The Ultimate Guide to Metamorph Birds of the Early Ultimocene

The Age of the Metamorph Bird

Clade Metamorphaves

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The world of the Ultimocene is dominated by strange and wild creatures, many of them utterly alien compared to their introduced ancestors, but of all the planet's abundant life possibly no creatures are more derived than the changelings, or metamorph birds, which have by this time reached their highest degree of diversity ever. Indeed the early Ultimocene could be considered the age of these extraordinarily varied animals, which can trace their ancestry all the way back to the Cryocene, 200 million years ago, where their long distant ancestors evolved from the strackbirds - thrush-sized songbirds which lived nomadic lives and associated with large terrestrial carnivores to find food. This lifestyle required significant adaptation in order to reproduce, as the parents were always on the move. This resulted in the strackbirds' evolution of increasingly self-reliant offspring that could take care of themselves from hatching, albeit in an entirely novel way from any bird before them. They evolved free-living larva-like chicks, fetal birds which hatched very early, with low metabolisms but strong jaws that could crawl and feed themselves independently of their parents, a process initially facilitated by the parent birds filling a secure tree nest with a larder of fresh meat for the nestling to feed on after hatching, by which time the parent would have moved on with its hosts to next again elsewhere. The evolution of a free-living larval form distinctly unique from the adult in habits and habitat opened up an enormous degree of opportunity for further specialization and allowed for their survival even through the harshest extinction event the world had ever seen at the end of the Thermocene. From this ancestral form, the changelings have by the Ultimocene since diversified incredibly in form and function, with their current descendants ranging in size from a few millimeters to more to tens of meters in length, and filling nearly every conceivable niche on land, air, and water. Today comprising as much as 85% of extant bird species, they are the most successful clade of birds ever to evolve on Serina. With multiple radically different representative groups extant in the Ultimocene but most of the ancestral forms with traits that allied them as kin since extinct, their alliances are now often difficult to see on the surface, especially as some have lost their larval stage completely while others, in reverse, have become completely neotenic and no longer have a distinct adult stage at all. With some of the living lineages having been separated from one another by more than 150 million years of unique evolution, the Metamorphaves are now collectively also the most ecologically and physically diverse vertebrate clade to ever exist, surpassing even the bony fishes. They include today both the largest terrestrial birds and the smallest birds of any kind ever to exist.

All metamorph birds of the Ultimocene belong to one of two very diverse clades that are only distantly related. From a common ancestor in the Thermocene epoch, many groups between them have evolved down very divergent paths and exploited often opposite strategies to succeed in the game of life.

The Incredible Shrinking Birds: The Verminfans

The most basal clade of metamorph birds extant in the early Ultimocene are the verminfans (literally "worm children"], which evolved in the Thermocene era. They are highly variable and include among them the most derived of all birds. They are allied by radically different adult and larval niches and forms, extensible cloacas that have adapted into ovispositors in the females, by a reduction in the complexity of the wing so that it consists only of the primaries, usually between only five and ten of them all originating from the manus but in some cases many more, and a tendency for clades to become smaller and smaller over time, culminating in the smallest birds ever yet to live. Verminfans engage in complete metamorphosis, and their flying adult sizes are usually limited to just a few ounces as a result.

Among the more primitive lineages of this clade are the squicks, including the descendants of the Pangeacene fleshborer bird, and the rootsuckers, similar to the common ancestor of the ninth-year canary, both groups having changed very little since the Thermocene when they split off from one another. All of these birds exhibit relatively typical adult forms that are very similar to ancestral perching birds, but very specialized, often worm-like larvae. They engage in rapid, dramatic metamorphosis after a period of larval feeding that varies from brief to extremely drawn-out. More divergent groups include the the pincerwings and the osteopulmas, which sport some of the most extreme anatomical changes of any bird.

