Birdbrain
I believe the answer is:
dodo
'birdbrain' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dodo that I've seen before include "bird long dead" , "Flightless bird, extinct since the 17th century" , "Native of Mauritius" , "Proverbially extinct bird" , "silly person" .)