Birds have notoriously small brains but many species have shown cognitive abilities in the same range as primates

Birds have been under much harsher selective pressure to reduce brain mass because excess weight impedes flying ability.

There's an issue that isn't really looked at with interspecies EQ calculations. Human brains have a certain amount if duplication of function on both sides of the brain, you can carry out a hemispherectomy (removing half of the brain) in a kid and still have a relatively normal functioning adult. An animal where weight/size absolutely has to be kept to a bare minimum is going going to lose that feature.

Humans have probably had a fairly strong selective pressure to minimize brain size, because we need to keep it down because of our bipedalism making our pelvic area narrow. What you'd see is improved neuron density to its theoretical maximum before any increase in brain size began. It's probably why the increase in brain size from australopithecus to AMH had a fairly flat curve for a very long time. We maximized packing density, and then brain size happened later.

As land mammals with a long life span, we need to keep the redundancy 'back up' function because we get banged on the head when hunting and fighting, and we reproduce slowly (unlike birds).

Ancient European Hunter Gatherers, for example, had far larger brains than the modern average.

They were also a lot heavier. About 60-70% of brain mass in humans deals with the body, not problem solving.

There's also been recent genetic selection that increases intelligence. Although that's been going the other way at since the fifties. Systemic welfare states. See the Icelandic study for that.