Aquatic Metamorph Birds
or, finding your inner fish through neoteny
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Changelings, or metamorph birds, are extraordinarily diverse in the Pangeacene as a result of their convoluted life cycles. In general, the larval form of the changeling, born at such a primitive state of development, is highly malleable, which has allowed them to colonize niches unavailable to other birds with more fixed body plans, including fully aquatic, ectothermic, quadrupedal, and endoparasitic lifestyles. The very short gestation period has allowed the group to abandon the constraints of the hard-shelled egg and in some cases the laying of eggs altogether. Because when young, the birds are only responding to the constraints of their environment as larvae, a general trend is the continuing derivation of the juvenile form but the retention of a very primitive adult appearance (species which mature without complete metamorphosis notwithstanding.) A good example of a changeling group with extremely disparate juvenile and adult forms can be seen in certain carnivorous parasites which spend their infancy as blind, featherless grubs boring through the body tissues of large animals but at maturity cocoon themselves and emerge as small, agile flying birds with acute vision and a typical distribution of plumage. Some species with very different juvenile and adult life stages, however, struggle to metamorphosize all at once in this way and instead develop through multiple distinct life stages which can broadly mirror their own evolutionary histories - for example some which spend most of their lives as animals broadly identical to sparrows or thrushes following a larval period during which they are completely aquatic, taking oxygen from the water and propelling themselves with a tadpole-like tail and then a "salamander" stage, where they are terrestrial but flightless, featherless, and ectothermic, before finally cocooning themselves and developing their adult traits. A similar but even more drawn-out development also occurs in changelings which are too large to metamorphosize quickly, which instead progress through their entire life cycle while still active and feeding, which has produced forms such as the serezelles and quadrupedal flying groups common to Serina today, which have retained some juvenile attributes into adulthood as a result of their incomplete metamorphosis.
Perhaps the most extreme example of incomplete metamorphosis, however, can be seen in one branch of changelings with aquatic larvae that specialized to such an extent that their anatomy became too far-removed from anything compatible with a terrestrial life style and which have survived only by developing their sex organs while still otherwise immature. Some examples of these species are among the least bird-like of any of their kin, their ancestry virtually unrecognizable and their appearance more like that of eels, fishes, and the larvae of amphibians, wholly adapted to living in water and with almost no adult traits other than the development of their gonads, while others are still vaguely bird-like in adulthood, developing certain avian elements to their anatomy - especially of the head region - while the development of others, such as lungs and plumage, is completely halted, normally by the lack of certain triggers which would otherwise activate them. They may swim with their forearms or with a tail, an organ which may or may not contain skeletal elements and is derived from a fatty food-storing structure in the earliest ancestral form but now muscled and able to undulate as a source of propulsion. The hind legs are either totally absent or present only as balancing fins, while respiration occurs within independently developed gills derived from inside the jaw which contains patches of heavily vascularized tissue over which water is pulled, coming through the mouth or nostrils, through the oral cavity, and being ejected from the hollowed-out remnant of the ear canal. The evolution of the gill in the changeling bird effectively removes the ability of the organ to function as an ear; while aquatic species can still pick up vibration and muffled noise through the bones of their skulls, this is likely a contributing factor to the loss of the terrestrial adult in species with highly derived, free-swimming aquatic young too large to respire through their skin or the inside of their mouths passively without actively pumping water through the ear canal. The alterations adopted in their infancy cannot be undone with maturity, meaning that any adult bird which matures from a gilled larval form will be deaf and thus especially vulnerable on land, where a sense of hearing is of much greater importance to avoid predators.
