Mysterious or just misunderstood? I don't know, but I sure don't want one knocking me in the noggin.
The amount of liquid goop that breaks through cracks in the Avantsphere's crust appears inexhaustible, with all variety of goo and other volatile matter. A few of the fauna native to the sphere have adapted to their new enviornment and indeed some even flourish there. Among them, only found in hot geyser ponds, are the sawna boids.
They clamor together in tight groups and use the rising steam as a camouflage to avoid getting messed with by the outside world. There eating habits remain somewhat of a mystery, though it is assumed by naturalists that they attain nutrients from the bacteria in the hot liquid they make their home. There numbers are most populous in the eastern areas of the Ava, though those that have immigrated far in search of steam may be found across the sphere.
The plural form of "boids" is boidses because some people can't talk good but think dey is wicked smaat.
Their settlement colonies, however, are temporary residences. Once the hot pond is dried up, they take to the sky, and won't land until they find another rising column of steam to circle down to and make their own.
Because of this nomadic lifestyle, all rearing of young must be done when grounded. The boids possess enough intelligence or keen enough instincts to know when mating is best to avoid complications of birth when they need to once more take to the sky after a hot lake exhausts itself.
However, there are mistakes. Occasionally, one of the boids will give birth during flight, but without landing, the chick plummets helplessly to the ground. This has led to people exclaiming "fuckin' sawna boids!" if something hits them unseen from above, referring to the chance of a plummeting chick smacking your dome.
It remains unknown why the birds refuse to make landfall outside of heated oases.
On rare occasions, if unable to find a serviceable molten puddle, they will continue to glide airborne until their lives are claimed by the elements and exhaustion. One by one, they will fall from the sky to ground, possibly maiming any dude or structure they strike below. Again, "fuckin' sawna boids!"