Birds can do most things just as well as mammals. Crows use tools to solve complex puzzles. Lyrebirds are expert vocal mimics, putting polyglots to shame. And even the humble pigeon is a discerning enough art critique to distinguish between paintings by Picasso and Monet. Nevertheless, birds still get a bad rap. Calling someone a ‘bird brain’ insults their intelligence due in part to bird brains not having the fanciful folds and layers found in mammal brains.

The neocortex, the outer layer of mammalian brains, is often touted as being the hallmark of a mammal's advanced cognition. It's the largest part of human brains, and is thought to give rise to primates’ advanced intellectual abilities. As bird brains are smooth and are without layers, it was long presumed they were behind the times and that their neurons didn't play a similar function or come from a similar evolutionary origin to mammals.

However, scientists have begun to peck away at this ancient notion of ‘bird brain’ being an insult. In a recent paper published in Current Biology, Steven Briscoe, working in the lab of Clifton Ragsdale at the University of Chicago, USA, demonstrated that birds, mammals and reptiles all share similar brain cells in the cognitive cortex, challenging old ideas about avian aptitude, anatomy and ancestry, and suggesting that ‘bird brain’ should really be a compliment.

To start, Briscoe and his team first evaluated whether any genes were specially enriched in the brain of an evolutionarily ancient bird: the chicken. Specifically, they focused on uniquely expressed genes in the mesopallium – a region of the brain involved in higher-order processing. Briscoe found a whopping 78 genes that were uniquely and highly expressed in the mesopallium. The team then focused on five of the more highly expressed unique genes and mapped out their expression visually to gain a bird’s eye view of how well it distinguished the mesopallium from surrounding brain areas. The stained brains revealed that the top five genes cleanly delineated the mesopallium from other surrounding brain regions, suggesting the genes are important genetic markers for the special cell types that comprise mesopallium.

One caveat is that chickens are lower in the cognitive pecking order compared to the linguistic abilities of vocal learning avians, such as songbirds. As a follow-up, the team confirmed that expression of the same genes delineated mesopallium in the European starling, a songbird with an extensive vocal repertoire, suggesting these genes are conserved across birds.

While the avian mesopallium and mammalian cortex originate from the same brain region early in life (telencephalon), they take on different shapes across development: the cortex becomes layered in mammals, whereas clusters of neurons assemble in birds (referred to as ‘nuclei’). Therefore, Briscoe and colleagues examined whether mouse cortex expressed a similar genetic signature to that which characterized the mesopallium. Lo and behold, expression of the same five genes that distinguished the mesopallium outlined the mouse cortex as well, suggesting the neurons are cut from the same evolutionary cloth.

The research is fascinating as it suggests that birds and possibly primates evolved intelligence independently and that cortical neurons may be as ancient as the last common ancestor shared by birds and mammals. In short, neurons just took on different architecture for bird brains, but it doesn't make them any less brilliant.

S. D.
C. B.
J. J.
C. W.
Neocortical association cell types in the forebrain of birds and alligators
Curr. Biol.