The ability of many birds’ feathers to shed water is remarkably effective; as a result, the phrase “like water off a duck’s back” is frequently used. Much more peculiar are the belly feathers of the sandgrouse, especially the Namaqua sandgrouse, which absorb and retain water so effectively that the male birds can fly more than 20 kilometers from a distant watering hole back to the nest and still retain enough water in their feathers for the chicks to drink and sustain themselves in Namibia, Botswana, and South Africa’s scorching deserts.
Although scientists had a general idea of how those feathers function, it took the most up-to-date microscopy tools and careful work with a collection of sandgrouse feathers to reveal the specific structural details that give the feathers their capacity to hold water. The research was conducted by Professors Lorna Gibson of MIT’s mechanical engineering department and Jochen Mueller of Johns Hopkins University, and it was published today in the Journal of the Royal Society Interface. Gibson holds the Matoula S. Salapatas Professor of Materials Science and Engineering title.
Gibson claims that E. first noted the special water-carrying capacity of sandgrouse feathers in 1896. G. B. Meade-Waldo, who was captive breeding the birds. Gibson claims, “He saw them acting like this, and nobody believed him! I mean, it just sounded so ridiculous.”.
“What we did was take measurements of the dimensions and perform some calculations to demonstrate that this is what is actually happening,”
Lorna Gibson, the Matoula S. Salapatas Professor of Materials Science and Engineering.
In a study that demonstrated the existence of the unusual behavior, Tom Cade and Gordon MacLean published their detailed observations of the birds at watering holes in 1967. After the male sandgrouse spent about five minutes dipping in the water and fluffing its feathers, the researchers discovered that the feathers could hold about 25 milliliters of water, or roughly a tenth of a cup.
When a male bird returns to its nest after a half-hour flight, about half of that liquid can evaporate. The remaining liquid is then drunk by the chicks, who are unable to fly for about a month, directly from the feathers.
Gibson claims that although Cade and MacLean “had part of the story,” the tools required to conduct the new study’s detailed imaging of the feather structures were not yet available.
Gibson and Mueller used micro-computed tomography, video imaging, and scanning electron microscopy in their study. They took Namaqua sandgrouse belly feathers from Harvard University’s Museum of Comparative Zoology, which houses specimens of about 80% of the world’s bird species.
The sandgrouse is shown in this image flying before and after being soaked. You can see how much bigger the sandgrouse’s underside is. Thanks to Lorna Gibson and others. Credit: Lorna Gibson, et al
Generally speaking, bird feathers have a central shaft from which shorter barbs and then shorter barbules radiate. However, the structure of sandgrouse feathers differs. The barbules in the inner zone of the feather have a structure that is helically coiled near their base and then extends straight. The barbules in the feather’s outer zone are straight instead of having a helical coil. The grooves and hooks that hold the vane of contour feathers together in the majority of other birds are absent from both components.
When the barbules are moistened, they uncoil and rotate so that they are perpendicular to the vane, creating a dense network of fibers that can hold water through capillary action. Additionally, the barbules in the outer zone curl inward to aid in retaining the water.
The dimensions of the various feather components could be measured thanks to the microscopy methods used in the new study. The barbules are small and flexible enough that surface tension is sufficient to bend the straight extensions into tear-like structures that can hold water in the inner zone, where the barb shafts are large and stiff enough to provide a rigid base about which the other parts of the feather deform. Additionally, because the barb shafts and barbules are smaller in the outer zone, they can curl around the inner zone and hold onto more water.
Researchers from Johns Hopkins University and Massachusetts Institute of Technology used high-resolution microscopes and 3D technology to capture an unprecedented view of the feathers of the desert-dwelling sandgrouse, showcasing their unique architecture and revealing for the first time how they can hold so much water.
According to Gibson, earlier research had suggested that the characteristics of water retention were caused by surface tension, but “what we did was make measurements of the dimensions and do some calculations to show that’s whats actually happening,” he says. The research done by her group showed that the various stiffnesses of the various feather parts play a significant role in their capacity to hold water.
Gibson claims that the study’s primary impetus came from an interest in this unusual behavioral phenomenon on an intellectual level. “We merely wanted to see how it functions. The entire tale just seemed so captivating. But she adds that there might be some practical applications. Some adaptation of this feather structure, for instance, might be incorporated into the systems of enormous nets that are used to collect water in desert regions where water is scarce but fog and dew are common, such as in Chile’s Atacama Desert. You could think that this could be a way to make those systems better, she says. This type of structure may make a material more efficient at capturing fog and holding water.
More information: J. Mueller et al, Structure and mechanics of water-holding feathers of Namaqua sandgrouse ( Pterocles namaqua ), Journal of The Royal Society Interface (2023). DOI: 10.1098/rsif.2022.0878
