The reason we hear bird cries (4)
I believe the answer is:
caws
'bird cries' is the definition.
'caws' can be an answer for 'cries' (caw is a kind of cry). I'm unsure of the 'bird' bit.
'the reason we hear' is the wordplay.
'the reason' becomes 'cause' (I've seen this in another clue).
'we hear' shows a homophone (sound like).
'cause' is a homophone of 'CAWS'.
Can you help me to learn more?
(Other definitions for caws that I've seen before include "Harsh cries of crows" , "Cries of murder" , "Rook calls" .)