Owls' calls (5)
I believe the answer is:
hoots
'owls' calls' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hoots that I've seen before include "Owls' noises" , "Jeers" , "Owl noises" , "Sounds of the owl" , "Loud raucous cries of an owl" .)