Unique molting patterns may have saved birds during dinosaur extinction

Researchers may be one step closer to answering the question of how the ancestors of today’s birds managed to survive the cataclysmic event that killed off the dinosaurs 66 million years ago. The study suggests that the survival of avian ancestors may be linked to their unique molting patterns.

A bird’s molt, as seemingly insignificant as it may appear, is a vital process linked to numerous important functions, explained Jingmai O’Connor, an associate curator of fossil reptiles at Chicago’s Field Museum.

Every bird is a living remnant of an era of dinosaurs. A bird’s plumage, comprised of complex structures of keratin protein (similar to our hair and nails), plays an essential role in flight, camouflage, thermoregulation, mate attraction, and protection against harmful UV rays. However, these sophisticated structures are incapable of repair, which means they must be frequently replaced in a process known as molting. 

“Molt is something that I don’t think a lot of people think about, but it is fundamentally such an important process to birds,” said O’Connor. Her most recent research was focused on a 99-million-year-old cluster of juvenile bird feathers trapped in amber, providing the first definitive fossil evidence of juvenile molting. 

Simultaneous molting

The specimen presented a unique combination of traits not seen in any extant bird species, demonstrating a simultaneous growth of all body feathers. This pattern is most typical in altricial bird species, which hatch naked and rely heavily on their parents for warmth and nourishment. However, O’Connor’s previous research suggested that this extinct bird belonged to the Enantiornithines, a group known for its precocial nature, meaning they were born with feathers and were fairly self-reliant from a young age.

Simultaneous molting, as observed in this juvenile bird, may have had severe implications for the Enantiornithines during the devastating asteroid impact that caused the mass extinction event. This extinction event would have resulted in a drastic plummet in global temperatures and a scarcity of resources. 

As a precocial bird undergoing a rapid molt, the energy demands to stay warm would have been steep. However, with limited resources, these demands would be difficult to meet, potentially contributing to their eventual extinction.

Evolution of molting 

In addition to this groundbreaking discovery, O’Connor, along with postdoctoral researcher Yosef Kiat, undertook another study that delved into understanding the evolution of molting in birds by examining its patterns in extant species. 

The research, published in the journal Communications Biology, revealed that most modern adult birds undergo sequential molting, replacing a few feathers at a time over several weeks. This method allows the bird to maintain flight capabilities during the molting period. Simultaneous molts, which involve shedding and regrowing all flight feathers at once, were less common and typically seen in aquatic species like ducks, who don’t depend heavily on flight for survival.

Interestingly, molting evidence in fossil birds and other feathered dinosaurs is a rare find. “We had this hypothesis that birds with simultaneous molts, which occur in a shorter duration of time, will be less represented in the fossil record,” explained O’Connor. This theory suggests that a shorter molting period equated to fewer opportunities for a bird to die and thus become fossilized during molting, preserving evidence of this crucial process.

“We tested more than 600 skins of modern birds stored in the ornithology collection of the Field Museum to look for evidence of active molting,” said Kiat. “Among the sequentially molting birds, we found dozens of specimens in an active molt, but among the simultaneous molters, we found hardly any.”

The findings appeared to align with their hypothesis, hinting at the possibility that the lack of molting evidence in the fossil record may be due to fewer instances of simultaneous molting in ancient birds.

Valuable insights 

Despite the fact that their study was based on modern birds, the scientists believe that these findings could provide valuable insights into the molting patterns of prehistoric birds. “In paleontology, we have to get creative, since we don’t have complete data sets,” said O’Connor. “Here, we used statistical analysis of a random sample to infer what the absence of something is actually telling us.”

In this context, the noticeable lack of molting fossil birds, despite the prevalence of active molting in modern bird specimens, indicates that fossil birds likely did not molt as frequently as today’s birds. It is plausible that they underwent a simultaneous molt, or they might not have followed the yearly molt schedule typical of most current bird species.

These groundbreaking studies on molting unveil a recurring theme: the molting habits of prehistoric birds and feathered dinosaurs, especially those from groups that did not survive the mass extinction, were distinctly different from today’s avian species.

“All the differences that you can find between crown birds and stem birds, essentially, become hypotheses about why one group survived and the rest didn’t,” said O’Connor. She proposed that the differential survival could be attributed to a multitude of factors, one of which could very well be the molting process. 

“I don’t think there’s any one particular reason why the crown birds, the group that includes modern birds, survived. I think it’s a combination of characteristics. But I think it’s becoming clear that molt may have been a significant factor in which dinosaurs were able to survive,” concluded O’Connor. 

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