It's part of the tail feather group, and a drake who has molted usually has two curls… multiples come from genetics or possible damage or splitting of the tail feather root.

Why do ducks have curls?

A mature drake will often display one or more curled feathers right at the top of his tail. Drakes usually develop this feather when they get their adult plumage, around 8-12 weeks of age. ... Freckles poses in the sunshine to show off both his tail curl and his great color!

Are mallard curls related to age?

Tail curls do not reliably indicate age of a mallard. Typically immature birds have no curl or only one curl, however this is not a given. To reliably age a bird you have to examine the plumage.