How molting helped birds survive the dinosaur extinction

Every avian creature you have ever laid eyes upon, be it a robin, a pigeon, or even a penguin at the zoo, is, in fact, a living dinosaur. Birds represent the sole group of dinosaurs that managed to survive the catastrophic mass extinction caused by an asteroid impact 66 million years ago. However, not all bird species that existed during that time were as fortunate. The reason why the ancestors of present-day birds managed to endure while many of their relatives perished has been an enigma that paleontologists have sought to unravel for decades. Recent studies offer a potential explanation: the contrasting methods of molting feathers between modern birds and their ancient counterparts.

Feathers constitute a crucial characteristic shared by all birds. Composed of a protein known as keratin—similar to the substance found in our hair and nails—feathers serve various purposes, such as enabling flight, facilitating swimming, providing camouflage, attracting mates, offering insulation, and safeguarding against the sun’s rays.

However, feathers are intricate structures that cannot be repaired. To ensure their feathers remain in optimal condition, birds undergo a process called molting, wherein they shed their feathers and grow new ones. Juvenile birds molt to replace their downy feathers with adult plumage, while mature birds continue to molt approximately once a year.

“Molt is an aspect that often escapes the attention of many people, but it is undeniably a vital process for birds because feathers play a role in numerous functions,” explains Jingmai O’Connor, an associate curator of fossil reptiles at the Field Museum in Chicago. “We aim to comprehend how this process evolved, how it varied across different bird groups, and how it influenced bird evolution and the survivability of various clades.” O’Connor has recently published two papers investigating the molting process in ancient avian species.

One of these studies, featured in the journal Cretaceous Research, documented the discovery of a cluster of feathers preserved in amber, originating from a baby bird that thrived 99 million years ago.

Illustration of what a newly hatched Enantiornithine bird may have looked like. Credit: Yu Chen and Shundong Bi.

Presently, newborn birds exhibit a range of developmental stages and levels of dependency on their parents. Some species are classified as altricial, wherein the hatchlings are born naked and helpless. The absence of feathers allows for efficient transfer of body heat from the parents to the babies’ skin. Conversely, there are precocial species that emerge from the egg with feathers and possess a certain degree of self-sufficiency.

All baby birds experience successive molts, shedding their existing feathers and growing new ones until they attain their adult plumage. Molting is an energy-intensive process, and the simultaneous loss of numerous feathers can make it challenging for a bird to regulate its body temperature. Consequently, precocial chicks tend to undergo a gradual molt, ensuring a consistent supply of feathers, while altricial chicks, which rely on their parents for food and warmth, undergo a “simultaneous molt,” shedding all their feathers in synchrony.

The amber-encased feathers examined in this study provide the first conclusive fossil evidence of juvenile molting, shedding light on the life history of a baby bird that does not align with any existing avian species.

“This specimen exhibits a peculiar combination of both precocial and altricial characteristics,” notes O’Connor, who collaborated with senior author Shundong Bi from the Indiana University of Pennsylvania on the paper. “All the body feathers are in a nearly identical stage of development, indicating that they began growing simultaneously or very close in time.”

However, it is highly probable that this particular bird belonged to an extinct group known as Enantiornithines, which O’Connor’s prior research has demonstrated to be highly precocial.

An illustration of a more mature juvenile Enantiornithine. Credit: Yu Chen and Shundong Bi.

O’Connor proposes that the challenges faced by precocial baby birds, which had to maintain their own warmth while undergoing a rapid molt, may have contributed to the extinction of the Enantiornithines.

“Enantiornithines were the most diverse group of birds during the Cretaceous period, but they perished alongside the non-avian dinosaurs,” explains O’Connor. “When the asteroid struck, global temperatures would have dropped dramatically, and resources would have become scarce. These birds would have had even higher energy requirements to keep warm, but lacked the necessary resources to meet them.”

In a separate study published on July 3 in Communications Biology, O’Connor and Field Museum postdoctoral researcher Yosef Kiat investigate molting patterns in modern birds to gain insights into the evolution of this process.

In present-day adult birds, molting typically occurs once a year in a sequential manner. They gradually replace a few feathers at a time over a few weeks, allowing them to continue flying during the molting process. Simultaneous molts, where all flight feathers are shed and regrown within a couple of weeks, are rarer and often observed in aquatic birds like ducks that do not rely heavily on flight to find food and evade predators.

Finding evidence of molting in fossil birds and other feathered dinosaurs is exceedingly rare. O’Connor and Kiat aimed to understand the reasons behind this scarcity. “We hypothesized that birds with simultaneous molts, which occur within a shorter time frame, would have a lower representation in the fossil record,” explains O’Connor. The rationale behind this hypothesis is that spending less time molting reduces the chances of dying during the molting process and becoming fossilized with molting-related features. To test their hypothesis, the researchers examined the collection of modern birds housed at the Field Museum.

Illustration showing a young Enantiornithine bird. Credit: Yu Chen and Shundong Bi.

Kiat, the lead author of the study, explains that they examined over 600 preserved skins of modern birds from the Field Museum’s ornithology collection to detect signs of active molting. Among the birds that undergo sequential molts, they discovered numerous specimens in the midst of molting. However, among the birds that undergo simultaneous molts, they found very few instances.

While these observations pertain to modern birds rather than fossils, they serve as a valuable proxy. O’Connor remarks, “In paleontology, we need to be resourceful since we lack complete data sets. In this case, we employed statistical analysis of a random sample to infer the implications of the absence of something.”

Based on the absence of molting fossil birds, despite the prevalence of active molting among the modern bird specimens, the researchers deduce that fossil birds likely molted less frequently than most modern birds. They may have undergone simultaneous molts, or they may not have molted on an annual basis like the majority of present-day birds.

Both the findings from the amber specimen and the study of molting in modern birds converge on a common theme: prehistoric birds and feathered dinosaurs, particularly those from groups that did not survive the mass extinction, exhibited distinct molting patterns compared to contemporary birds.

“All the differences observed between crown birds and stem birds essentially become hypotheses about why certain groups survived while others did not,” explains O’Connor. “There isn’t a single specific reason why crown birds, which encompass modern birds, managed to survive. It’s likely a combination of factors. However, it is becoming increasingly evident that molting may have played a significant role in determining which dinosaurs were able to endure.”

Source: Field Museum

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