The Pangeacene was notable for ushering in a new age of birds, where the aberrant changelings, or metamorph birds, became the most abundant bird clade, almost completely displacing other groups of flying birds. Until the end of the Thermocene, there had always remained at least a few groups of primitive passerines, lineages which persisted almost as living fossils through the ages, with a body plan basic but highly effective. Though the basic form survived through the adults of some changeling birds, there were no egg-laying songbirds. The small birds which flitted through the trees through the early Pangeacene began their lives as some variation of larvae, and reached their adult state only after a period of metamorphosis.
In the extreme environmental conditions of the Thermocene-Pangeacene boundary, it was a species of these metamorphosing perching birds that happened to be best suited to survive, but until that time the changeling birds were not inherently more adaptable than egg-laying forms and did not compete with them heavily. As the environment stabilized and life recolonized a largely sterilized world, it was almost inevitable that at least one of the surviving shelled-egg-laying birds, which included the flightless water snuffle, ground-nesting waterfowl, and a single species of cliff-nesting seabird, would re-diversify to at least some extent. Evolution was not a linear process with a set endpoint, but a here-and-now selective process, the pressures driving it constantly changing. And if those conditions were to change in the right way, an adaptation formerly deleterious can become advantageous again.
This is is exactly what has occurred among the flying birds in the late Pangeacene. 52 million years have now amassed since the end of the Thermocene, and the demographic of the flying bird clades have seen significant shift, as Serinan gulls - not actually gulls in the strict sense of course but an ancient lineage of generalist seabird canaries - have commanded an impressive radiation back into inland environments. In a stable, warm environment, nesting and raising eggs was not inherently inefficient in small flying birds and this method of reproduction has come back into fashion with the evolution of the sparrowgulls.
Sparrowgulls
Sparrowgulls are, in a sense, a reversion of form back to traditional perching birds, with some small differences. Their underlying forms are conservative and familiar, having changed very little over the ages, and though a small number of species of these seabirds had returned inland by the early Pangeacene, they were greatly outnumbered by an explosive diversity of metamorph birds and did not amount to anything significant. Toward the middle of the period, however, they had found an environment where they had an upper hand competitively: the dry southern desert. In very dry conditions, changelings do poorly. While it is true that forms are adapted which can develop in vernal pools and rapidly develop into adults before the waters dry up - like certain desert amphibians - there are very few other mediums available in a very dry environment in which the changelings, with their gelatinous eggs and moisture-dependent larvae, can reproduce. And just as there are a handful of frogs which can survive in an Earth desert but a much greater diversity of hard-shelled, egg-laying reptiles, the dry interior was a great place for a bird which laid watertight eggs.
The exact sequence of habits which led an ocean-feeding seabird to life in the dry interior desert is not entirely known, but can be inferred. The ancestors of the sparrow gulls surely evolved in the southern hemisphere, and likely initially began hunting abundant insects in the coastal grasslands that sprawled in the far south between the desert and the coasts, and only from there gradually moved further inland, where they found minimal competition from other birds. They did not initially return to trees or develop any perching adaptations save for losing the webbing on their toes, as trees were few and far between in these environments, and so they remained ground-nesters which often sheltered their eggs under patchy desert brush or outcroppings of stone. The first sparrowgulls were well-established across the arid interior by 228 million years PE, but did not venture significantly beyond it.
In the last twelve million years, the climate became wetter and the desert receded. Grassland and vegetation moved inland, and with it a larger number of egg-eating predators, molodonts, bumblets, and early carnivorous serezelles among them. If the birds did not rapidly adapt, they would die out, and surely many could not adapt their behavior in time to survive. The sparrowgulls which only laid their eggs in a bare scrape on the ground, as they'd done for millions of years, were selectively culled, while more adventurous individuals which built more complex nests of grasses and twigs and placed these nests higher up on the branches of trees and plants experienced greater success in rearing their offspring. It became necessary to be able to build their nests on thinner branches in order to avoid climbing predators, but having totally lost their hallux toe they were at a disadvantage to redevelop a secure perching grasp to hold onto such thin footing as their ancestors could. The result, an adaptation which occurred six million years ago, was the movement of the next inner digit backwards to replace it, giving them two toes facing forward and one set back - a bit less stable than the four toes of their ancestors, but a functional system.
At the same time, many of the different lineages of changeling birds had already become extremely specialized, with many such as the serezelles and ornimorphs, let alone aquatic neotenic forms, moving dramatically far and away from any semblance to their original adult perching bird forms and niches. By the late Pangeacene, the metamorph birds had largely evolved away from traditional songbird niches into highly specialized new forms, with vastly unrelated molodonts and other tribbets now having monopoly of food sources such as grass seeds that were formerly relied upon by a huge variety of birds. This process was gradual and not a direct result of competition. The result, however, was that many of these niches had been left open to be reclaimed by different birds.
The newly-arboreal sparrowgulls were now well-suited to that task and began to move out of their dry homelands towards the forests of the equator and beyond. In six million years, this third wave of generalist perching birds has dispersed over most of the world, taking advantages of a great variety of food sources. The primitive songbird form which had stood the test of time until so recently would not be lost from the world of Serina just yet. As almost all other life on Serina was evolving down increasingly strange and specialized paths, one group of birds were doing the total opposite and returning to their ancestral forms. The plain, but persisting, shell-egged birds were reclaiming the world that could never have existed in the first place without them.