Legless bird

"...a flock of white birds, about the size of a thrush, appeared, flying quickly along the side of the lake in a very swallow-like manner. One of them, Graham was fortunate enough to shoot; and we were surprised beyond measure to find that it was without legs, and pouched, like the Marsupalia, or kangaroos of Earth. In the pouch were several pinkish eggs; and we inferred that this creature hatched them as it carried them about the air, requiring no nest for the purpose. It was clothed in plumage more like scales than feathers, the filaments being similar to fine shreds of horn."