Birkans

Bilkans are the race that came originally from Bilka.

Appearance
Like Skypieans and Shandians, they are endowed with wings on their backs. What differentiates them from the other two races is that their wings face downward. One notable exception is Enel, who doesn't have wings at all: in place of them he instead has a ring with drums on it. Bilkan people are quite various in their appearance. Some of them possess goat-like facial features such as floppy organs on both sides, resembling goat ears (though they also have normal human ears) and small horns. Unlike the Skypieans and Shandians, their people tend to dress in long white robes and gowns or other religious or partly religious looking attire. That gives them them a religious look to some extent. Their race appears on a drawing in Enel's mini-series on an image that represented the three races of the Sky Islands; the drawing depicting a race with the same wings. Urouge, a pirate in the Blue Sea, also has a Bilkan appearance.

Strength
Enel, Ohm, Gedatsu, Shura and Satori all had access to mantra with Enel's mantra noted to be the strongest due to his Devil Fruit powers, while Satori's was the weakest.

Departure from Fairy Vearth
Their known history is that they were one of three races that once lived on the Moon and aided in the creation of the Spaceys. When resources on the moon dried up, they along with the Shandorians (ancestors of the Shandians) and the Skypieans left the moon and the Spaceys behind. They abandoned their city of Bilka to settle life anew on the "Blue Star".

The Tides of History
At some point in history, the three races became separated from each other and fell into the individual races that are seen in the current storyline of One Piece. The Bilkans settled on a Sky Island apparently named after their former home, Bilka. They also appeared to "forget" their origins, although the moon, known to them as the "Fairy Vearth" remained a folk tale amongst their people, believing it to be a place of dreams.

The End of Bilka
Eventually, Enel came along and, using his Devil Fruit powers of the Goro Goro no Mi, destroyed their homeland of Bilka. He then traveled to and took over Skypiea and made himself God there.

Trivia

 * The ancient drawing depicts the 3 races of the sky. However, the drawings also appear to depict a group of people working together towards one goal. With this noted at some point the Shandorians, Skypieans and Bilkans may have even been one and the same race (more believable since the Skypieans and Shandians wings are almost identical). Regardless of whether or not they were the same people, to date the history of them separating (or maybe just losing contact with each other) has not been mentioned in the storyline. In fact the only mentioning of a link to the three races past appears to be the Bilkan's folklore about the Fairy Vearth.