Rise of the Metamorph Birds
The evolution of the metamorph birds, also known as changelings, began relatively early on in Serina's history, no more than 75 million years PE. The common ancestor to all changelings was a small predatory perching bird which you may recall as the strackbird, a close relative of the parasitic bloodpeckers which lived in close mutualistic associations with large carnivores of the Cryocene epoch, which they assisted in finding food in return for the gleaning of scraps. It was this association which was likely the driving factor for the "larder and larvae" reproductive strategy that eventually gave rise to the group, in which the young hatch very rapidly at a mostly underdeveloped state but with large jaws, and can quickly feed themselves on stashes of food provided by their parents in the short window of a few weeks that their host would remain in a given area before moving off in search of new prey. The large kills taken by their hosts also made it easy for the parents of the earliest changeling ancestors to rapidly fill a larder with stocks of meat to last their young after they had moved on. Though the adult birds had begun to evolve away from their hosts altogether into a variety of new niches and some groups had even appeared where these "larvae" became entirely free-living and independent before the end of the Thermocene, it would not be until after the great ecological upheaval of the Thermocene-Pangeacene boundary that the group would truly take off. With the stark decline of more primitive bird perching bird groups as well as large megafaunal lineages, the changelings were now set to take over.
Metamorphs have undergone an incredible radiation since the start of the Pangeacene, reaching levels of ecological and morphological diversity unrivaled by any other animal group currently alive. Before the extinction event, changeling birds were common but did not enormously diverge from their ancestral niches in adulthood, coexisting alongside more primitive birds for tens of millions of years. This has changed markedly since the end of the Thermocene, with the changelings now having produced some of the most aberrant bird lineages ever to walk, fly, or swim their world.
~~~
Metamorphs are of course distinct from other birds for their two-stage life cycle; ancestrally, they hatch out from a small, soft-shelled egg as a very small and very underdeveloped worm-like creature, which is capable of feeding from birth. The first changelings were nest-bound carnivores, which fed on larders of protein provided for them by their parents, which would then disperse before they ever saw their young. These earliest species had little need to move far or quickly in their juvenile stage, and had only to pull themselves along at a leisurely pace with their stubby forearms, which eventually developed clawed spurs through the activation of latent genes that had lain dormant for hundreds of millions of years - often many more than were present in any of their ancestors, but which originated from the same genes and were fundamentally the same structures. They needed only to eat their way around their provided stash of food until they were large enough to cease feeding, enter a state of torpor, and use up their fat reserves as they developed into feathered, typical adult birds. In this process they lost much of their body weight, absorbed their fatty tails, shed the claws from their arms and the hooked tips to their beaks, opened their eyes for the first time, and developed their hind limbs. They would then take their first flight out of the nest as a sexually mature adult, the sexes of which in some groups of which would then evolve to be highly distinct to avoid competition for food resources.
Two groups diverged from the early nest-based carnivorous metamorphs. One subset began to lay their eggs directly in food sources without building a nest at all. Their first choice were carcasses, which would provide a great supply of food, but at the risk of all the larvae being killed if the entire carcass were consumed by a larger scavenger. This selection gave rise to birds that specialized to lay their eggs in the tissue of living animals, which provided safer refuges for their young, as they could avoid predators on their own. These metamorphs relied on sharp jaws to cut a wound into the body of the host and then deposited their tiny eggs into the tissue, where they would hatch into blind and ravenous grub-like parasites that would crawl through their host's flesh, just under the skin, and gnaw at the flesh or simply bite down and grow nourished on their blood alone, hanging off the prey like a sessile tick with no limbs at all to speak of - simply a sharp biting jaw attached to a fat worm-like body - until they dropped off and pupated on the ground into an exceedingly small and short-lived adult. Though some adopted other diets, many of these tiny, insect-like birds remain parasitic even in adulthood, following herds of large animals over the grasslands in dense swarms and feeding on blood and tissue with a sharp hooked beak in the way of their similar though larger Cryocene relatives, the bloodpeckers.
