Feature Article

9-Jun-2020

Hooks on the feathers stick together: Visualizing how birds form continuous wings in flight

DOE/Argonne National Laboratory

By studying bird feathers at the Advanced Photon Source, scientists have observed the mechanism that connects those feathers when birds fly.

Ever since the first humans saw birds gracefully soaring through the sky above their heads, we have been asking two questions: how do they do that, and how can we learn to do that?

Centuries of research later, we know quite a lot about how birds fly, but there's enough we don't know that we can still be surprised. A team of researchers has just proven this point with the first 3D visualization of the mechanism birds use to keep their flight feathers (the larger ones on the wings) together in flight, a mechanism that allows them to combine their disparate feathers into a continuous, flexible wing.

A cross section from an APS image stack of a pigeon wing feather. The red arrow points to an inter-feather hook for reference. (Image by Teresa Feo / Smithsonian National Museum of Natural History.)

"Birds are cool and fascinating They do a lot of amazing things, things that we would also like to be able to do." -- Teresa Feo, Smithsonian National Museum of Natural History

This pivotal discovery has already led to new technological innovations and gives us more insight into how we might emulate bird flight in our own flying machines.

At the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) User Facility located at the DOE's Argonne National Laboratory, researchers used ultrabright X-rays to scan feathers from many different species of birds. According to Dr. Teresa Feo, research associate at the Smithsonian National Museum of Natural History, she and her colleagues visited the APS five times with box loads of feathers.

"We took every feather we could, since the scans only take five or 10 minutes," Feo said. ?"We would show up with a couple hundred feathers of every species we could think of."

What they found was a complex system of hook-shaped microstructures on the tops and bottoms of feathers, which lock with the corresponding hooks on adjacent feathers to form a single-direction fastener. When birds stretch their wings, these hook-like structures lock together to prevent gaps, and they unhook when the wings are retracted. The research team published their work in Science.

Scientists have long understood that there is some mechanism that connects bird feathers in flight, Feo said, though the common term for these structures -- friction barbules -- turns out to be a misnomer. Rather than using friction, these tiny, unidirectional hooks physically connect with one another through a complicated yet passive system triggered by movement of the bones of the wing.

"We knew the feathers were connected to the wing at one end, and pretty flexible, and we knew that birds can separate their feathers," Feo said. ?"We were able to show the mechanism that keeps the feathers from going past one another. The hooks on the feather branches keep the wing itself a coherent surface, one feather connected to the next."

Picture of a pigeon feather mounted for scanning in the beamline at the APS. The laser level shows approximately where the scan will be taken. (Image by Teresa Feo / Smithsonian National Museum of Natural History.)

The interlocking cilia were found on feathers from more than a dozen species of birds. Interestingly, silent flyers like owls were found not to have the same locking mechanism, and Feo said that is likely because the process is noisy, and the birds rely on a quieter system to allow them to hunt.

Feo said that the APS provided her and her colleagues fast, accurate scans of as many feathers as they could carry, which provided the backbone of this research. Previous attempts to scan feathers in university environments were less successful, since scans take longer with a lower-intensity beam and keeping feathers still for hours proved difficult.

The scans were completed using high-resolution computed-tomography X-ray imaging at beamline 2-BM, operated by Argonne's X-ray Science Division (XSD). Francesco De Carlo, physicist with XSD and group leader for 2-BM, noted the unusual nature of this research, but touted the versatility of the APS and those who run the beamlines to support many different types of science.

For this paper, Feo worked with Stanford University Assistant Professor David Lentink, who runs a lab dedicated to studying biological flight as an inspiration for mechanical engineering design. As part of that research, Lentink and his colleagues designed a drone robot with real bird feathers that simulates avian flight. (It's nickname: PigeonBot.) Lentink said they built the robot based on earlier measurements of feathers and how they interact, but found that PigeonBot acted differently than they expected.

"Based on the literature we expected there to be some form of enhanced friction," Lentink said. ?"When we measured the forces we found the feathers actually locked together. The APS helped visualize the locking mechanism. Both the locking force and the locking structures were scientific discoveries."

That locking mechanism, Lentink said, has no equivalent in technology, and replicating it could lead to many different advancements.

"We discovered ?'directional Velcro,' which can inspire the design and manufacturing of the first ?'directional Velcro' fasteners," he said. ?"The directionality means releasing them is very easy, while they will hold snug under tension. This has a wide range of applications including medical bandages."

Learning more about the way birds fly can also lead to innovations in the way airplanes are built, Feo said. She pointed out that birds can morph their wings with great agility and control, something that mechanical airplanes so far cannot do.

"Birds can fly through narrower areas by pulling their wings in," she said. ?"Birds fly in complicated environments, and their wings run into things and recover all the time. That capacity is something we would love to be able to replicate in our flying machines."

At heart, though, scientists like Feo and Lentink have that same fascination that those early humans felt watching birds take flight, and learning more about how they do what they do.

"Birds are cool and fascinating," Feo said. ?"They do a lot of amazing things, things that we would also like to be able to do."

About the Advanced Photon Source

The U. S. Department of Energy Office of Science's Advanced Photon Source (APS) at Argonne National Laboratory is one of the world's most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation's economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.

3D rendering of the same feather scan at the APS. One red arrow points down to the inter-feather hook in the cross section for reference, while other arrows point up to other hooks. (Image by Teresa Feo / Smithsonian National Museum of Natural History.)

This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

The U.S. Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://?ener?gy?.gov/?s?c?ience.