Gloomswallow

Gilltail metamorph birds are a group not rare in the hothouse, but small and cryptic and so rarely seen. Most examples of these birds are little changed from their early Ultimocene roots, exhibiting simple bauplans (for metamorph birds) that need few changes to endure. Adults are usually insectivorous and associated with water, as this is where they lay their eggs, while larvae are free-living, respire underwater with many sets of fleshy gills along their long cartilaginous tails, and have grasping fore- and hind legs that make them resemble salamanders. Metamorphosis is complete, in contrast to larger metamorphs like aukvultures and skuorcs with stunted development, and the swimming larvae produces a cocoon of silk-like saliva fibers after burying itself safely in the sediment near the shore of the pond or lake it lived in. The mature bird takes around four weeks to develop, the tail (lacking any bone) being reabsorbed for energy used to grow flight feathers and muscles for flight. When it emerges, it is completely, utterly transformed and few traces of its infancy remain, save for a jaw which is usually still very wide, as the larvae are predators which often use suction feeding to collect small invertebrate prey. Gilltails are found on both continents in Serina in the hothouse, but are restricted to freshwater habitats.

Some gilltails have, however, adapted to live in more unusual sorts of those habitats. The gloomswallow is a relatively large species which has adapted to live in the damp, extensive system of caves under Serinarcta. Adults, mostly black in plumage, exhibit six strange, wiry whiskers around their very wide mouths and spend much of their time clinging to steep cave walls in total darkness. They roost in large aggregations, often touching one another in close association. In such a setting, each birds' whiskers can be seen to twitch, flicker, and move with a small muscle at its base, as if receiving signals from the birds around it. This is exactly what is happening, for the gloomswallow's whiskers are not mere feathers but a set of complex, derived organs with a keratinous root surrounded by highly innervated tissue. Olfactory receptors line the tissue, functioning as an extension of the bird's sense of smell, and allowing birds to communicate their social status, sexual receptivity, and other attributes instantly to one another simply by brushing whiskers; in function, these structures function much like insect antennae.

The larval gloomswallow also exhibits these whiskers, and it is in this life stage that the organs originated as sensory structures used to detect water currents and scent traces which could belong to potential prey. Living deep underground, in calm pools that form off to the sides of flowing subterranean rivers, the larval gloomswallow never sees sunlight, and so is mostly depigmented. Its eyes, too, remain undeveloped until pupation, though they begin to show through the skin of the face in the weeks leading up to this process. Solitary and sometimes cannibalistic, it moves slowly along the rocky or muddy bottom of the cave, catching any small animal it finds within range by rapidly opening its expansive mouth and sucking in whatever hapless food item has come too near. To pupate, it climbs to the top of the water along a rocky wall and sticks its cocoon to the hard surface just above water, where it can later climb out as winged bird and join its parents living higher up the cliff. When the instinctive drive to leave the water begins in a mature larvae, it normally follows the scent signals of roosting adults to find a suitable place to pupate where it will be able to fly out of the cave, but some fail to do so, and leave their cocoons to find themselves trapped in tiny air pockets with no escape; such unfortunate birds starve to death in a few days or drown in their efforts to find an escape route.

Adult gloomswallows are insectivores which leave their cave roosts at night to hunt insects over wetlands and forested areas near water; they may travel up to 20 miles in a night to reach a suitable feeding site if their roost is not ideally situated. They navigate in flight out of the caves by sight, their wide eyes being sufficient to let them spot and catch their prey under most conditions, but they may remain in their roosts during cloudy moonless nights where visibility is worst. Within the darkness of the cave, a simple form of echolocation is used to navigate and avoid hitting one another or the walls of the cave. Gloomswallows generally lay their eggs in underground caves, but not always. Some adults breed in above ground water sources, possibly being unable to tell the difference at night when they are also dark. Gloomswallow larvae can, remarkably, survive here too - eggs exposed to sunlight develop melanin, an example of epigenetics. Though they hatch without visible eyes, if exposure to melanin continues, they will develop them prematurely, before pupation. Despite the very different appearance of their larvae, gloomswallows are very closely related to a number of species which roost in trees and have fully-sighted, pigmented larvae, and these species can also hybridize. The changes which have rendered most gloomswallow larvae pink and blind are not yet irreversible, if they find themselves in other condition, though these larvae are generally less competitive than their relatives and eat and thus grow more slowly, so that a smaller percentage will survive outside their ideal habitat in the caves.