They are from an oceanic planet, where endothermic, feathered flyers are one of the most successful land-based lifeforms. Since land is limited to a scattering of small islands, the feathered flyers can travel the distances between dry land more easily than flightless or exothermic creatures.

The evolutionary ancestors of the sapient genus of avians are tree climbing critters with grasping hands on both their fore and back limbs. In the sapients, the hindlimbs are specialized to a point where they are used almost entirely for grasping, tool manipulation, and launching into flight. The dramatic speciation of the sapient avians was caused by hundreds of thousands of years of separation by ocean, and fostered by a lack of traumatic events like Earth’s ice age or K-Pg asteroid impact to wipe any of them out. Besides the five major species, there are many subspecies that can successfully breed with one or more of the major species.

The avians are a technologically advanced genus. After the industrial revolution, aggressive colonization by the skimmer species, and the globalization of information, the avian genus achieved interstellar flight. The first aliens they made contact with were the ferrets, then later humans (after the ferrets found us.)

Note: Despite the fact I drew all of them naked, avians have very strict decency standards (well, the skimmers do, but they have forced their cultural standards upon the others) and they wear quite a lot of clothing; usually covering as much as they can without restricting flight. They are like little space Victorians.