why do people say "bird brain" ? Birds are smart!

My Quaker shocked me with singing "Happy Birthday" a few months after hearing me sing this to my other parrot. The Quaker was brought into our house after living in my art gallery, which then went out of business. So it picked up the song, without being "taught". It was just so surprising to hear her(him) sing it months later, when it had only heard it for the first time that ONE day!
She often picked up words and phrases at the shop where she lived the first five years of her life. One of the funniest stories was one my mother told me. Mom had been managing the gallery for me while I stayed home to get some other stuff done. A customer dropped something and my Quaker said very loudly, "WHOOPS!" The woman turned on my mother in annoyance and exclaimed, "I didn't break it!" And then she promptly walked out. She didn't know that the whoops came from a bird, not my mom being a smart alec! haha
(I told mom she should have TOLD the customer it was the bird! I didn't need to lose any customers! But it was still funny!)