The Ultimocene is a time of change for the changelings, or metamorph birds. Primitive egg-laying groups radiating from the sparrowgulls are experiencing a resurgence and taking back niches lost at the end of the Thermocene, pushing them further into ever more aberrant forms. Metamorphs now survive by capitalizing on forms, behaviors and niches unattainable to other birds still constrained by the hard-shelled egg.
Most extant metamorphs are derived significantly from their original form, which included a carnivorous larval form and flying reproductive adults - though some do still persist, mainly as very small bug-like creatures with worm-like offspring that live in carcasses or as parasites of live animals and mature into short-lived flying adults which mate, lay eggs, and expire. The rest have adopted niches very different from any birds ever before them. Placental birds, which experienced a short heyday in the early Pangeacene, have secondarily lost their larval stage and the external egg entirely and developed true live birth as well as a fully quadrupedal gait. Just one group survives to the Ultimocene, but they are very diverse; these are the flightless, cosmopolitan ornkeys - arboreal wingless birds similar to primates and possums.
Non-placental changelings comprise several other very divergent lineages. Archangels, quadrupedal flying birds which lay large soft-shelled eggs and have also lost their larval stage, have experienced great success as semi-aquatic waterfowl in addition their original large grazing forms. Instead of losing their larval stage, however, other metamorphs have capitalized on their highly adaptable larval forms and evolved to retain juvenile traits into adulthood, usually through a stunted metamorphosis where only some adult traits develop and maturity is reached in a still juvenile condition. One such group of these particularly aberrant birds known as the aquamorphs, by now widespread across the world of birds, long ago evolved swimming, tadpole-shaped larval forms which live and feed in water. Though the most basal still retain the ability to morph into a short-lived flying reproductive stage, most by now have totally lost any ties to the air. Instead of a full metamorphosis as they grow, they develop only a few notable avian traits - some details of the skull, the forearms (which become paddles), and the reproductive organs. They retain their gills - formed from the ear canal of the neonate - and mature in an aquatic state.
Ornimorphs comprise their own distinct branch of the aquamorph clade. The ancestral condition of these animals is a modified variation on the original aquamorph life cycle. The small, shell-less eggs are laid in the water in the manner of a frog; the eggs hatch into aquatic larvae which grow and feed for anywhere from a few weeks to a few months before morphing into a terrestrial form. They differ from their ancestors in that instead of burying themselves in the mud and morphing in a cocoon of mucous, like an insect, the process is gradual and occurs while the animal is still active and mobile. Legs develop slowly over time, and the larva becomes more terrestrial. The larval forms of this group do not develop proper ear gills and retain the primitive skin-driven respiration of the earliest changelings, so that as they move onto land their ears develop normally. Gradually their skin thickens and their lungs develop along with their limbs so that they are soon fully terrestrial quadrupeds superficially resembling the earliest tetrapods. Featherless and unable to regulate their own body temperatures yet, they at first hunt on the ground, eventually developing longer legs and moving into forests. Here they gradually mature over a longer period, until eventually they climb into the canopy and mature as fully-feathered flying birds. Now distinct from the outside only on account of their quadrupedal posture, with wings adapted to bear their weight, they fly far and wide. Unlike the fleeting, short-lived flying morph of the efts, the ornimorphs have comparably long adult lifespans usually much longer than their juvenile stages. They reproduce and scatter their eggs in freshwater pools many times throughout this period, and the cycle continues.
By the Ultimocene, the ornimorphs have also given rise to another distinct group, the squaves, which again exhibits neoteny by capitalizing on the adaptability of the terrestrial life stages and forgoing their maturity into a flighted adult. Very different from most birds, squaves finish their development as quadrupedal lizard-like animals that have also begun to give birth to live young rather than lay eggs. Some simply hatch their eggs internally and then release the live tadpoles into the water, protecting them from predation while in the egg, but the most successful have taken this further by nourishing their young internally via placental ties through the aquatic stage until their legs and lungs are fully developed, thus freeing them from a dependency on standing water. They become the third live-bearing bird group through a similar series of events to their placental cousins. Their skin is thick, holding in water, and protected with small scales like a reptile's. Their forearms also sport claw-like projections and they retain a lengthy tail for balance - traits which together lend them a lizard-like appearance - but the mouth retains a distinctly avian, toothless beak for grabbing prey and some forms still attain a sparse degree of plumage as adults. With a slow metabolism, squaves require less food than comparably-sized birds of other groups and compete minimally with most other birds - instead, they compare much more closely with tribtiles, the reptilian ancestors of tribbetheres which have until this time had little other competition. In addition to competing for space and food sources, some representatives of each group adapt to prey on the other, as the ecosystem adapts to just another change of countless in the marching scales of time.
A second ornimorph group which exhibits neoteny into adulthood are the avimanders. These animals mature at an earlier life stage than the squaves, before they become terrestrial. Though some do develop a single lung, allowing them to gulp atmospheric to breathe and so to survive in stagnant pools, the more derived ones continue to respire through their skin. To increase their surface area and therefore the efficiency of the process their skin develops abundant hair-like cilia which function like gills to passively absorb oxygen from the water column. This allows them to spend their entire lives underwater.