Most birds can fly, but flightless birds like penguins and ostriches have adapted to life without wings. Scientists, though, have much to learn about differences between flying and non-flying birds.

In a new study published in PNAS, researchers examined hundreds of bird specimens from museum collections and uncovered common traits all flying birds share. These “rules” provide clues about how dinosaurs originally evolved flight and which ones were capable.

Not all dinosaurs evolved into birds, yet all living birds are dinosaurs belonging to the group that survived the asteroid impact 66 million years ago. Well before then, some members of the Penneraptoran dinosaur group began developing feathers and flight abilities. Members developed feathers before flight, possibly for insulation or mating displays. Velociraptor had feathers but couldn’t fly.

The wing, highlighting the flight feathers, of the Temminck's lark
The wing, highlighting the flight feathers, of the Temminck’s lark. Credit: Yosef Kiat / Field Museum

Scientists cannot travel back in time to observe dinosaurs, so they rely on fossilized skeletons, limb bones, and feather traces to determine flight capabilities.

For example, long asymmetric feathers on birds’ wingtips indicate flight, while symmetrical ones signal non-flight.

This led to collaboration between Jingmai O’Connor of the Field Museum and Yosef Kiat. Kiat studied feather traits like wingbone attachment sites and asymmetries in living birds’ 346 species.

The primary feathers of a penguin
The primary feathers of a penguin. Credit: Yosef Kiat / Field Museum

Thanks to our collaboration, Yosef can trace these traits in fossils dating back 160-120 million years to study early feather evolution, said O’Connor.

Kiat found a consistent rule – all flying birds have 9-11 primary feathers. Non-flyers vary, from over 40 in penguins to none in emus. Applying this to the bird family tree, changes take long periods.

They also examined 65 fossil specimens representing 35 extinct dinosaur and bird species. By connecting feather patterns to proportions, they inferred flight abilities like the early bird Archaeopteryx and four-winged Microraptor. Caudipteryx had flyer ancestors but lost flight features over time.

Fossil showing the wings and feathers of the dinosaur Microraptor
Fossil showing the wings and feathers of the dinosaur Microraptor. Credit: Yosef Kiat / Field Museum

These data inform dinosaur flight origins. Recent findings show birds weren’t the only feathered flyers, said O’Connor. Debates surround single or multiple origins. Our study suggests single origin, but understanding is still emerging.

Combining paleontology and living avians provides insights into feathers and flight, key to dinosaur success. More research will refine our knowledge of how characteristics coevolved.

Dinosaurs including birds comprised one of Earth’s most prolific lineages, and flight was central to their dominance.

Sources

Field Museum | Yosef Kiat, Jingmai K. O’Connor, Functional constraints on the number and shape of flight feathers. PNAS 121 (8) e2306639121, doi.org/10.1073/pnas.2306639121

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