Merriam-Webster
Word of the Day

December 8

rectrix \REK-triks\

DEFINITION noun

:any of the quill feathers of a bird's tail that are important in controlling flight direction

EXAMPLES

In general, your only chance to see most or all of a bird's rectrices is when the bird is in flight.

"As in other passerines, the juveniles of many species show more pointed rectrix tips than do adults." — From National Geographic's 2009 book Complete Birds of the World

DID YOU KNOW?

Although "rectrix" (from the Latin word "rectrix," the feminine of "rector," meaning "one that directs") has been an English word since the late 18th century, it has clung to its Latin plural "rectrices." This is not terribly unusual for a technical term like "rectrix." (Note also "cilium," whose more common plural "cilia" refers especially to the hairlike extensions on the exterior of some cells.) "Rectrix" has another meaning wholly unrelated to birds. It's also used (albeit rarely) to refer to a woman who rules or governs.