The Incredible Shrinking Archangels of the Ice Age

Archangels, placental birds which walk on all fours but retain powers of flight, are best known for being the largest flying animals ever to live. Soaring with two sets of wings, they evolved as humongous, nomadic migratory birds that grazed primarily on vegetation. Archangels were metamorphic birds, but had evolved to retain their young inside their bodies throughout their larval stages and nourish them with a placental connection. Unlike more advanced placentals like serezelles however, archangels did not give birth to well-developed miniatures of themselves but rather to the pupal sac of their young; laying this structure, formed by the larva in utero, the chick then finished its development outside the mother's body and the sac functioning like a new external egg, usually being buried in warm, loose soil. After initially developing at ambient temperatures, when ready to hatch the chick would emerge, now a fully-independent and flight capable miniature of its parent, and then rapidly vibrate its muscles, warming itself and kick-starting the warm-blooded metabolism it would carry with it through its adult life.

This life strategy, convoluted though it might be, has worked well for the archangel clade since their appearance in the early Pangeacene. Yet today the biggest archangels, those who have never strayed from this life history, are in trouble. The world is rapidly cooling, making breeding difficult. Species that once migrated to the tropics to breed and give their young the best conditions for growth now struggle to find warm enough places, and their young are taking longer to develop, not always taking flight before the ground freezes and kills them outright.

Fortunately, throughout the early Ultimocene the archangels were not all staying with the old, tried and true ways. The clade has diversified markedly since their first forms, and in particular a trend can be seen toward a significant reduction in size. Though truly gigantic species still survive at the start of the middle Ultimocene's first ice age, it is these small species which have figured out perhaps the most straightforward solution to raise their young in a cooler world. Though some species might adapt to harsh conditions with complex physiological adaptations, sometimes the best solution is the simplest one. A recently-diverged clade of small waterbird-like archangels known as seraphs re-evolved brooding, sitting on and so warming their young in their pupal sacs and ensuring a stable, consistent environment for them to mature into independent flying juveniles much like how their ancient ancestors, the strackbirds and the canaries before them, incubated their hard-shelled eggs. Evolving incubation independently a second time, over a hundred million years after its loss, came about rather simply from birds making mounds of loose soil to lay their eggs, shaping it with their bodies as they laid upon it and deposited them, and gradually spending more and more time around them. Those pupa whose mothers sheltered them better survived better, and it did not take long at all to select for parents that sat over the sacs after laying and warmed them against their own bodies when faced with harsher and colder environmental conditions.

Seraphs would be large flying birds by Earth standards, running a gauntlet from ten to forty pounds in weight, but they are pipsqueaks among archangel birds. Yet it is this smaller size that lets them brood their young and now improve their survival in unpredictable conditions versus their more vulnerable larger relatives. This lost-and-found adaptation has been so successful that the seraphs are one of a relatively small number of animal groups that are rapidly speciating in the midst of a global mass extinction, as they displace a variety of other small archangels with less developed parental care. Some still graze on grass, but others now branch out into other diets their giant relatives never adopted: meat. This dietary shift begins small, with invertebrates, but will likely lead to bigger things as environmental conditions only become harder, and normal food supplies more dwindling.

Pteese

Pteese (singular ptoose, pronounced toose) are the most basal seraphs, very functionally and behaviorally similar to the larger archangels except of course much, much littler, no larger than earth swans. They are still primarily herbivores, grazers mostly but sometimes dabbling for water plants, and they remain highly migratory. Pteese are super gregarious, gathering in flocks of more than one hundred million on the northern grasslands in the summer to breed and take advantage of the seasonal flush of vegetation. They birth and brood their pupating young beneath their bodies on sheltered sites where ground predators are few, including river or lake islands and rocky hillsides, but like earlier archangels they still provide no care to the young once fledged and the chicks fly off in age-segregated flocks to survive entirely on their own, not joining with the adults until almost fully grown in one to two years. The young are more insectivorous and proportionally different, with very long wings and a shorter neck; they often fly behind large grazing animals to snap up disturbed insects, and rarely land until several months of age when they begin to become more terrestrial.

The Sandpiping Seraph

The sandpiping seraph is a more derived seraph species from the ptoose, about as big as a swan, and is adapted to probe-feed on coastal beaches. The bill is highly elongated and rod-like, able to dip deep into the sand when the tide recedes to pluck out hidden shellfish and other invertebrates. With fully feathered legs and arms the sandpiper has no adaptations to wade in water and so when the tide comes in, they take flight and bide their time soaring along the coast, landing in large numbers in a number of hours when the tides recede again and reveal exposed mudflats rife with prey. Seraphs move inland to breed near marshes, as their newly-flighted young are far smaller than themselves and cannot initially survive along the exposed ocean beaches. They feed in calmer freshwater ponds and wetlands until they are half grown, at which time they migrate back to the coasts and take up their adult niche.

The Skydiver

Skydivers are one of the most derived archangel species, though ironically this has resulted in the development of a more conventionally avian bodyplan when viewed at a glance. Skydivers are the first truly carnivorous archangels, sharing a common ancestor with the sandpiping seraph, but adapting to feed in deep water rather than at the shore and so make use of a completely new food source, one which is now highly abundant in the cold, productive ice age ocean. Strong flyers, these seraphs have long, pointed beaks with fine serrations and no longer eat a significant amount of plants. Instead they feed on small fishes by plunge-diving from a height, then swimming after prey with their wings folded. Their hind wings are lost to improve their hydrodynamics, with the plumage having been replaced with leg scales in a repeat of a similar mutation which turned the leg plumage of ancient earth birds into scales. Though the elongated spur on the ankle which supported the hind primary feathers is still present, it is now vestigial and kept folded against the leg. The nostrils of the skydiver face backward to keep from filling with water when diving.

Skydivers have the most advanced parental care of archangels. In addition to brooding their pupating young, their chicks remain close to their mothers for months after hatching and are provided supplementary food in addition to foraging for small prey on their own. This allows young skydivers to fill a more similar niche to the adult and move right out to the open sea earlier in life, without a young shore-scavenging phase as in their relatives. While other archangels are immediately independent upon pupation, skydiver chicks spend about a week at the nest site before flying, during which time they imprint upon their mother.

Primarily the female skydiver cares for her young, and only she broods them, in contrast to many other species of archangel. This has allowed the two sexes to differentiate, with the male being larger and slightly more colorful, with a heavier beak. Males attempt to form harems, fighting one another for prime breeding territory on rocky coasts. Males may help locate food sources for both mates and young and are not totally absentee parents, being aggressive and protective of them against predators.

The trend that unites this particular branch of metamorph birds, to re-develop many of their ancestrally lost behavioral traits in regards to parental care, is most strongly demonstrated in the skydiver. A metamorph bird which appears shockingly normal, they remain fundamentally distinct nonetheless in their viviparous reproduction, lack of a proper egg (the 'egg' instead being a soft-shelled pupal sac produced by embyros in utero), and quadrupedal movement still allowing massive potential body size... as the ice age sea provides abundant food, and the troubles of keeping their chicks warm are better solved.