What distinguishes flight plumage

This wing of a species of lark illustrates the rule: nine flight feathers with an asymmetrical structure emerge from the “hand segment” of the wing. © Yosef Kiat

Everyone has feathers - but as we know, not every bird uses them to fly. Now researchers have revealed a characteristic plumage feature of the flying birds that has remained hidden until now: They have nine to eleven asymmetrically structured feathers on the “hand section” of their wings. Flightless birds, on the other hand, have fewer or significantly more. Paleontologists then used this rule to shed light on the evolutionary history of birds and their feathered dinosaur ancestors.

Not extinct: If you want to see a dinosaur, you often just have to look out the window. Today's birds belong to this group of animals - they arose from ancestors that survived the asteroid impact around 66 million years ago. The secret to their success was a “patent” that had long been developed in dinosaurs: the feather. It is assumed that in the Jurassic period, representatives of the group of Pennaraptora dinosaurs eventually developed more complex feathers from down-like structures. Initially they may have served various purposes. But as development progressed, their aerodynamic effects became more and more important. This led to complex flight abilities in the Pennaraptora early on. However, as this group and later birds continued to evolve, the ability to fly again began to be lost - as was the case with ratites or penguins.

Plumage characteristics in sight

When researching the evolutionary history of feathered dinosaurs and birds, paleontologists have so far focused primarily on the characteristics of fossil arm and wing bones. Yosef Kiat and Jingmai O'Connor from the Field Museum of Natural History in Chicago, on the other hand, are now focusing on the development of plumage structures. According to the two scientists, it has hardly been systematically recorded which plumage features are typical for the wings of birds capable of flight.

The basis of their study was therefore a comprehensive examination of the feather structures of today's birds using hundreds of museum specimens. When Kiat looked at the wings and feathers of hummingbirds, falcons, penguins, etc., a well-known feature of the flying species became clear: their primary flight feathers along the wing tips are asymmetrically structured. The feather vein is offset laterally, while the primary feathers in flightless birds are constructed symmetrically. But the scientist also discovered a characteristic that had previously remained undiscovered: all birds capable of flight have nine to eleven flight feathers that emerge from the final wing segment, which corresponds to the hand.

An old principle

It's a simple rule that no one seems to have noticed before: "It was really surprising to find that with so many different ways of flying that we find in modern birds, they all follow this rule - they have between nine and eleven primary feathers. I was surprised that no one seemed to have noticed this yet,” says Kiat. In flightless birds, however, the number of feathers in this area varies greatly - in penguins, for example, there are more than 40 small feathers, while emus have no primary feathers on the wing segment. Apparently these different forms emerged in the evolutionary history of flightless birds.

In addition to the information about the feather morphology of modern birds, Kiat and O'Connor then integrated the plumage characteristics of 35 species of dinosaurs as well as extinct birds. The analysis results showed that the possession of nine to eleven primary feathers was apparently an original characteristic of birds and Pennaraptora dinosaurs. The different plumage features on the wings only developed in the flightless development lines and apparently only slowly. “Basically, it was shown that the number of primary feathers and the shape of the feathers can be used as an indication of whether a fossil creature could fly, or whether its ancestors could,” says O'Connor. For example, the researchers found evidence in a representative of the Pennaraptora dinosaurs that although it was itself flightless, it may have already emerged from ancestors capable of flight. However, other fossil species with feathered wings once again confirmed their ability to fly.

“Our study, which combines paleontological data based on fossils of extinct species with information from birds living today, provides interesting insights into the evolutionary history of plumage and the evolution of the dinosaurs that gave rise to it,” said Kiat. As for the origins of dinosaur flight, O'Connor concludes: There is debate about whether flight evolved in dinosaurs just once or at multiple points in time. Our results seem to indicate that flight evolved only once. “But we must remember that our understanding of flight in dinosaurs is just beginning and we are probably still missing some of the earliest stages of feathered wing evolution,” he said.

Source: Field Museum of Natural History, specialist article, PNAS, doi: 10.1073/pnas.2306639121

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