Also known as Trollcuckoos, Changelings are an sophont species capable of taking the form of other creatures as they develop. Unlike most other creatures, Changelings have 5 nuclei. The first is the basic nucleus of the species, while the other ones are empty at birth, and are filled with copies of hosts.
Born from an egg as small (1 cm) larvae, their growth is plastic and they may grow much larger if their host is also large. A Changeling larva will drink genes from its host (then usually a baby) and incorporate it into one of their spare nuclei. After acquiring enough nutrients, the Changeling larva will wait until its host and parents are expected to sleep and will start secreting anaesthetic gas and will soon eat its host from the inside and spew a coccon around itself. Over the course of a few hours, it will pupate and metamorphize into the likeness of its host.
If successful, the Changeling will be raised by its "adoptive" parents like a cuckoo. As a defense mechanism, young Changelings constantly secrete mind-numbing pheromones so the parents will have a harder time noticing their child has been swapped.
As they grow Changelings are usually obedient (due to their lower placement on the Scale of Being), but upon reaching sexual maturity at around 15, passionate urges will become stronger. A mature Changeling will seek another of its own to mate. They usually have a hard time, since the disguises are genetically perfect, but pheromonal cues can guide them. Some Trollcuckoos express a need to isolate themselves from civilization upon maturity.
Changelings can acquire the genomes of up to 4 hosts to help in their disguises and unique life cycle. They must pupate whenever they change form. Depending on the size of the new form, the pupation may last many days or be impossible thanks to a lack of nutrients; it would be nigh-impossible for a Trollcuckoo to pupate from the shape of a mouse into the shape of a kraken. It is worth noting the genomes of Ishim are too different from a Changelings for them to take the form of one.
Female Changelings have a long stinger with which they can inoculate eggs into hosts painlessly. Many mothers feel bad at the idea of changing innocent children, while many fear changing wild animals, where the chance of success would be much lower. Changeling larvae cannot grow within other changelings.