This is the end of the line, birdbrain! (8)
I believe the answer is:
railhead
'this is the end of the line birdbrain' is the definition.
'railhead' can be an answer for 'is the end of the line' (I've seen this before). I'm not sure about the rest of the definition.
'line birdbrain' is the wordplay.
'line' becomes 'rail line' (rail is a kind of line).
'birdbrain' becomes 'head' (I am not sure about this - if you are sure you should believe this answer much more).
'rail'+'head'='RAILHEAD'
Can you help me to learn more?
(Other definitions for railhead that I've seen before include "end of the line - with 26d" , "End of the train line" , "Terminal of the track" , "You've reached the end of the line here" .)