Squicks

Squicks are very small, parasitic or carrion-feeding birds which are no larger than one inch in length as adults, with highly reduced vermiform young that are blind and deaf, that feed on dead or living tissue. Adults are fully developed but short-lived and may or may not feed (when they do, they usually feed on blood), and lay their small, very numerous and shelless eggs directly in raw flesh where the young hatch and feed in a very similar manner to their ancestors. These larvae respire through their skin until adulthood and are highly vulnerable to desiccation outside of a moist carcass or body; they feed and grow quickly, then pupate in a ball of hardened mucous into the flying adult form in a span of just a few weeks. In most respects they are the most primitive living metamorph birds, though the adult's wings are highly reduced in structure as a result of their extremely small size and support only five primary feathers, the least of any flying bird. Though some larval squicks sport spines or claws on their wings in infancy, these are not retained in any of the living species into adulthood. Parasitic adults are usually colored cryptically and most often fly by night, where they are less easily detected by their host species, while diurnal forms may be brightly colored.

The herdsbane is a nocturnal, blood-feeding squick that predominately targets circuagodonts, alighting upon and biting them in the night when they are less able to defend themselves. Its young mature rapidly in carcasses and festering wounds.

Rootsuckers and Butterbirds

Rootsuckers (with their adult stages known as butterbirds), unlike squicks, are herbivorous as larvae. Instead of flesh, they feed upon sugars in the roots and tubers of plants in forest environments, which they suck with a sharp upper mandible which is adapted to pierce into the tissues (the lower mandible is vestigial in the larvae, and absent in the adult altogether), and a long, highly flexible tongue which has become a hollow siphoning tube to pull out fluids. To prevent the beak from becoming damaged while the larvae burrows in the soil, it has evolved a very high degree of cranial kinesis and a joint at the base that allows it to fold down flat against the chest while digging; this flexion is retained into adulthood, allowing the butterbird to fold down its beak when not in use. The larvae are less worm-like than in most of their kin; though blind and with permeable skin, they have only a very short tail and are born with fully developed front and hind limbs with which to crawl and digs beneath the ground; two of the three large wing claws of the larvae are shed during metamorphosis, with the smallest and first digit being retained and used in clinging to surfaces when roosting.

Plant sap is a stable but very low-nutrition food source compared to animal flesh or blood, and so rootsuckers may spend many years before they have accumulated enough energy to metamorphosize, very similarly to the ninth-year canary of the Thermocene, to which they are convergently evolved but not directly descended. Unlike that species, some adult butterbirds are now fairly long-lived not just in infancy but also in adulthood. As a perquisite to survive for many years on a low-calorie diet of root sugars, they have developed an extremely fine control over their metabolic rates into adulthood and are able to switch back and forth between a high-energy rate and a very low one that allows them to survive widely varied conditions throughout the year. They are able to take advantage of abundant flower nectar in the summer in the temperate regions in which they are most abundant, and then to hibernate through long, cold winters when food is scarce. In addition to nectar, adult butterbirds consume animal prey by spearing small insects and other metamorph birds with their spear-like mandible, and then inserting the proboscis, which is sharply pointed at its tip, into the body and first siphoning out the blood and other bodily fluids and then scraping away the meat inside in thin strips which can be passed down through the proboscis. This animal food is gleaned from in and among the flowers they feed from, as well as from tree bark, and provides the necessary nutrients for long-term survival which are absent in nectar, and the fat and protein necessary for females to produce their eggs.

To avoid competition from other nectivorous animals, including tribbats and other bird groups, butterbirds are frequently nocturnal and marked in subtle earthy patterns that let them blend in seamlessly with wood or rocks when roosting during the day.

The common switchbeak is a widespread butterbird across the forests of the northern hemisphere. It feeds equally on flower nectar and animal prey, including other metamorph birds, and by folding its bill tightly down against its chest is able to squeeze itself into the tiniest crags to roost during the day, safe from predators or harsh weather.

