Gnomish
bascovas
noun. flapping and beating (especially of wings)
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
A word appearing as G. bascovas “flapping and beating, especially of wings” in the Gnomish Lexicon of the 1910s, a noun form of G. basga- “flap” (GL/22).
Neo-Sindarin: I would adapt this word as ᴺS. blabed “flapping, beating (especially of wings)”, based on the later verb [N.] blab- “flap, beat” (Ety/PALAP).