Scientists uncover the amazing way sandgrouse hold water in their feathers

Newswise —

The feathers of numerous birds possess an extraordinary ability to repel water, often likened to the well-known expression "like water off a duck's back." However, the belly feathers of the sandgrouse, particularly the Namaqua sandgrouse, exhibit an uncommon trait of absorbing and retaining water with remarkable efficiency. This enables male sandgrouse to fly over 20 kilometers from a remote watering hole back to their nests, while still retaining enough water in their feathers to provide for their chicks and sustain them in the scorching deserts of Namibia, Botswana, and South Africa.

The unique structural details that allow sandgrouse feathers to efficiently hold water were uncovered through the use of advanced microscopy techniques and careful examination of a collection of sandgrouse feathers. The findings, published in the Journal of the Royal Society Interface, were authored by Lorna Gibson, the Matoula S. Salapatas Professor of Materials Science and Engineering and a professor of mechanical engineering at MIT, and Professor Jochen Mueller of Johns Hopkins University. While scientists had previously inferred a general understanding of how these feathers work, it was the latest tools of microscopy and diligent research that revealed the precise structural characteristics that enable sandgrouse feathers to absorb and retain water effectively.

The remarkable ability of sandgrouse feathers to carry water was initially documented as far back as 1896 by E.G.B. Meade-Waldo, who observed the birds in captivity. Despite Meade-Waldo's account, his observations were met with skepticism as the concept seemed far-fetched at the time. "He saw them behaving like this, and nobody believed him! I mean, it just sounded so outlandish," remarks Gibson, highlighting the initial disbelief surrounding the unique water-carrying capability of sandgrouse feathers.

In 1967, Tom Cade and Gordon MacLean conducted a groundbreaking study that provided detailed observations of sandgrouse at watering holes, confirming the authenticity of the birds' unique behavior. The researchers discovered that male sandgrouse feathers were capable of holding approximately 25 milliliters of water, which is equivalent to about a tenth of a cup. This water retention occurred after the bird had spent approximately five minutes dipping in the water and fluffing its feathers, demonstrating the remarkable capacity of sandgrouse feathers to carry and store water.

About half of that amount can evaporate during the male bird’s half-hour-long flight back to the nest, where the chicks, which cannot fly for about their first month, drink the remainder straight from the feathers. 

Cade and MacLean “had part of the story,” Gibson says, but the tools didn’t exist at the time to carry out the detailed imaging of the feather structures that the new study was able to do.

For their study, Gibson and Mueller utilized a combination of scanning electron microscopy, micro-computed tomography, and video imaging techniques. They obtained Namaqua sandgrouse belly feathers from the collection of specimens at Harvard University's Museum of Comparative Zoology, which houses a vast collection of bird specimens, covering approximately 80 percent of the world's bird species. These advanced imaging techniques and access to the extensive bird specimen collection allowed the researchers to conduct a detailed analysis of the structural characteristics of sandgrouse feathers, shedding light on their unique water-carrying abilities.

Unlike typical bird feathers that have a central shaft with smaller barbs and barbules extending from it, sandgrouse feathers have a unique structure. In the inner zone of the feather, the barbules have a helically coiled structure close to their base, followed by a straight extension. In the outer zone of the feather, the barbules are straight and do not have the helical coil. Importantly, both parts of the barbules lack the grooves and hooks that are present in most other birds' contour feathers, which are responsible for holding the vane of the feather together. This distinct structural arrangement of barbules in sandgrouse feathers is thought to be crucial in their ability to efficiently absorb and retain water, enabling them to carry water back to their nest for their chicks, even in arid desert environments. The detailed imaging using advanced techniques has allowed scientists to unravel the unique structural characteristics of sandgrouse feathers that contribute to their exceptional water-carrying abilities.

When wetted, the coiled portions of the barbules unwind and rotate to be perpendicular to the vane, producing a dense forest of fibers that can hold water through capillary action. At the same time, the barbules in the outer zone curl inward, helping to hold the water in.

The advanced microscopy techniques used in the recent study enabled precise measurements of the dimensions of different parts of the sandgrouse feather. In the inner zone of the feather, the barb shafts are relatively large and stiff, providing a rigid base around which the other parts of the feather deform. The barbules in this zone are smaller and more flexible, and the surface tension of water is sufficient to cause the straight extensions of the barbules to bend into tear-like structures that are capable of holding water.

In the outer zone of the feather, both the barb shafts and barbules are even smaller, allowing them to curl around the inner zone. This further aids in retaining water within the feather structure. The combination of large, stiff barb shafts in the inner zone and smaller, flexible barbules in both the inner and outer zones of the feather work together to create an intricate structure that is highly effective at capturing and retaining water, enabling sandgrouse feathers to efficiently transport water for their chicks in arid desert environments.

Although earlier research had proposed that surface tension was responsible for the water retention properties of feathers, the recent findings by Gibson and her team at the University of East Anglia shed new light on the underlying mechanisms. By conducting measurements and calculations, they confirmed that it was actually the varying stiffness of different feather parts that played a crucial role in their ability to retain water. This breakthrough provides a deeper understanding of the intricate interplay between feather structure and water retention characteristics.

The study was mostly driven by intellectual curiosity about this unique behavioral phenomenon, Gibson says. “We just wanted to see how it works. The whole story just seemed so interesting.” But she says it might lead to some useful applications. For example, in desert regions where water is scarce but fog and dew regularly occur, such as in Chile’s Atacama Desert, some adaptation of this feather structure might be incorporated into the systems of huge nets that are used to collect water. “You could imagine this could be a way to improve those systems,” she says. “A material with this kind of structure might be more effective at fog harvesting and holding the water.”

The work was partly supported by the National Science Foundation and the Matoula S. Salapatas Professorship in Materials Science and Engineering at MIT.

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