Pincerwings and Bumblebirds

The pincerwings are a clade of verminfans which evolved from the early ancestors of squicks, which places them as less derived than the modern representatives of this group so far as when they split off the tree of life, though they are more derived in so far as having changed more drastically in form and behavior. As larvae they have adapted their forearms into a second set of jaws which project forward and flex on a horizontal plane and are used in predation. These birds are worm-like and primitive as larvae, like those of the squiks, often with long, roughly cylindrical bodies, but unlike the squicks they have become free-living and mobile in the environment, and are active predators (albeit very small ones), which use their pincers to grasp molluscs, worms and other bird larvae and hold them to the mouth to facilitate consumption. As the wings are adapted into pincers, they are unusable to pull the body along and so these birds have adapted an entirely new and novel muscle system consisting of an outer ring of longitudinal muscles which stretch, and an underlying band of circular muscles which contract, allowing an earthworm-like squirming locomotion that makes them excellent burrowers. Larvae have almost no skeletal elements outside the skull and forearms and most of the body is purely supported by muscle. As larvae, they are blind - there is little need to see, as they burrow beneath the surface of the soil or in the sediment underwater - and rely on touch and smell to hunt; adults, known as bumblebirds, are sighted and relatively normal but are clumsy, bumbling flyers due to the specializations of their wings which limit the flexibility of the wings to just the shoulder joint, effectively producing an insect-like wing which limits their size and maneuverability. The wing is so simplified that it has become more energetically efficient for the bird to simply repeatedly contract and stretch the paired muscle bands around the torso in order to flex them, and as adults the muscles controlling the pincers actually degrade, with flight occurring entirely by flexion of the abdominal muscles. Larvae are small enough that they still respire via their skin and as such can survive in soil or in water, anywhere that is sufficiently moist, while the adults which grow up to an inch in length are short-lived and feed on vegetation, fruit, nectar or carrion. Both larvae and adults of many species of pincerwing are poisonous as a result of storing toxic compounds from certain insects the larvae feed upon, and are brightly marked with warning coloration to advertise their toxicity.

The varicolored bumblebird is a diminutive creature of the equatorial rainforests, its body scarcely larger than a man's thumbnail; as a larva it feeds upon toxic beetles, sequestering deadly toxins in its flesh as an adult, which it advertises in bright warning coloration.

Osteopulmas

Most lineages of verminfan birds diverged tens of millions of years ago, some more than a hundred million. There is one group however which only appeared at the very end of the Pangeacene, roughly 245 million years PE. Possibly the most derived of all birds, indeed vertebrates ever to evolve, this strange, still young clade would be the osteopulmas, or bony-lunged birds - incidentally, the smallest birds ever.

The osteopulmas are most closely allied to the pincerwings, with which they share the specialized muscle groups as larvae and retain into adulthood, also putting them to use in flight, but their larvae lack the adaptation of their forearms into pincers - indeed, they no longer develop forearms as larvae at all - and so their wings as adults are less constrained. They exhibit remarkable specialization of their bodies to facilitate even smaller sizes than their ancestors - at their smallest, just two centimeters in wingspan - which most notably include having switched from active to almost completely passive respiration. The lungs of the smallest osteopulmas have ossified and become inflexible, and the primary means of oxygen absorption in these birds now occurs via an extremely pneumatic skeleton and intricately connected air sac system. The vertebrae on the spinal cord have become hollow tubes which just breach the skin on the birds' back and function as spiracles to passively diffuse oxygen into and carbon dioxide out of the air sac system. Oxygen travels down through a system of trachea directly into the blood stream. The lungs no longer serve any role in moving air into the respiratory system; only minor muscular flexion of air sacs directly beneath the spine provides some pumping of air into and out of the system.

The osteopulmas have also experienced significant changes to their metabolism that set them apart from their relatives. They are thus far the only verminfans which are not endothermic in adulthood. Rather, they are poikilotherms; animals with temperatures that vary widely depending on environmental conditions. This means that their energy needs are dramatically reduced, at the cost of being forced into torpor in cold conditions. The wing muscles of osteopulmas must reach around one hundred degrees Fahrenheit before powered flight is possible; like insects, these birds can fly at considerably lower ambient temperatures, however, by vibrating their abdominal muscles rapidly in order to generate this heat only where necessary. To power their flight strokes, the bird normally rapidly contracts one set of muscles which circle the torso while simultaneously stretching an overlying set forward, which pulls the wing forward in a flight stroke, and then by reversing both muscle groups which pulls the wing backwards and repeats the cycle. In order to warm their muscles, however, the bird rapidly stretches its longitudinal muscles both forward and behind the wing simultaneously, resulting in little to no wing movement but producing a large amount of heat.

