Think about every bird you've ever laid eyes on – whether it's a robin in your backyard, a pigeon in the city, or even a penguin at the zoo. Surprisingly, each of these birds is essentially a modern-day dinosaur. They're the only group of dinosaurs that managed to survive the massive extinction caused by an asteroid impact 66 million years ago.

However, not all the birds that existed during that period managed to survive. The reason why the ancestors of today's birds lived while many of their relatives perished has puzzled paleontologists for years. Recently, two new studies suggest a potential explanation: the contrasting ways in which modern birds and their ancient counterparts molt their feathers.

Feathers are a fundamental characteristic common to all birds. They're composed of a protein called keratin, the same substance found in our nails and hair. Birds depend on feathers for various purposes such as flying, swimming, blending into their surroundings, attracting mates, staying warm, and shielding themselves from the sun. Despite their importance, feathers are intricate structures that cannot be fixed. To maintain their feathers in good condition, birds undergo molting – a process where they shed old feathers and grow new ones. Baby birds molt to replace their juvenile feathers with adult ones, and fully grown birds molt approximately once a year.

"Molt is something that I don't think a lot of people think about, but it is fundamentally such an important process to birds, because feathers are involved in so many different functions," says Jingmai O'Connor, associate curator of fossil reptiles at Chicago's Field Museum. "We want to know, how did this process evolve? How did it differ across groups of birds? And how has that shaped bird evolution, shaped the survivability of all these different clades?" Two of O'Connor's recent papers examine the molting process in prehistoric birds.

In May 2023, the journal Cretaceous Research featured a paper describing a fascinating find: a bunch of feathers trapped in amber, revealing a snapshot of a baby bird's life from 99 million years ago.

In the bird world today, baby birds come in a range of developmental stages and parental dependency. Some, known as altricial birds, are born without feathers and utterly dependent on their parents for warmth. The absence of feathers allows parents to directly transfer body heat to their babies' skin, ensuring their well-being. On the flip side, there are precocial species, born with feathers and relatively self-sufficient right from the start.

Every young bird experiences a series of molts, shedding old feathers and growing new ones until they reach their mature plumage. This process demands a significant amount of energy. Losing a substantial number of feathers simultaneously can pose a challenge for birds, making it difficult for them to maintain their body warmth.

Because of this, young precocial birds go through a gradual molting process, ensuring a constant supply of feathers. In contrast, altricial chicks, which rely on their parents for nourishment and warmth, experience a simultaneous molt, shedding all their feathers around the same period.

The collection of young feathers trapped in a fragment of amber from the Hukawng Valley in Kachin Province, northeastern Myanmar, stands as the initial concrete fossil proof of juvenile molting.

The 99-million-year-old finding unveils a young bird whose life story doesn't align with any birds currently existing.

"This specimen shows a totally bizarre combination of precocial and altricial characteristics," Dr. Jingmai O'Connor, a researcher at the Field Museum of Natural History, shared.

"All the body feathers are basically at the exact same stage in development, so this means that all the feathers started growing simultaneously, or near simultaneously."

Yet, this bird almost certainly belonged to an extinct group known as Enantiornithes, which were remarkably self-sufficient from a young age.

The researchers suggest that the challenges faced by a precocial baby bird, trying to stay warm while quickly molting, might have played a role in the extinction of Enantiornithes in the end.

"Enantiornithines were the most diverse group of birds in the Cretaceous, but they went extinct along with all the other non-avian dinosaurs," Dr. O'Connor added.

"When the asteroid hit, global temperatures would have plummeted and resources would have become scarce, so not only would these birds have even higher energy demands to stay warm, but they didn't have the resources to meet them."

At the same time, another study released on July 3 in Communications Biology, conducted by O'Connor and Yosef Kiat, a postdoctoral researcher at the Field Museum, explores molting patterns in present-day birds. Their aim is to gain insights into the evolution of this process.

In present-day adult birds, molting typically occurs once a year in a step-by-step manner. They replace only a few feathers at a time over several weeks, enabling them to keep flying during the process. Simultaneous molts, where all flight feathers shed and regrow at the same time within a few weeks, are less common. This pattern is observed more in aquatic birds such as ducks, which don't rely heavily on flight to find food and escape threats.

It's very rare to find evidence of molting in fossil birds and other feathered dinosaurs, and O'Connor and Kiat wanted to know why. "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," says O'Connor – less time spent molting means fewer opportunities to die during your molt and become a fossil showing signs of molting. To test their hypothesis, the researchers delved into the Field Museum's collection of modern birds.

"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," the first author of the study, Kiat, explains. "Among the sequentially molting birds, we found dozens of specimens in an active molt, but among the simultaneous molters, we found hardly any."

While these are modern birds, not fossils, they provide a useful proxy. "In paleontology, we have to get creative, since we don't have complete data sets. Here, we used statistical analysis of a random sample to infer what the absence of something is actually telling us," says O'Connor. In this case, the absence of molting fossil birds, despite active molting being so prevalent in the sample of modern bird specimens, suggests that fossil birds simply weren't molting as often as most modern birds. They may have undergone a simultaneous molt, or they may not have molted on a yearly basis the way most birds today do.

The amber finding and the research on molting in present-day birds highlight a shared pattern: ancient birds and feathered dinosaurs, especially those from groups that didn't make it through the mass extinction, had distinct molting habits compared to today's birds.

"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. "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."