Efts
There are, however, some species which, though they may spend most of their life in their aquatic form, nevertheless can still be induced to transition into a flighted form by certain environmental conditions. Living in pools of water which may occasionally dry up, they can spend many generations reproducing in their aquatic form, but when a certain environmental trigger occurs such as the water becoming warmer and lower in oxygen - signalling that the pool could begin to dry up and become uninhabitable, a very dramatic change driven by hormonal changes occurs in the offspring produced by the population dwelling there. They continue to breed and lay their eggs, but these now develop rapidly into a highly stunted variant of their normal appearance with more developed forelimbs and small lungs as well as gills, which is known as an eft. After only a few weeks, when they are just a few inches long and would normally still be very immature, the efts begin to breed with one another. Males and females gather in orgies in shallow water, the males fertilizing the females internally with a mobile penis-like lip of the cloaca - a trait absent in the previous generations which simply broadcast their eggs and sperm into the water. The male dies after mating but the female lives on and buries itself in the sediment inside a cocoon of mucous. It emerges in just twenty-one days as a flight-capable feathered bird. Its eyesight is acute, but it is indeed totally deaf, with its ear canal sealed shut and lacking an ear drum entirely. For most birds this would be a handicap, but the lifespan of the flying eft is so short already that it does not greatly hinder it. The bird is even smaller than the aquatic creature it developed from and weighs just one to two tenths of an ounce, but they pack into their tiny bodies everything they need to find their way to a new body of water, including enough fat to supply its journey, and establish a new population - including fully developed ovaries full of fertile eggs. Its sole goal in life now is to find a place to deliver them safely, where they can hatch and produce the next generation, and she will not deviate from this goal for any reason - even to eat. They break through their cocoons and burst from the mud in huge numbers that serve partly to overwhelm opportunistic predators - since they otherwise lack any defense - but also to increase the odds of at least one individual carrying this populations' genes finding a new suitable body of water to colonize. The method of any one bird is simply to take off and fly in one linear direction, not stopping until it either passes water or dies of exhaustion in about 24 hours; with a few exceptions, the flying life stage of the eft doesn't feed and usually lacks a stomach altogether. With the number of birds that emerge from any given drying pool, though, this almost always means at least a handful do come across another pond or lake in which to deposit their eggs; their duty fulfilled, they frequently settle on the water and drown shortly afterwards or may linger for several hours on the shore until they die of starvation. But their eggs will have survived. If the pool is well-oxygenated, their eggs will develop into the large aquatic adult form and remain as such as long as conditions allow. If it's transient and likely to be short-lived like the one their parents came from, however, they will hatch immediately into another generation of efts, which will still stand a chance in another few weeks of finding a more permanent source of water. Depending on environmental conditions, a population can thus exist and reproduce for multiple generations in either form, with the alternate lying dormant in their genes until it is triggered by the onset of conditions which make it the more likely to permit future survival.
Efts cannot mate with aquatic generations; the latter lack the ability to impregnate the former internally. Likewise, male efts cannot fertilize the free-floating eggs of aquatic females. Nonetheless, both alternate reproductive forms carry the genetic blueprint to produce the other in their own genome and will do so if environmental conditions are likely to favor it. Because they also pass accumulated mutations onto each other, both variants of a given population are considered a single species, even though the alternating generations are sexually incompatible.
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A common occurrence in Serina's arid interior in the Pangeacene is the annual emergence of swarms of flighted efts from the dwindling pools at the end of the rainy season, which disperse by the millions to find new pools to release their eggs in and spread their genes. The ability of these most unique birds to spread far over land and colonize isolated bodies of water, incidentally, gives them an edge over true water-bound fishes in colonizing both seasonal pools and newly-formed ponds and lakes, and the diversity of freshwater fishes in Serina by 228 million years PE is in decline as a direct result of their competition, especially in isolated bodies of water. Fish can still colonize inland water sources, mainly when their eggs are carried on the feet of water birds, but this is only possible with fishes that have reverted to oviparity and does not work with live-bearing forms, which usually must swim actively from one waterway to another. By the time fish of any sort are likely to reach a new water source, it will most often be populated already with fish-like changelings birds hatched from eggs dropped by efts, which are likely to severely limit their populations feeding on their eggs and young before they can establish a population.
The process of evolution begins to come full circle on Serina in the Pangeacene, as fishes in the form of tribbetheres begin to displace large terrestrial avians and birds in the form of changelings start displacing freshwater fishes in inland environments. Birds still dominate the air, however, being more efficient flyers than the tribbats, and the ocean is still the domain of the ray-finned fish, which easily limit the success of less specialized fish-like neotenic birds in environments where distribution is possible by way of water. Eft-based reproduction is extremely inefficient at sea, or indeed in any large and established body of water, where the relatively few eggs which a flying carrier can release are so few that in an environment with established predators few are likely to survive - there are thus no marine efts. A few groups of completely aquatic changelings have infiltrated the sea, however, being able to produce numbers of offspring more comparable to competing fishes, but without any especially advantageous adaptations to put them any more than equal to true saltwater fishes, which remain as diverse in the Pangeacene as previously, they do not seem set to overtake them in this environment any time soon.