The plumage of adult osteopulmas is highly reduced among birds; the primary feathers of the wing are much more numerous than in other verminfans(and indeed, any other bird thus far to exist), numbering up to seventy per wing - but simplified into little more than barbs which zip together, almost turning the entire wing into a single feather. The body plumage is reduced to tiny, iridescent scales and small insulating hairs. They are capable of bearing weight and walking on their wings as a result of their diminutive sizes, the only verminfans so able, and the adult has spines along the manus used to grip and to climb.

Their size also means that osteopulmas are also able to grow extremely quickly and reproduce with a shorter generational turnover than any other bird, sometimes just three weeks. Eggs are laid upon a rich food source, often flesh, fermenting fruit or animal droppings, by the female, who has uniquely evolved a keratinized ovipositor that remains always extended from the cloaca, allowing her to inject her eggs into suitable food sources directly. The maggot-like larvae hatch in just a couple of days and feed aggressively, then drop off onto the ground and pupate in a sac of hardened mucous, emerging in as little as ten days fully formed and able to fly and to breed.

By shrinking into ever smaller and more specialized forms, the osteopulmas seem set for big things. Originating in the tropical rainforests, in the past ten million years they have developed an almost cosmopolitan distribution.

The zebra tweezle, no bigger than a honeybee, is perhaps the archetypal osteopulman bird, exceedingly small, bizarrely derived, and with similar forms found worldwide; it feeds on overripe fruit, feces, and carrion.

Onto Bigger Things: The Ambuloptera

Only slightly less basal than the verminfans, the second major clade of metamorph birds are the ambuloptera (walking-wings), or quadruped birds, which split off from the verminfans very early, and are descended from the first metamorph birds to evolve truly free-living larval stages which began adapting to more varied diets than carcasses, and which became free-living in the forest floor. Though less successful by number of species, they comprise the most morphologically diverse clade of all the metamorph birds, which is extremely varied in form, niche, size and behavior. Most broadly, they are distinguished from the verminfans by more developed larval stages, born with open eyes and able to move more freely in the environment to forage for food. In the Thermocene era, while the ancestors of verminfans were becoming more worm-like, the ancestors of the ambulopterans redeveloped long-lost musculature in their forearms in the larval stage, allowing them to go from merely pulling the body forward to actively bearing weight, adaptation made possible by their neoteny and the still very malleable nature of their anatomy at this early stage of development. These changes greatly improved their freedom of movement over their competitors by allowing them to walk and not merely crawl worm-like on their bellies, and forever changed their path of evolution.

From this ancestral form, the ambulopterans gave rise to many distinct descendant groups, each of them capitalizing on their own strategies and adaptations to follow very different paths over evolutionary time.

Into the Water

All ambulopteran larvae normally show a progression of complexity as they mature. Like verminfan larvae, they are born with very thin, moist, and permeable skin through which they respire, but as most groups grow their lungs develop within a few weeks and become their primary source of respiration. Early species would have laid their amphibian-like eggs in damp leaf litter to prevent them from dehydrating, and when the eggs would hatch, the larvae would scurry away and fend for themselves there until they were larger and more tolerant of the drier conditions above the forest floor, when they would metamorphosize in a similarly rapid and complete manner to the verminfans. These early forms did not develop lungs until adulthood, and were thus similarly constrained to small adult sizes, as the larvae had to stay small to effectively respire.

It was not a far jump for the adult birds to actively begin laying their eggs in freshwater to ensure they stayed moist, though, and it was in this way that some ambulopteran larvae became semi-aquatic. Very young birds, able to absorb all their necessary oxygen through their skin, could breathe underwater and indeed, their method of absorbing oxygen worked more effectively there. Not only were able to exploit food resources unavailable to other metamorph birds at the time, but they could grow far larger before metamorphosing. The development of their walking forelimbs was delayed, and instead at first they now developed long embryonic tails, which propelled their round, fat bodies through the water like tadpoles. Before the Thermocene-Pangeacene boundary, a new clade had developed; the aquamorph birds, defined by their obligately aquatic larvae. Only when several weeks to months of age, when they were often quite large, did their lungs then become active and their hind- and forelegs develop into more than vestigial nubs, and they began to move out of the water.

