Bird to get down suddenly (4)

I believe the answer is:
duck
'bird' is the definition.
(duck is a kind of bird)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for duck that I've seen before include "Quacker?" , "Dear; bird; fabric" , "Bob" , "eg Jemima, Donald" , "Shoveler, for example" .)
