Moonlarks (or "Suldreen" in the Enlightened Language) are an uncommon species of bird. Moonlarks lay their eggs in the ocean and allow the tide to carry the eggs away, leaving the baby moonlarks to fend for themselves without a family. However, a parent is always secretly watching out for them to make sure they are safe. They sing melodies of guidance and comfort to assure the baby moonlarks, reminding them that while they are physically alone, they are never truly so.
Moonlarks have silver or blue feathers arranged in a sweeping, peacock-like train, long legs like a crane, and a neck similar to a swan's. Sophie notes that they are beautiful.
Because of the method these birds use to raise their young, the Black Swan named the project of making an elf embryo to be born in a human family "Project Moonlark". This elf, which is Sophie Elizabeth Foster, was tossed out into the sea of humans, other than Mr. Forkle secretly watching over her. Calla, a gnome part of the Black Swan and an assistant to Sophie’s creation, suggested the name.
As a baby in the human world, Sophie Foster was able to speak in the Enlightened Language (which is instinctive) before learning to speak English. One of her early words sounded similar to "soybean", which her human family interpreted as an attempt to pronounce her own name "Sophie". The word "Soybean" was then attached to Sophie as an annoying nickname, mainly used by her human dad William Foster and her sister Amy.
When Alden Vacker heard that story, he suggested that Sophie was actually trying to say the word "suldreen", and then he immediately tried to cover the implications of this idea. Sophie's birth, unusual raising, and special genetics are all a result of a project the Black Swan named Project Moonlark, so it is likely that the thought about "suldreen" was related to that. However, at the time the Black Swan were still considered criminals, so Alden did not elaborate on this thought until later.
After discovering Sophie in Keeper of the Lost Cities, Alden borrowed two moonlarks from the Sanctuary to study their habits. Due to their aquatic natural habitat, he kept the birds in the pond behind Everglen. In Nightfall, Mr. Forkle says the only place that he ever found moonlarks in the wild is Brumevale.