Bird having fun (4)
I believe the answer is:
lark
I believe this is a double definition.
'bird' is the first definition.
(I know that lark is a type of oscine)
'fun' is the second definition.
(I've seen this before)
'having' is the link.
(Other definitions for lark that I've seen before include "Mischievous activity" , "Jape; bird" , "One gets up early" , "Songbird - 3" , "Fool" .)