As a native speaker, "birdbrain" means stupid.

The term implies something that feels like "dodo" or imbecile.

Corvids and parrots and raptors are remarkably smart, yet not every bird species is. Some are incredibly stupid.

There's also the anecdote (maybe just in the American South?) of a chicken running around with its head cut off.

Also think of the "Always Sunny" meme where they call Dee a "bird" all the time [1]. It's meant to mean stupid, amongst other things.

[1] https://youtu.be/60D7SN06A8M

Chickens will run around with their heads cut off. It's the nerves going haywire after being chopped. Similar observations were made of guillotined corpses twitching after death (and heads trying to speak, but that's just until the blood runs our), human bodies are just too heavy and unbalanced to move effectively without coordination.

Birdbrain can also mean "flighty"; which corresponds to fast (and wrong) decision making.

> Some are incredibly stupid.

That's from an anthropomorphized angle though? In their evolutionary niche, their level of expressed intelligence seems to be spot on, otherwise they'd have evolved or been outcompeted long ago.

In fact, some of those species that you might consider "incredibly stupid" may have existed long before humans and might easily outlive us if we follow some not-very-unlikely paths that we have opened up with technology and (lack of) social maturity. Who is "incredibly stupid" then?

The chicken with its head cut off saying extends beyond the south. I grew up hearing it in the Midwest. Since chickens do run around when their heads are cut off, I think anywhere with chickens would end up with this phrase.

The Always Sunny thing is more about her looks. When they call her dumb, they tend to just call her dumb/stupid/etc.

I would imagine the Dodo is a big reason for the birdbrain insult meaning dumb. Since Dodo itself can be used to mean dumb, and since they weren't afraid of humans they were easy targets and hunted to extinction. That may say more about humans than the Dodo.

Sure, and you can reach a stupid decision quickly. "Birdbrain" has never had a connotation of being slow to react.

Some sayings are wrong or half-truth but they stick anyway e.g. "blind as a bat".

The chicken tale predates discovey of the new continent.

You will find similar middle age tales.

Mike the headless chicken was extensively documented, if an incredible fluke. He also had an intact brainstem, even if not a full brain. https://en.wikipedia.org/wiki/Mike_the_Headless_Chicken

> Corvids and parrots and raptors are remarkably smart, yet not every bird species is. Some are incredibly stupid.

Raptors are pretty stupid. They don't _need_ to be smart. They're solitary and they are typically apex predators.

I have a great view out of my window, and I have a family of crows living nearby. They cooperated to drive out owls and eagles that were nesting in a nearby park. Right now it's raining with pretty strong gusty winds and the crows are flying outside, doing pretty crazy aerobatics. They are clearly having a lot of fun!

I have never seen pigeons or seagulls do that.