Moonlarks, or as their proper name in the Enlightened Language "Suldreen", are highly uncommon birds. Moonlarks lay their eggs in the ocean and allow the tide to carry the eggs away, leaving the baby moonlarks to fend for themselves in the world without a family. However, a parent is always secretly watching out for them to make sure they're safe. They sing melodies of guidance or comfort that 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. They are described to be quite beautiful and rare.
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.
As a baby, Sophie was used to express a word that sounded like "Soybean", and interpreted by her human family as an attempt to express her name "Sophie". But apparenly, it was probably the word "Suldreen" in the Enlightened Language, a result of the secret knowledge the Black Swan inserted into Sophie's brain.
In Book 1: Keeper of the Lost Cities, after discovering Sophie, Alden borrowed two moonlarks from the Sanctuary to study their habits. It turned out that they live in habitats with water, such as lakes and ponds, and Alden used the lake at Everglen for this matter. In Nightfall, Mr. Forkle says the only place that he ever found moonlarks in the wild is Brumevale.