While virtually legless birds have existed for some time in the form of the most derived verminfan larvae and certain extinct eargills, skuwyrms are the first terrestrial birds to evolve a lack of limbs through all of their life stages, giving them a snake-like appearance and behavior. Unlike snakes however, all species retain feathered plumage.
Skuwyrms are a clade of legless skuorcs descended from the skuzzards, animals like the woozle, which began evolving around 280 million years PE and are now widespread and diverse with a worldwide distribution. They are now highly distinctive animals with elongated worm-like forms and only two claws midway down their length, which are the vestiges of their back legs. They are usually small and very gracile, being active pursuit hunters that chase prey rather than ambushers that lie in wait. They are primarily carnivorous, with some omnivorous outliers, and have lost their legs to facilitate easy entrance of molodont burrows to hunt prey, but many species now hunt above ground, often in dense tussocks of grass where their serpentine shape allows them to slip through the vegetation while chasing prey with the least resistance; this is the primary habitat of the ratsnakes, which have excellent eyesight and hunt by vision. The belly in all skuwyrms is scaly and rough to provide traction as they move forward via serpentine undulation, in which the body is contorted into a series of S-shaped horizontal loops, and each loop pushes against the ground alternately.
The dorsal surface of most skuwyrms is still feathered; this hair-like integument provides insulation and protection from the sun, but one of its primary roles is to soak up scent secretions produced from glands near the animal's tail and deposit them like a paintbrush as it moves along in its environment, claiming its territory. Others of the same species can read these cues, with males determining whether a female may be receptive to mate and females judging a male's size by the strength of his marks. Some skuwyrms have crests of feathers which can be raised in threat displays or in courtship dances, while others display warning coloration in the form of black, white, or red bands on their feathers that betray a venomous bite. Venom glands are ancestral to this clade, but have been lost in many derived species including the yellow-eared ratsnake of Serinarcta, which is non-venomous and feeds on insects, smaller skuwyrms, and nestling vertebrates that do not require toxins to overpower. The bite is rarely medically significant to animals more than a few times their own weight, being sufficient mainly to help immobilize prey animals that are small enough to swallow whole. Its food is held in the mouth with hook-like keratin teeth as well as a novel structure unique to the non-venomous ratsnake subclade - four bony spurs, two each on the top and bottom jaw, which function like fangs to ensure a strong hold on struggling prey and prevent its escape.
At least half of the body length and sometimes more is made up of a long tail which is hairless at its distal half even on top, and which can be released via autotomy if the animal is grabbed by a predator, allowing the owner to flee while its attacker is left with a squirming tail that will jump on its own via muscle contractions for as long as ten minutes before dying. The tail can grow back almost flawlessly afterwards in a matter of weeks, without any scar tissue, and even large body wounds can heal in a similar way. Offspring are born alive and fully precocial; no parental care is provided. Adults of some species may be highly cannibalistic, and the yellow-eared ratsnake is particularly inclined to kill its own young if it comes across them. To escape their elders, juveniles will play dead; this species is a visual hunter with eyesight attuned to motion. If something stops fleeing, it will often be overlooked. This trick doesn't work so well with burrow-hunters that find food with their noses and sensitive whisker hairs in an environment too dark to see in, and like most small animals, skuwyrms are still common prey to other animals.