Amber with feathers from a baby bird that lived 99 million years ago and an illustration of a bird. Photo by Shundong Bi.

Why some dinosaurs survived as birds

Are birds dinosaurs? Essentially yes.


Fossil studies have unlocked an intriguing answer to a long-standing mystery – the survival mechanism of birds’ ancient ancestors that allowed them to weather the asteroid-induced extinction that obliterated the other dinosaurs 66 million years ago. The distinguishing factor might be in how these predecessors of today’s birds molted their feathers.

Dinosaurs and Birds

Every bird, from the common robin to the adorable penguin, is a breathing testament to dinosaur survival. Feathers, made of the same protein, keratin, as our nails and hair, serve essential functions in birds’ lives. From flying and swimming to camouflage, mate attraction, insulation, and solar protection, feathers play a pivotal role. However, their complexity prohibits repair, prompting birds to routinely molt – replacing old feathers with new ones to maintain optimal performance.

The Forgotten Phenomenon of Molting

While most people pay little attention to molting, it is a vital bird function, says Jingmai O’Connor, an associate curator of fossil reptiles at Chicago’s Field Museum. O’Connor is dedicated to exploring the evolution of molting, its variation among bird groups, and how it influenced bird evolution and survival. She recently published two papers delving into prehistoric bird molting patterns.

A paper published in Cretaceous Research in May 2023 unveiled a unique discovery – a cluster of feathers from a 99 million-year-old baby bird encased in amber. This find is the first concrete fossil proof of juvenile molting.

Precocial and Altricial: The Two Extremes of Baby Bird Development

Birds, when hatched, fall into one of two categories: altricial, who are naked and helpless, relying on parents for heat and food; and precocial, who are born with feathers and are relatively independent. These birds go through several molts to reach their adult plumage. For precocial chicks, molting is a slow process, maintaining a steady feather supply to keep warm. In contrast, altricial chicks, who rely on their parents, undergo a “simultaneous molt,” shedding all feathers simultaneously.


An Extinct Lineage Unveiled: The Enantiornithines

The amber-preserved baby bird shows an unusual blend of precocial and altricial characteristics, offering an insight into a now-extinct group called the Enantiornithines. The bird exhibited simultaneous feather growth, an indication of rapid molting. O’Connor postulates that the combined stress of rapid molting and self-insulation might have contributed to the Enantiornithines’ extinction.

Linking Survival to Molting Patterns

A subsequent study in Communications Biology, co-authored by O’Connor and Yosef Kiat, investigated modern birds’ molting patterns to understand the process’s evolution. Generally, modern adult birds molt sequentially over weeks, keeping flight possible during the process. Simultaneous molts, common in aquatic birds, are rarer.

A Gap in the Fossil Record: Where are the Molting Birds?

Molting evidence in fossil birds and feathered dinosaurs is scarce. O’Connor and Kiat speculate that birds molting simultaneously, a shorter process, might have fewer chances to get fossilized during molting. The researchers tested this hypothesis by examining over 600 specimens of modern birds from the Field Museum’s collection.

A Lesson from Modern Birds: Filling the Fossil Gap

Though they couldn’t find many examples of simultaneously molting birds, the study gave some important clues. This lack of evidence led the researchers to infer that ancient birds either underwent simultaneous molts or didn’t molt annually, unlike most modern birds.

Redefining Prehistoric Bird Molting

Both the amber fossil and the modern bird molting study suggest that prehistoric birds and feathered dinosaurs, particularly the ones that didn’t survive the mass extinction, had distinct molting patterns from today’s birds.


Feather Molting: A Survival Key for Modern Birds

Differences between today’s birds and their ancient counterparts offer possible explanations for the survival of some and extinction of others. According to O’Connor, there may not be a single reason why modern birds survived while others perished. However, it’s becoming clear that molting patterns could have significantly determined dinosaur survival.

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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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