After analyzing the brain volumes of dinosaurs, as well as those of both extinct birds and modern birds, scientists were able to reconstruct the evolution of the avian brain.

The new evolutionary models -- detailed this week in the journal Current Biology -- showed birds and non-avian dinosaurs boasted similar relative brain sizes prior to the mass extinction event that ended the Cretaceous Period. Once the dinosaurs were gone, researchers found, some groups quickly evolved larger brains.

Increases in avian brain sizes coincided with an explosive radiation of birds species as new lineages filled the niches left empty by extinct species.

"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," lead study author Daniel Ksepka, curator of science at the Bruce Museum in Connecticut, said in a news release. "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."

For the study, paleontologists used CT scans to create 3D models of the skull cavities of hundreds of birds and dinosaurs, allowing them to estimate the brain sizes of ancient species. After combining the data with existing brain size measurements for modern bird species, scientists analyzed how brain size scales with body size in different dinosaur and bird lineages.

"There is no clear line between the brains of advanced dinosaurs and primitive birds," said study co-author Amy Balanoff of 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."

While several groups of birds expanded their brain size in the wake of the extinction event that wiped out the dinosaurs, two of the smartest modern birds -- parrots and corvids, the group that includes crows and ravens -- only recently evolved larger brains.

One recent study showed corvids were able to adapt to life in cities by evolving larger brains and boosting their cognitive abilities.

"Crows are the hominins of the bird kingdom," said co-author Jeroen Smaers of Stony Brook University. "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."

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