Science suggests we should stop using ‘bird brain’ as a barb

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The writer is a science commentator

The New Caledonian crow can lay claim to being one of the world’s cleverest birds. It can put two sticks together to make a rod to “fish” for food. The corvid’s dazzling propensity for strategic planning and problem-solving mirrors the cognitive skills of some great apes. 

Observations like these have stoked a decades-old debate about whether birds and mammals enjoy a shared cognitive legacy from a common ancestor, which lived more than 300mn years ago — or whether their skillsets evolved separately. Now, research points to the latter, with birds and mammals developing similar cognitive abilities independently, via different evolutionary paths.

In other words, it is not common ancestry but “convergent evolution”, in which unrelated species develop comparable behaviours and traits, that gives birds and mammals their superficially similar cognitive firepower. That finding matters to the understanding of our own evolution: if wildly disparate lineages of living things take different evolutionary routes on their way to ending up with similar neural circuits and behaviours, it hints that there may be limited ways of building a sophisticated brain.

The new research, comprising a trio of papers published in Science last week, takes an exceptionally deep dive into the pallium, the bulbous chunk of avian brain that houses neural circuits underlying cognition. The pallium is very broadly equivalent to the cerebral cortex in humans.  

One key technique, called single-cell transcriptomics, was used to unveil which genes matter to individual neurons (brain cells) in the avian pallium. “All cells in an organism contain the same DNA but each cell reads different parts of that DNA,” explains Giacomo Gattoni, an evolutionary biologist at Columbia University who was not involved in the research but co-authored an accompanying commentary article.

That selective reading by cells, he adds, allows them to flower and function differently, ending up, say, as muscle or liver cells: “The technique allows you to go neuron by neuron, and see what genes each neuron is reading, and then compare across species.” This was a focus of one of the three papers, which individually profiled thousands of neurons in chickens, mice, lizards and turtles (reptiles, birds and mammals all share a common ancestor dating to around 320mn years ago); compared them to see which were conserved, or not, across species; and mapped them spatially. Another paper scrutinised how the neurons choose which genes to selectively read, by studying “regulatory” DNA sequences that act like switches; the third focused on how neural sensory circuits in a chicken, mouse and gecko assemble during embryonic development. Overall, more than 50 scientists — drawn from institutions including Heidelberg University, the Catholic University (KU) Leuven in Belgium, and the Achucarro Basque Center for Neuroscience in Spain — contributed to the work. 

Collectively, their research revealed that, while there were likenesses in neural circuits between species, superficially similar neurons sometimes mapped back to different parts of the avian, mammalian and reptilian brain, and did not necessarily connect up in like-for-like circuits; were regulated in different ways; and that brain circuits underlying similar cognitive traits form at differing times and speeds across birds, mammals and reptiles. Some avian pallial neurons did not have counterparts in mammals at all.

It is persuasive evidence that birds and mammals have taken separate evolutionary paths that converge to surprisingly similar neural architecture, giving rise to behaviours that look alike. “This convergence suggests there are finite ways, and perhaps only a few, of building a complex brain,” Gattoni says, adding that a next step will be to explore how neural connectivity evolved in different species.

Despite evidence that animals like octopuses are intelligent, we humans tend to overlook the cognitive abilities of other animals. But birds, descended from flying dinosaurs, have also had to battle to survive, and the cognitive gap may be narrower than we assume. Corvids can count, fashion and use tools, and engage in social play; parrots can mimic language and even learn the meanings of words.

Nutcrackers can cache thousands of morsels, retrieving them later when food is scarce — a mighty feat of spatial memory. That some of us still throw the term “bird brain” around as a barb points to poor cognition in our own species, not theirs.