Synonyms for Birdbrain:

n.

birdbrain (noun)
addle-head, loon.
dunce (noun)
blockhead, dullard, buffoon, nitwit, dolt, scatterbrain, dimwit, ignoramus, fool, bonehead, imbecile, moron, lame-brain, simpleton.
fool (noun)
dunce, fool, loon, simpleton, clod.
ignoramus (noun)
dolt, dimwit, simpleton, nitwit, bonehead, fool, dullard, dummy, ignoramus, imbecile, moron, blockhead, buffoon, clod, scatterbrain, lame-brain, dunce.
imbecile (noun)
dummy.
stupid person (noun)
dunce.

Other synonyms:

Other relevant words:
addle-head, loon.