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

No, birds definitely cannot be stupid. When a bird flies in the air, for example, there are so many factors involved. If the bird's brain is not capable of assessing all the information, it would not be able to fly at all.

One other thing I have noticed is that birds can adapt to the environment very well. I used to wake up very early at one time. Birds like chicken and ducks are not supposed to wake up until the sun rises. However, after just a few times of me waking up so early, my chickens and ducks also wake up at the same time and start making noises, expecting to be fed.