One bird or another dropping head (3)
I believe the answer is:
owl
'one bird' is the definition.
(owl is a kind of bird)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for owl that I've seen before include "Fly low like a bird" , "Low kind of wise bird" , "eg Snowy, Tawny" , "Traditionally wise bird" , "paragon of wisdom?" .)