[Sound description] A series of 6–9 (sometimes 15) calls lasting 2–3 seconds each and spaced 2–10 seconds apart. The general tone is a "oo" tone which begins in a low pitch and ends with a high note. This sound can be heard more than 5 km away. [Posture] Usually done standing, with the mouth opened slightly and the head bent down [Context] Used by both sexes when alone or in a group, and appears to be done spontaneously without external cause [Name] 
Whoop