A sound reason for the bird’s cries (4)
I believe the answer is:
caws
'bird's cries' is the definition.
'caws' can be an answer for 'cries' (caw is a kind of cry). I am not sure about the 'bird's' bit.
'a sound reason' is the wordplay.
'a sound' indicates a 'sounds like' (homophone) clue.
'reason' becomes 'cause' (I've seen this before).
'cause' sounds like 'CAWS'.
'for the' is the link.
Can you help me to learn more?
(Other definitions for caws that I've seen before include "Rook calls" , "Harsh cries of crows" , "Cries of murder" .)