There are more than two-thousand species of bird present throughout the Great Agarthan Jungle, from the minuscule anteater hummingbird to the greater spherical penguin. Despite the extraordinary diversity present, however, all of their eggs, no matter the parent, are outwardly identical. Each egg is recognizably Agarthan by its signature gömböc curvature, gumdrop size, and transparent shell that reveals nothing but green jelly inside. If appearances are to be believed, there is no embryo within at all- just the same undifferentiated ooze.
This is because the contents of each egg are biologically encrypted. During the course of its development, each bird exists as a confused gelatin of organic substances, rather than as a recognizably developing fetus. Not even the mother is certain of the contents of her own eggs; she knows that some of them will eventually hatch and be revealed as her children, but her eyes cannot pierce the veil of secrecy any more than any other creature. All she knows is that the radiation of her body is slowly deciphering their yolks, allowing them to eventually come alive.
The whole process exists as a form of Müllerian mimicry: because all eggs are encrypted, and all bear the same general appearance, predators are incapable of knowing which species it is that they are preying upon. They do not even know if an egg is genuine, or if it instead contains a false, venomous yolk, until it is too late. Only a handful of birds, such as the widower macaw, actually lay eggs intended to kill, but enough do so to act as a deterrent for the rest of their feathered kin. To ensure the effectiveness of this trait, nests tend to be communally shared among multiple species.
The biological encryption exhibited is remarkably strong. Human experimentalists have attempted to break it by copying the radiation signature from the mothers among these birds, then exposing their eggs to an artificial signal bearing the same properties. Though these experiments have been far from what might be considered successful, the results have certainly been interesting: human-induced hatchings of Agarthan bird eggs have only ever managed to result in the emergence of human infants from them.