From this stage, three distinct clades subsequently diverged which can be seen today; the gilltails, the ornimorphs, and the eargills.

Gilltails

The gilltails are the most basal aquamorphs. The larvae of this group often grow relatively large, up to 14 inches in length, and have had to evolve a more efficient respiration system than the skin alone to support life at this scale. Their solution was to utilize the the fleshy locomotory tail,which has become lined with small fleshy protuberances, each lined with folded, hair-like cillia which are densely packed with hemoglobin and which pulse to pass water through the tail, forming a gill - thus their common name, and allowing them to survive underwater at larger sizes than skin-based respiration alone allows. Larval gilltails, in common with other aquamorphs, are active and highly mobile versus verminfan larvae, resembling salamanders or newts, and hatch with well-developed eyes and acute visual acuity made possible by relatively larger eggs. They are omnivorous and feed on algae, detritus and any small animals that they can snap up; their beaks are reduced to little more than a horny covering around the jaw, even in the adult, which is carnivorous and feeds on insects and small animals found near water, by swallowing them whole. Larvae initially have only a tail and no limbs, but within weeks will have developed both fore and hind legs used to grip the stems of water plants and clamber through water vegetation. Adults retain the two digits of the wrist and bear two small wing claws put to the same use in clinging to reeds. Metamorphosis is complete, and occurs when the larvae forms a cocoon of mucous and buries itself in the mud near the shore of its pond or stream habitat.

The tufted frogjar of the northern marshes is a typical gilltailed bird, with an aquatic, newt-like larval stage which is much larger than the small, generalized bird it grows into, due to the re-absorption of the tail which occurs in metamorphosis.

Ornimorphs

Ornimorphs are the second most basal clade of aquamorph changelings. They begin life the same as gilltails, as limbless, tadpole-like creatures which breathe through their skin until they become too large for this to still suffice for their needs. Evolution has come up with its own solution to this crisis, however, and in their case, their lungs develop and they become air-breathing very early, long before maturity - uniquely among their fellows, their larvae do not have any sort of gill at all. By breathing air, ornimorph larvae thus too lost their restriction of size.

The larvae of ornimorphs could therefore grow even larger than those of gilltails, sometimes to twenty four inches or more - tens of times the size of any of their terrestrial relatives, and well beyond most of the normal predators of changeling larvae. They faced a new issue now, however, and that was that by becoming so large in their youth that their final metamorphosis into the adult, flying life stage would take much, much longer. When the time came to change, the young would have to hole themselves up somewhere safe, sometimes for months as they produced their mucous cocoon and went into a state of torpor as their bodies changed and transformed into feathered, winged creatures at last. The process was risky, and many would be found by predators in this lengthy and defenseless state, which is why a few initially mutant larvae that didn't respond to the hormonal cues of their kin and go into torpor to transform, but which rather remained active and matured more gradually while awake and feeding, quickly gained the competitive advantage.

Maturation in these birds became more prolonged than in their ancestors, but in exchange, they did not have to sit vulnerable for months in a cocoon to reach maturity. Now instead, they grew and changed gradually over months to years, transitioning first from an aquatic, then to a terrestrial and finally to a winged, warm-blooded and normally long-lived adult stage. By the early Pangeacene, this new clade known as the ornimorphs were widespread. Notable for symbolically re-living their own distant ancestry with each generation - from a fish-like larval stage, a reptile-like juvenile which becomes increasingly arboreal, and finally a feathered adult that takes flight from the treetops and into the open sky - these birds are now common across the world in the Ultimocene. Able to spawn in water, adults of most species have become the most airborne of any life forms, and may never touch ground again after their maiden flight at sexual maturity.

