The Bird race (aka Avian) is a class where formidable bosses can be found. Enemies that belong to this class are weak to Bird Killers. Notable Bird Bosses include Glacial, Fat Chocobo, and the Esper Phoenix. Bird Killer skills are a passive effect when equipped by a unit.
To find the killer gear for the Bird race click here to open the Killer Gear Finder
These trial bosses can be found in the Vortex. The Esper battles can found in the world map. See wiki page for more details.