How birds evolved big brains

Birds and non-avian dinosaurs had similar relative brain sizes prior to the mass extinction at the end of the Cretaceous Period, according to a major new study.

An international team of 37 evolutionary biologists and palaeontologists reconstructed the evolution of the avian brain using a large dataset of brain volumes from dinosaurs, extinct birds and modern birds.

This revealed that after the extinction the brain-body scaling relationship shifted dramatically as some types of birds underwent an explosive radiation to re-occupy ecological space vacated by extinct groups.

The study’s findings are published in the journal Current Biology.

“One of the big surprises was that selection for small body size turns out to be a major factor in the evolution of large-brained birds,” says lead author Daniel Ksepka, from the Bruce Museum, US.

“Many successful bird families evolved proportionally large brains by shrinking down to smaller body sizes while their brain sizes stayed close to those of their larger-bodied ancestors.”

Ksepka and colleagues used CT scan data to create endocasts – models of the brain based on the shape of the skull cavity – of hundreds of birds and dinosaurs, which they combined with a large existing database of brain measurements from modern birds.

They then analysed brain-body allometry: the way brain size scales with body size.

“There is no clear line between the brains of advanced dinosaurs and primitive birds,” says co-author Amy Balanoff, from Johns Hopkins University. “Birds like emus and pigeons have the same brains sizes you would expect for a theropod dinosaur of the same body size, and in fact some species like moa have smaller-than-expected brains.”

The two groups of birds with what the researchers call truly exceptional brain sizes – parrots and corvids (crows, ravens, and kin) – evolved relatively recently. Their cognitive capacity includes the ability to use tools and language and to remember human faces. 

In the study, parrots and crows exhibited very high rates of brain evolution that may have helped them achieve such high proportional brain sizes. 

Jeroen Smaers, from Stony Brook University, US, even suggests crows are “the hominins of the bird kingdom”. 

“Like our own ancestors, they evolved proportionally massive brains by increasing both their body size and brain size at the same time, with the brain size increase happening even more rapidly,” he says.

In their paper, the researchers say their data reveal the complex and dynamic history of avian evolution. 

“This history includes high early rates of evolution that stabilised across the theropod-bird transition, a subsequent series of profound grade shifts as crown birds adapted to myriad ecologies early in the Cenozoic, and a culmination in which two groups – parrots and corvid – independently acquired relative brain sizes, neuronal densities, and sophisticated cognitive potential near the pinnacle of the vertebrate world,” they write.