Why do mute swans make a creaking sound with their wings?

The "sighing creak" produced by the wings of the mute swan (Cygnus olor) during flight is a well-known characteristic of the species. This sound is generated by the structure of the bird's wings, which feature modified vanes on the outer primary feathers that create noise as the bird flies.1

While many anseriform birds rely on vocalizations for communication, some species, including the mute swan, produce these distinct nonvocal sounds through their wing movements.12