Why did most birds die out?

Roughly 66 million years ago, an asteroid strike triggered a mass extinction that wiped out nearly every dinosaur lineage, including the dominant Cretaceous birds known as enantiornithines. Only one bird branch, Neornithes, survived and gave rise to all 10,000-plus living bird species.

Lead author Jingmai O'Connor, associate curator of fossil reptiles at the Field Museum in Chicago, has spent years searching for an explanation. The new study, she said, points to plumage.

"We think the types of feathers that these birds had, and/or the way they molted those feathers, may have been one of the underlying causes of the selectivity of the end-Cretaceous mass extinction — essentially, why some birds died out and why others survived," O'Connor said.

Did water access help birds survive?

One earlier hypothesis suggested Neornithes survived because many lived in aquatic habitats that may have buffered them from the post-impact chaos. The hesperornithiform fossils complicate that picture. These birds also depended on water, yet their entire lineage vanished.

That contrast suggests habitat alone cannot account for the survival of modern birds' ancestors. Something else, possibly insulation, must have given Neornithes an edge.

What made Neornithes different?

The newly identified hesperornithiform feathers are the first ever recovered from that group. Their structure sits between the plumage of enantiornithines and modern birds: some feathers looked modern and waterproof, while others were smaller, fuzzier body feathers resembling those of non-avian dinosaurs.

Body feathers help birds retain heat. After the asteroid impact, dust and debris likely blocked sunlight and triggered a long "impact winter." Birds with better insulation would have had a survival advantage.

"The hesperornithiforms retain primitive feather types that may not have been as efficient for insulation as modern plumaceous feathers, and that could explain why they went extinct along with the enantiornithines," O'Connor said.

Could more coprolites hold similar secrets?

The study marks the first systematic effort to examine feathers preserved in coprolites, or fossilized feces. O'Connor hopes other scientists will start CT scanning similar specimens in museum collections.

"This opens up a whole new avenue for investigation," she said. "We only knew to look at this one because of how it happened to be split open, with the feather exposed — it was literally a lucky break."

Co-author Gregory Wilson Mantilla, a University of Washington professor and curator at the Burke Museum, called the find unusually valuable because bird fossils and their feathers are exceptionally rare. The coprolite, he added, "gives us an incredible window into predator-prey interactions 66 million years ago."

The research team included 13 scientists from the Field Museum, Burke Museum, Carter County Museum, University of Colorado Boulder, the Natural History Museum of Los Angeles County, and other institutions. The study's DOI is 10.1016/j.cub.2026.08.054.

Researchers caution that the conclusions rest on a single specimen. More coprolite scans will be needed to test whether primitive insulation truly separated survivors from the extinct, and to clarify how feathers evolved in the bird lineages that vanished.