Why were nightjars once called goatsuckers?

The name "goatsucker" for nightjars stems from an ancient superstition that these birds used their exceptionally wide mouths to suck milk from the udders of goats at dusk. This misconception was likely reinforced by the birds' habit of flitting around livestock, where they were actually hunting insects attracted to or disturbed by the animals.12

The term is derived from the scientific name of the type genus, Caprimulgus, which translates literally to "goatsucker". Because this name is based on a myth, there is a modern preference for using the term "nightjar," which refers to the birds' distinctive, jarring vocalizations.132