[pcrh]

This might be what that article referred to [0]?

tl;dr, the higher cognitive abilities of birds are centered in a different region of the brain compared to mammals, the pallium vs the cortex. Neuron density in the bird pallium is also higher than the comparable density in the mammalian cortex.

[0] Developmental origins and evolution of pallial cell types and structures in birds https://www.science.org/doi/10.1126/science.adp5182