Owl wing velvet may quiet a sound most airfoil diagrams ignore. Which one?

Answer: Feathers rubbing together

Large air vortices — Large vortices matter for many wings, but this answer misses the surprising biological twist. Owls are not rigid airfoils; their wings are layered feathers that slide, overlap, and rub during flapping. Recent ecology work argues that velvet and fringes may reduce this structural noise. That means an owl wing is closer to a moving fabric surface than to a single airplane wing.

Air scraping the wing — Air scraping over a surface is the standard airfoil story, so this is an understandable guess. But the question asks for a sound most airfoil diagrams ignore, and air-over-wing noise is exactly what those diagrams usually emphasize. The surprising biological source is feather against feather. Owl velvet may act more like a dry quieting layer between moving fibers.

Feathers rubbing together ✓ — Right. The velvet-like surface may reduce frictional sounds from feathers sliding over one another, a source that clean airfoil sketches tend to leave out. That is a good reminder that biology often solves multiple noise problems at once: airflow noise, rubbing noise, and impact noise. It also shows why copying only the outline of an owl wing misses part of the living surface.