As sung when the birds are on key? (4,4)
I believe the answer is:
swan song
'as sung when the birds are on key?' is the definition.
I can't tell whether this definition defines the answer.
'when the birds are on key?' is the wordplay.
'when' becomes 'on' (I've seen this in other clues).
'the birds are' becomes 'swans' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'key?' becomes 'g' (musical key).
'on' after 'swans' is 'swanson'.
'swanson'+'g'='SWAN-SONG'
Can you help me to learn more?
(Other definitions for swan song that I've seen before include "Final performance, as the air of an aquatic bird" , "Musician's last work" , "A dying performance" , "Musician's final appearance" , "Bird music for last act before death or retirement" .)