A person who lacks intelligence or who makes stupid decisions. You're such a bird-brain. I can't believe you got stranded on the highway because you didn't put enough gas in your car!
birdbrain
1. n. a stupid-acting person. I’m such a birdbrain. I forgot my driver’s license, officer.
2. and birdbrainedmod. stupid. Look, you birdbrained idiot, you are dead wrong!