- Estrildid finch chicks have unusual mouth markings, such as black-lined beaks, glow-in-the-dark beads, and hole-covered roofs, which some find unsettling.
- These markings may be linked to brood parasites that lay eggs in the finches' nests, but the evolutionary order is debated—whether finches evolved markings as defense or parasites evolved mimicry.
- Some finch species without brood parasites still have markings, suggesting other functions like helping parents judge chick health, age, or location in dark nests.
- Research shows that altering mouth markings reduces feeding by parents, indicating the markings stimulate feeding behavior.