The second major group, and the much more morphologically diverse of the two, are those with mobile, free-living larvae. As some of the early nest-bound changelings became more independent and able to move freely outside of nests to take care of themselves, they had to had to develop additional adaptations to find food and avoid predators. Though they still started life as a tiny worm-like larvae without vision and only a single pair of limbs, their developmental pattern changed. Now, some quickly developed their hind legs and opened their eyes well before they began to truly develop into adults, combining juvenile and adult traits. New musculature had developed in their wings that allowed them to support weight and they crawled on all four legs, something their ancestors had not done for many hundreds of millions of years. Worm-like larvae which respired through their skin and could only find their food through touch and scent could survive only in sheltered, moist environments rich in food. Though this initially meant a nest lined with meat, as their diets became more adaptable, it came to include almost any warm, moist environment, such as the damp leaf litter of the forest and even the shallows of a vegetated pool of water - both of which became some of the most common breeding sites for adult changelings to deposit their eggs. Now, however, as some groups began to develop their hind legs and eyesight at increasingly tender ages and lengths of just a few millimeters, they could leave these nursery environments soon after birth and exploit a greater variety of food sources. Some of these minuscule insect-like infants adapted their clawed forearms to climb up into the branches of trees and bushes to graze on new sources of food such as buds and leaves, while others used theirs to actively hunt, catching and killing smaller invertebrate prey. As the quadrupedal larvae moved into new niches and adopted new strategies to move, find food, and avoid predators, some became too specialized to lose all of their juvenile adaptations and develop into a typical bird as their ancestors did; some also had become too large in their larval states to simply hole up and spend a few weeks transforming into an adult without being highly vulnerable to predators and injury during the lengthy and defenseless intermediate stage. The process of maturation had to become longer and more gradual; rather than a dramatic change from one form to another, it now involved a steady progression of development while the animal remained active and feeding. Because it could no longer involve as dramatic of a change, the mature adult often ended up retaining many of its juvenile features; plumage grew, and there would perhaps be some changes to the structure of the beak and the proportions of the limbs, but the diet rarely changed drastically - if it did, only after a long period of transformation during which it fed omnivorously. Perhaps most noticeably, they retained the clawed digits on their wrists and the abilities to climb and even walk on their wings as quadrupeds even after maturity. The result was an adult bird with a broader range of available niches. Quadrupedal flyers could utilize a quadrupedal launch, putting the wings to use in take-off and greatly increasing the size they could attain while still being able to enter the air. Large herbivores with ruminating stomachs could become enormous, simply dropping onto all fours when their guts became too awkward to support on their hind legs alone. For some groups, developing traits related to being a flying adult was no longer important and the transition from larvae to adult consisted of nothing more than the development of the sex organs while the bird retained its juvenile habits and form, never flying or even developing the specialized feathers that would allow it.
All the while, another group was going down an opposite path by specializing in a way completely opposite from its adult niche: they became fully aquatic. Breathing through their skin, very young metamorphs could survive either in moist environments on land or in oxygenated water, so long as they could find organic matter to eat. Some retained their permeable skin past the first few days of life and adapted their stunted arms into fins, elongating their worm-like bodies into a vertically-compressed eel-like form that could undulate to propel them through the water. Using the whole of their skin to absorb oxygen worked for small species with sedentary lifestyles, but to achieve greater sizes and maintain higher activity levels, specialized gill tissue was needed which could more efficiently pull oxygen from the water. The mouth was the most available region for its development, and the lower jaw expanded outward and back with cartilaginous plates that supported a highly vascularized pad of such tissue, consisting of a mass of highly folded, blood-rich filaments on either side. Water was sucked in through the mouth, over the filaments, allowing gas exchange, and expelled through a pair of spiracles at the back of the jaw which developed from the ear canals, in much the same was as occurs in fishes.
Few traits of the adult bird were now of any use to the larvae, and many ceased to appear - the hind legs stayed vestigial nubs, which then became streamlined steering fins. Eyesight, of course, proved important for most and was an evolutionary priority, with vision now developed at hatching.
The end result were creatures which could not in any respect to be said to resemble birds at all; finned, featherless, and fully aquatic, taking oxygen from the water, they had effectively reverted back to the form of the common ancestor to all land vertebrates; the fish. Specifically, with elongated bodies and only small fins, they first resembled the most primitive of all fishes, the lampreys. As their bodies in their juvenile state became increasingly specialized and - most important - larger than a few inches in length, just as with their cousins on land it became impossible for them to transition smoothly into a flying adult without an extremely awkward, hazardous intermediate stage. Massive populations and countless opportunity for chance mutation struck again, and a small percentage happened to develop their sex organs without attaining the rest of their adult traits. Just as their predecessors - both their most recent flying ancestors and long, long before, their aquatic piscine ones, they simply broadcast their eggs and sperm into the water. And with that final advancement, they lost their final ties to the skies. Forms evolved with better streamlining; air sacs in their tissues specialized into a series of swim bladders that lifted them from the bottom into the open water and left them neutrally buoyant. Now able to compete actively with other aquatic life thanks to the influx of oxygen they could now take in with their specialized gills, they spread out from calm inland pools down tributaries and estuaries, or with their eggs adhered to the legs of water birds, and they colonized the sea alongside the ray-finned fishes in perhaps the most extravagant example of convergent evolution ever to occur.
Back on land, their relatives were now adopting a life cycle that could not be more dissimilar. While the sea changelings embraced their larval stage, producing smaller and smaller young and in extraordinary numbers, the terrestrial quadrupeds had begun to revert away from their larval stages at all. Some flyers began to provide a larger store of nutrients within their eggs so that their young could remain inside, relatively protected, until they were better developed and has passed the fetal larval stages - not unlike the ancestral birds they displaced. Others, however, adopted a method more unique and hatched smaller numbers of their young in their bodies, nourishing them with a placenta, so that they progressed rapidly through their worm-like infancy in a fetal state once more and were born - live, in the manner of a mammal - only once they were much more developed. Many also redeveloped parental instincts that included guarding and even actively feeding their chicks, behaviors lost in their ancestors. In this way they produced far fewer offspring, but proportionally many more would survive. Vivas were the first birds to develop live birth, and mastered it many tens of millions of years ago. It would be the metamorphs, though, which perfected the process.