Up The Long Ladder")
The starship USS Raven was named after the bird of the same name. (VOY: "The Raven")
When Leonardo da Vinci had trouble understanding what had happened to him when he was transported onto Tau's planet, Captain Janeway likened his situation to that of a sparrow, only interested in hunting insects and singing for a companion and not knowing anything about the politics of Florence. (VOY: "Concerning Flight")
Early models of da Vinci's flying machines were based on bats or sparrows. Janeway suggested, however to base them on hawks instead. (VOY: "Scorpion")
Bird-like species could be seen flying across the skies of Mintaka III, Tarok, and an unnamed planet in the Delta Quadrant. (TNG: "Who Watches The Watchers"; VOY: "Initiations", "Time and Again")
A humanoid bird-like creature was on display in Trelane's mansion. (TOS: "The Squire of Gothos")
Bird was one of the words Miles O'Brien used due to his infection of the aphasia virus. (DS9: "Babel")
Major Kira Nerys told Mullibok a story from her childhood about a huge ugly tree in front of her window, so big and ugly even the birds won't sit on him. (DS9: "Progress")
Birds in art
Birds were often seen incorporated into pieces of art, like sculptures. (TNG: "Encounter at Farpoint", "Too Short a Season", "The Bonding", "Unification II", "Starship Mine", "Preemptive Strike", "All Good Things...")
A bird-like creature was seen on a Vulcan wall decoration on the planet Dekendi III. (ENT: "Stigma")
The Romulan logo consists of a bird-of-prey holding Romulus and Remus in its talons. (TNG: "The Neutral Zone")
Da Vinci was so fascinated by Tuvok's Vulcan ears that he drew a picture of a caged bird with pointed Vulcan ears as wings. (VOY: "Concerning Flight")
Sentient avian species
The Baneans may have evolved from avians as well, due to the fact that they have feathers.
Other avian species