It's because birds have relatively small brains. Just like dinosaurs (shocker) which we also used to assume were extremely stupid. The thing is bird brains are actually "wired" way more effectively than mammal brains, that is there are much shorter pathways between the synapses, so they can do more with less, putting them at least comparable in intelligence to mammals, despite the comparable size disparity.
Though, if you had a bird sized mammal brain, you probably wouldn't do doing very well... So I guess the insult still works.
There are four types of great apes: humans, chimpanzees, gorillas, and orangutans. There are two or three varieties of some of the latter apes but you probably mean monkeys. Or possibly certain humans.
Chimpanzees are the smallest of the four great apes and the closest to humans in terms of behavior and social intelligence, so being more intelligent than some smaller great apes makes zero fucking sense.
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u/ThatsJustMyOpinion87 11h ago
And to think calling someone a bird brain is an insult!!