A small bird mating (2,2)
I believe the answer is:
at it
'mating' is the definition.
'a small bird' is the wordplay.
'small bird' becomes 'tit' (I've seen this before).
'a'+'tit'='AT IT'
(Other definitions for at it that I've seen before include "Making love" , "Engaged in something particular" , "engaged in dubious activity?" , "Occupied (in crime?)" , "Active" .)