Ounce for ounce, bird brains have significantly more neurons than mammal or primate brains
Given that birds fly, giving a very high cost to mass, it is no surprise that there has been strong selection pressure for low mass computation.
Given that flight is a high energy activity, it is no surprise that there has been less pressure on energy efficiency vs mass content in avian brains. (Given it takes so much energy to fly, a little bit more for computation isn’t a huge cost.)
The cost benefit matrices between avian and mammalian systems are very different. Horses for courses as the old saying goes – which takes on new overtones in the contexts of complexity theory and evolutionary theory.