The shadowskimmer, a very primitive ornimorph that has an air-breathing aquatic larval stage but which never evolved a terrestrial intermediate, is also the largest species. With a wingspan that can reach up to thirty feet, they are specialized skim-feeders of the open sea, where they surf along just above the surf and snap up small fish and other prey in their narrow, elongated bills under cover of darkness when such prey rise near to the surface from the depths to feed on plankton. Hunting at night and resting on the wing at high altitude by day, it sports unusual reversed countershading, with a black belly to blend in with the night sky and a white dorsal side, to reflect excessive solar heat during the day. It is so highly adapted toward an airborne existence that it has no hind legs at all; in an emergency, it can roost and take off from the surface of the sea by its powerful wings alone, though it will never touch down intentionally once mature, spending its entire adult life in the sky.

Only a very small percentage of individuals reach maximum size and reproductive age; the skimmer's life as a flying creature begins modestly. The shadowskimmer is one of the last ornimorphs that still retains some semblance of a cocoon, and the predatory aquatic larvae spin one in the thick reeds at the edge of a stream from which they emerge several weeks later as a small, fully-formed bird. The juveniles, no larger than swallows, emerge from the water in large flocks and feed in a similar manner to the adults for several years in freshwater before, if they survive these first perilous seasons, heading out to sea once their wingspans reach five feet.

All early ornimorphs were relatively large birds, but competition with sparrowgulls gradually encouraged the evolution of smaller and more specialized species throughout the later Pangeacene. Their larval and juvenile stages are now often highly specialized, allowing them to avoid competition with other birds, and in a reversal of natural selection, smaller adult sizes mean they can develop through their various juvenile stages more quickly.

The spectacled puddlebrood is a derived ornimorph. Unlike the shadowskimmer, its development is clearly demarcated into distinct specialized stages,beginning with an aquatic tadpole and progressing through an amphibian, reptile, and dinosaur-like life stage before becoming a flighted bird.

This species reproduces by laying its eggs in the water-holding leaf axils of nanboo trees, from which the larvae eventually sprout legs and emerge in the form of frog-like creatures that feed on ants, sequestering poisonous chemicals in their tissues that protect them from predators. Over several months, the growing juveniles become larger and more explicitly arboreal, with their bright warning colors fading into camouflaging stripes as their diet changes to a more generalized insectivore's and their innate toxins diminish. With their metabolic rate still slow, they are now ambush predators; their tongues by now have elongated and are used to snap up prey until, finally, they near their adulthood and develop a more active metabolism and can capture insects actively and the tongue reduces. They now scurry up higher into the trees and will, over the next few months, becoming first leapers, then gliders, and finally take flight over the forest canopy and transition completely to a diet of agile flying insects which they hawk down with great agility. As adults, they will land only to lay their eggs in the small leaf-born pools of water in which the cycle will begin again.

Additionally, some ornimorphs have experienced even greater adaptability by prolonging their metamorphosis indefinitely; some clades are neotenic, and reproduce in a more primitive juvenile state without reaching the final flying adult form. They were not the only aquamorph clade to do so...

Eargills

The eargills are the most derived aquamorph changelings. Rather than evolve new ways to facilitate their eventual metamorphosis, during the Thermocene they capitalized on neoteny and gave rise to permanently larval species which were able to become sexually mature without ever transitioning out of their larval forms at all. To facilitate respiration as they became too large to efficiently breathe via their whole skin alone, they evolved an entirely new respiratory organ - a true gill - by channeling water through their mouths and out of their ear canals, where the skin specialized into a matrix of convoluted folds with a large amount of surface area, which were extremely efficient at absorbing oxygen directly into the blood stream. To facilitate this, their ear drums became permanently ruptured, and their ability to hear became reduced to sensing vibrations in the water column via the bones of their jaws. Eargills, so freed from the need to leave the water to breed and with this efficient manner of respiring, rapidly diversified from the ancestral tadpole form into various fish-like creatures, some flawlessly hydrodynamic and others elongated and eel-shaped, and spread across global waters as the first completely and utterly aquatic birds, independent not only of land, but of air. With their ties to the sky severed completely, they could grow enormously large - the largest eargilled changelings now grow to tens of meters in length.

