Moonlarks, or "Suldreen" in the Enlightened Language, 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.
Because of the method these birds use to raise their young, the Black Swan named Sophie "Project Moonlark". She was tossed out into the sea of humans, other than Mr. Forkle secretly watching over her. When Sophie was a baby, her family thought that she was trying to pronounce her name, but instead of saying Sophie, she said "soybean". This ended up being an annoying nickname throughout her time living with humans. After moving to the Lost Cities, she learned that she was most likely saying "Suldreen", or Moonlark in the Enlightened Language.
Alden borrowed two moonlarks from the Sanctuary to study their habits in Book 1: Keeper of the Lost Cities. They live in habitats with water, such as lakes and ponds; Alden used the lake at his home. In Nightfall, Mr. Forkle says the only place that he ever found moonlarks in the wild is Brumevale.