"It seems to be the area that's involved in more complex behavior and cognition, such as being able to learn about your environment and remember it," Milner said.
So after the K-T event, she said, these birds "were just better equipped to deal with challenging physical conditions."
Fossil bird skulls that have not been flattened out over time are extremely rare, and no examples are known from the time of the K-T event.
But Milner says the brain advances seen in the 55 million-year-old birds would probably have begun more than 65 million years ago.
And fossils of the oldest known bird, Archaeopteryx, which lived 147 million years ago, reveal its brain was "nowhere near as well developed as the ones we looked at," she said.
Desperately Seeking Fossils
Julia Clarke, a geoscientist at the University of Texas at Austin who was not involved with the study, says there are various competing theories to explain why birds outlived the dinosaurs.
One idea is that the ancestors of all living birds came from the southernmost part of the southern supercontinent Gondwana, where they escaped the worst of the environmental fallout from the K-T event.
Another theory is that modern bird lines evolved in coastal habitats that also were less heavily impacted.
As well as providing valuable new evidence for the evolution of birds, she said, the latest study offers an intriguing new theory that will motivate paleontologists to look harder and farther to find more fossils.
"We still desperately need good fossils sampling brain and skeletal features in the species that are very close but outside the [evolutionary tree] of all living birds," Clarke said.
"We can only get so close to understanding the brains of the earliest birds with the sample of known species currently available."
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