Pikebirds, such as this species, are widespread freshwater predators in northern freshwater systems and prey upon anything they can swallow whole.

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Yet the tie to the sky was not cut in entirety by the eargills. One more primitive branch still survives which still retains an adult form; they are the dayflight birds, all of which live only long enough to disperse away from one body of water and lay eggs in another, making them vernal pool specialists, especially in arid regions.

Though most dayflight birds survive in vernal pool habitats by eating algae, detritus or small invertebrates, the largest pools support their own endemic predators. Cannibal dayflights, their larvae larger and more aggressive than the others that share their pools, eat almost exclusively other dayflight larvae. The adults are highly sexually dimorphic, allowing them to easily find each other and mate in their brief lives which, though longer than most dayflights due to their richer larval diets and higher energy stores, still last just a couple of days.

Though all dayflight genera are capable of metamorphosis into a flying adult, this does not mean all do so regularly. Some species are able to be aquatic through their entire lives, reproducing in the larval form as in other eargills, and doing so for generations. Yet even in these apparently flightless, neotenic species and population remain hidden latent genes which hold secrets until they are needed again. Dormant genetic pathways can be expressed through certain environmental triggers to induce the ancestral metamorphosis. Known as efts, these creatures spend most of their lives as "fish" in freshwater ponds and pools, but if these pools seem destined to dry out are triggered to lay eggs which will hatch into smaller, fast-growing larvae which then metamorphose into a winged dispersal stage - birds - which can save the population by searching out a new source of water, breeding and then laying another clutch of eggs. If conditions are good and oxygen levels high in the new pool, these will grow into another fully aquatic generation, but if they are poor will mature into a second generation of dispersing flyers who will hopefully have better luck finding a stable water source. Some eargill populations produce efts almost annually and have specialized to make use of short-lived vernal pool habitats which dry up after a few months, but most can go years, sometimes decades and possibly centuries before a particularly dry year results in an incredible migration of flyers which set out in search of greener pastures - or rather, wetter ones. Because these species have specialized to live in water, and retain these flighted life stages only as a last-measure unit of dispersal, eargills which enter the eft stage do not develop into completely functional birds. They lack stomachs, their mouths may be partly or totally sealed, and they are deaf as a result of the transformation of the ear canal into the gill of the aquatic form. They rely on energy gained in their brief larval stage to power their migrations, and fly until they find water, or die trying.

The giant eft is a common eargilled bird of southern rivers and lakes. Most populations are totally neotenic and do not morph, yet in the driest plains regions, where pools sometimes threaten to dry up completely in summer droughts, the efts still retain the ability to spawn a population-saving flying dispersal generation that escapes to find a new, wetter home to lay their eggs.

A Loss of Larvae

The evolution of a larval life stage is what defines the metamorph birds. The verminfans capitalized on this, allowing the young and adults to specialize more than would ever otherwise be possible into different ecological niches. But for some members of the ambulopterans, the larval stage eventually became seemingly no longer of any benefit at all, and was subsequently lost altogether.

Placental Birds

Following the Thermocene-Pangeacene extinction event, one group of early ambulopteran birds rapidly speciated in the resulting evolutionary vacuum in the opposite way of the aquamorphs. Their evolution based around the further specialization and adaptation of their weight-bearing forelegs for locomotory and manipulatory purposes. Such an adaptation, evolving first purely to facilitate more freedom of movement in the young life stages carried no loss of fitness if retained into adulthood and indeed conferred a fitness gain, allowing the adult bird to grow to larger size than was ever possible as a biped whilst still retaining freedom of flight by allowing for a powerful quadrupedal launch in the manner of the pterosaurs. Having a larval stage at all was, for some, no longer necessary, and one clade of ambulopterans subsequently lost it completely. The placental birds did this by retaining their very small eggs internally and having them hatch and develop through a simplified larval stage in the womb, attaching to the uterine lining and sharing a flow of nutrients with the mother until being born alive and very highly developed, often at a very large size, which required or at least strongly selected for a loss of flight abilities altogether.

