In order to form a complete history of the birds, Gessner brought together as many sources as he could. He included one of the first extant scholarly descriptions of a bird of paradise, found in the expansive De Subtilitate (1550), a book by the Italian mathematician and astrologer Jerome Cardan. Cardan reasoned that, because these birds were never seen alive and could not land without feet, they must exist perpetually airborne in the highest reaches of the sky. Cardan argued that nothing solid was ever found in their bodies, so they must be like the mythical rhyntace, “a little Persian Bird which has no excrement, but is all full of fat inside, and the creature is thought to live upon air and dew”. He also suggested that males had a cavity in their backs in which females laid eggs and incubated them, and he coined the Latinate term Manucodiata, drawn from the Malay name Mamuco diuata (birds of God). Gessner also used the description by French naturalist Pierre Belon, who, in his L’Histoire de la nature des oyseaux (1555), described the headdresses of the Janissaries he had seen in the Levant. They contained “plumes of a bird called the Rhintace…” from “a small creature of which only the skin is left” that he believed “may be the Phoenix”.