From the article:

Flight imposes strict weight limits, so birds pack enormous numbers of very small neurons into very small brains. Their axons, the long cable-like fibers that transmit electrical and chemical signals between cells, are shorter.

“Long lines take time. Short lines speed everything up,” Nieder said.

My first speculation after reading your comment was that because they fly, they would have to process far more visual stimuli and other sensory inputs (aerodynamics) within a certain time frame compared to mammals most of which are mainly terrestrial. Probably a silly thought, maybe not, I have no idea if this is paralleled in flightless or aquatic birds (if that’s been studied).