[vbezhenar]

You can't compare humans with elephants or birds. We are extremely different species with different evolution paths. It is very possible, that bird brains is much more optimized, but that tells nothing about our brains.

We can compare humans with our ancestors and trend is clear: our skull was getting bigger, despite the fact that it's not good for evolution, until some point after which it started to shrink.

Whether it translates to cognition increase or decline - we probably will never find out, we don't have time machine. But that's the most obvious assumption. Big skull is bad for birth, so evolution pressure on making skull smaller is enormous. Big brain is bad for energy efficiency, because brain consumes a lot of energy and food is scarce in the wild, so that's another evolution pressure. The fact that our ancestors kept growing head despite that evolution pressure meant that advantage was bigger. And intelligence is the only factor that comes to mind which can offset the negatives.