RE: why do people say "bird brain" ? Birds are smart!
Since there are so many species of birds, their level of intelligence can be vary. The differences between different birds can be as big as monkeys compare to humans. Some birds are definitely smart. No less smart than a cat or dog, if not smarter.
Many of the talking birds require extensive training. Most recently a friend told me there is a bird in her neighborhood that can pick up any random words it heard. Like when two neighbor kids argue with each other, the bird could repeat what they just said after just heard it once.