The blue-throated boomsinger of south Serinarcta is one of the largest birds ever to walk the land, weighing over fifteen tons and standing up to fifty feet tall. It is one of the last serezelles, which survived the evolution of more efficiently-feeding circuagodont herbivores by growing to enormous sizes and utilizing food resources out of their reach. Boomsingers are so named for their otherwordly, resonating songs which can be heard for miles, and even further as infrasound through the ground, letting herds communicate over vast differences. Boomsingers are so large as to be virtually gigantothermic, and have lost almost all of their feathers save for sparse plumes on the face. The young are born extremely large - already hundreds of pounds at birth - and are cared for by the herd for a very prolonged period.

Perditacunaves: Archangels and Kin

The perditacunaves ("birds of lost childhood"), placental birds with less specialized wings which retained powers of flight, were more constrained to reduce their weight than species which became flightless early on and so did not retain their young as long. The embryos initially attach to the uterine lining by their tails, but detach before full growth is completed and spin an especially thick, sturdy pupal sac of mucous and collagen while still within the mother. This cocoon has evolved into a new, second egg, which is then laid and develops further for another few weeks to months outside the mother's body. This pseudo-egg is different from the ancestral egg in being soft-shelled, rubbery and not calcified, but serves the same purpose. Unlike most birds before them, they are also buried in the earth rather than incubated by the parent. Instead of a true yolk, the larvae absorb their tails for energy as they finish development; as the tail is no longer utilized for any other purpose, it has swollen into nothing more than a bag of fat and nutrients upon which the developing chick relies until fully developed, ready to pierce the egg's shell and hatch into the world.

Perditacunaves retain the slower metabolism of larval metamorph birds in their embryonic stage, which allows them to develop slowly but steadily at lower temperatures than would have been possible for the ancestral warm-blooded bird. These birds' eggs are large and equipped with massive stores of energy that are not fully absorbed until just before hatching, when the vestiges provide a quick source of energy to kickstart the chick's metabolism; in just a few minutes, the hatchling's body warms rapidly to an endothermic condition, and it is able to take flight and begin foraging for food for itself. Some birds of this clade are known for the ability to reduce their metabolisms back to an embryonic level when necessary to conserve energy, allowing certain species to hibernate and enter torpor in conditions of low food availability.

The glidegander is a typical basal archangel. With a 25 foot wingspan, these highly social grazing birds still migrate across the southern plains in search of tender new grasses in the early Ultimocene, as their ancestors have now done for tens of millions of years.

Together the ambulopteran birds comprise the majority of terrestrial megafaunal birds on Serina today. Placental birds are flightless animals which have lost their wings entirely in order to adopt their front limbs into dexterous arms or weight-bearing limbs, allowing them to grow larger than any land-living birds before them. Perditacunaves are almost all flying animals, but they are often larger than the majority of their winged competitors, sparrowgulls and tribbats, and notable in that while usually strong flyers, they spend much or even most of their time on the ground or on water and find their food there, using flight only for dispersal and long-distance travel. A far cry from their tiny beginnings, today they include among them the largest flying animals ever to exist.

The Stormsonor, a highly migratory giant archangel which feeds on the Serinaustran tundra in summer and in tropical floodplains by winter, is the largest bird ever to fly on Serina. With a wingspan in excess of sixty feet and a height of up to thirty at the head, these 1,000 lb leviathans are larger than some air craft but are gentle giants, feeding on soft vegetation, often in wet areas. They are named for their eerie, rolling calls in flight, which are heard long before they are seen flying in flocks high overhead, and their tendency to follow storms to feed on the resultant grass and other plants that grows after rainfall on the plains. 

In Conclusion

Between the ambuloptera and the verminfans, the early Ultimocene is truly the age of the metamorph bird. Not since the hypostecene, and the introduction of the first canaries to Serina, has one animal clade seen such incredible and widespread success. No matter what the future will bring, the incredibly varied and adaptable metamorph birds will, in one form or another, be ready to exploit any opportunity life offers for some time yet to come.