The Evolution of the Placental Bird

The Placental Bird

Live-bearing placental birds could be considered the peak of avian reproduction, the first and only bird to completely remove itself from the constraints of the egg completely. Such an adaptation did not appear in a single step, however, but is rather the culmination of many adaptations developed by their ancestors over countless generations.

Ornkeys

Ornkeys are a group of arboreal prosimian-like changeling birds native to Serina's tropics in the early Pangeacene. They are very similar to the ancestors of all other flightless placental birds.

Female ornkeys produce very small (about .5 cm diameter) and soft-shelled eggs like other changelings, but fewer than most others, only between twenty and fifty. Most other changelings retain clutches of eggs in their oviducts for up to a day as they are produced, then releasing large numbers of them simultaneously. Ornkeys, however, retain their eggs for much longer, long enough that the eggs settle and implant themselves into the uterus of the mother and - within several days - hatch internally. It's likely than in an even earlier ancestor to the ornkey, these babies would then be born as per usual, simply as newborn larvae. The ornkey, however, has already evolved beyond that first stage and its young are not immediately voided but retained longer still, feeding for the first day or so on their egg yolks during which time their tissues fuse with the blood vessels of its mother's uterine lining, providing ready access to nutrients and oxygen in the mother’s blood and the facilitation of waste removal so that they continue to receive nourishment even after their food supply is used up. The embryos are then retained in their bodies for several more weeks while they develop and are nourished by the parent through the earliest stages of their life cycle, so that by the time the young finally lose their tie to the parent and are born, they have skipped the bulk of the larval stage altogether. Their eyesight by then is thus already acute and their arms and legs are both well-formed and able to climb, jump, and walk, so that even though fewer young overall will be produced in the end, they will all be more likely to survive. Though barely three centimeters in length at birth - still smaller than a great many insects - the babies are thus immediately capable of fending for themselves and are abandoned by their parent. They are carnivorous and begin to eat immediately, feeding mostly on insects and smaller animals that they could catch in the raptorial claws that line their wrists through a combination of ambush and active hunting in the branches. Some will succumb to starvation, unable to catch enough food, but the most successful hunters can eat up to its own body weight in flesh in a day.

At this stage a young ornkey is still bald and featherless, however, and though its lungs are fully-formed and its skin is watertight it cannot maintain its own body temperature and is an ectotherm, relying on warm environmental conditions to survive. To avoid predators and hide from prey, it is often colored cryptically in green or brown for camouflage. Within a month, however, it begins to develop its first feathers in the form of a fuzzy coat of "peach fuzz" that covers its head, neck, and torso. By two months of age, the animal has put on substantial size as a result of its insatiable appetite and is now as large as a hamster - four to five inches in length and several ounces - and able to keep itself warm. It is also now well-feathered with an insulating coat of plumage and has become especially agile, leaping and climbing with great speed through the branches. Its hind legs are now nearly as long as its forearms, while the bones in its wrist have split into two grasping digits, each tipped in a curved claw, that give it a grasping grip on the branches as it moves through the canopy. By this time their diet broadens significantly to include not only animal prey but also fruit, flowers, and even the occasional bite of salad. Sometime between five months and a year of age, the animal has reached its full size of a few pounds and also now reaches sexual maturity, with males developing brightly colored wattles and exposed skin along their faces and brightly colored plumage. It is now completely warm-blooded and fully feathered, except for along its arms and legs which still sport bare, scale-less skin, but retains many aspects of its appearance from its infancy. Ornkeys have lost their flight feathers entirely, but retain a small patagia of skin between their arms and their torsos, as well as a propatagium in front of their arms, that give them some gliding capability. Their wrists have become more mobile than other birds, able to rotate and be pronated, to better provide a grasp in the canopy and the shoulder has become more flexible with a range of motion near to 180 degrees, enabling the bird to brachiate.

above: Life stages of an ornkey (not to scale), from egg to adult. Stages shown in circle are those which are passed through in utero, including newborn with unabsorbed egg yolk and fetus adhered to the uterine lining of the mother.

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Birds like the ornkeys are considered to be the direct ancestors of serezelles and other flightless, four-legged placental birds alive today, which built upon the adaptations gained by their ancestors and adapted to retain their offspring for longer in the womb so that they could be born at an even more developed state. Approximately twenty-five million years ago, one group of ornkey-like changeling birds began specializing toward a more herbivorous diet of leaves and twigs. They grew larger and developed a bulky stomach to better process it. At first their young remained carnivorous, transitioning gradually to a vegetarian diet with age, but as the adult grew larger they became able to retain their young internally for much longer. Chicks were now born fully feathered and already able to digest the plant-based diet of the adults. As the adults became more herbivorous, they were no longer a potential predator to their own offspring, and it would benefit young to stay near their mother for protection; over time, the mothers began to guard their young, protecting them from predators and indirectly showing them how to find food. They began to cling to their parents' backs and even to take food that spilled their mouths while they fed.

Over time, some of the tree-dwelling herbivores became so large that as adults, they had an easier time feeding on the ground. To protect their claws, they walked on their knuckles, foraging on the forest floor and in grassy clearings. As they spent more time on the ground and their wrists specialized towards supporting weight at the expense of a gripping hand, the knuckle became a weight-bearing hoof-like structure and the wrist elongated significantly, developing an ungulate-like form. They eventually produced forms akin to deer and antelope, adapted to run at high speeds on all four limbs - the serezelles. To survive on the plains, where cover was minimal, they retained their offspring for even longer, giving birth only when their young were sufficiently developed to be able to walk and run alongside them almost immediately.