That's surprising bird sound (3)
I believe the answer is:
coo
I believe this is a double definition.
'that's surprising' is the first definition.
(expression of surprise)
'bird sound' is the second definition.
(sound made by a pigeon)
(Other definitions for coo that I've seen before include "Murmur amorously" , "Sound made by dove" , "Murmur lovingly" , "Speak lovingly -- cry softly" , "well I never!" .)