Flamingos make underwater vortexes to suck up prey

If you've ever really looked at how flamingos eat, you know how captivatingly peculiar it is. They bob their inverted heads in the water and do a kind of waddle cha-cha as they inch their way across shallow water, filter-feeding small crustaceans, insects, microscopic algae and other tiny aquatic morsels.

Victor Ortega-Jiménez, an integrative biologist at the University of California, Berkeley, remembers being fascinated by this behavior the first time he saw it in 2019, during a trip with his wife and child to the Atlanta zoo. Ever since, he has been wondering what, exactly, was going on beneath the surface.

"The birds looked beautiful, but the big question for me was, 'What's happening with the hydrodynamic mechanisms involved in flamingos' filter feeding?'" he said.

Back home, he was surprised to find no explanation in the scientific literature -- so he decided to produce one himself. Several years of meticulous research later, he and his colleagues arrived at a surprising discovery, described Monday in the Proceedings of the National Academy of Sciences. Flamingos, they found, are active predators that harness the physics of how water flows to sweep up prey and funnel it directly into their mouths.

"We are challenging the idea that flamingos are just passive filter feeders," Ortega-Jiménez said. "Just as spiders produce webs, flamingos produce vortices."

Ortega-Jiménez's collaborators included three exceptionally cooperative flamingos from the Nashville Zoo: Mattie, Marty and Cayenne. Zookeepers trained the birds to feed in a clear container, which allowed the researchers to record what was happening using high-speed cameras and fluid dynamic methods. The scientists generated oxygen bubbles and added food particles to measure and visualize the flow of the water as the birds fed. After initial observations with the live birds, the team built a 3D model of a flamingo head and used it to more precisely explore the birds' biomechanics.

Flamingos, they found, frequently and quickly retract their heads as they feed. Each of those motions creates a tornadolike vortex and an upwelling of particles from the bottom toward the water's surface. Further observation and experiments with the mechanical beak revealed that chattering, in which flamingos rapidly clap their beaks while their heads are lifted but still underwater, is responsible for causing the mini-twisters to flow directly toward the birds' mouths, helping them capture prey. Their bent, L-shaped beaks were also critical for generating vortices and recirculating eddies as they fed at the water's surface, reaping the rewards of those engineered flows.

Another "amazing finding," Ortega-Jiménez said, was what the birds do with their feet, which the researchers explored using a mechanical flamingo foot and computational modeling. The dancinglike motion of their webbed appendages underwater produced yet more vortices that pushed additional particles toward the birds' waiting mouths as they fed upside down in the water. Taken together, these findings suggest that flamingos are "highly specialized, super feeding machines that use their entire body for feeding," Ortega-Jiménez said.

Sunghwan Jung, a biophysicist at Cornell University who was not involved in the study, praised the work for being "an outstanding demonstration of how biological form and motion can control the surrounding fluid to serve a functional role."

Alejandro Rico-Guevara, an evolutionary biologist at the University of Washington, Seattle, also not involved in the work, agreed that the new paper puts to rest the notion that flamingos are passive in the way they filter feed. "There have been many hypotheses surrounding how their odd bills could work," he said, "but until recently we didn't have the tools to study it."

In addition to solving that mystery and revealing "a uniquely evolved way to capture tiny and evasive prey," he continued, the research suggests another evolutionary reason for webbed feet in birds, beyond just being good paddles.

Now that Ortega-Jiménez's curiosity about flamingo-instigated fluid dynamics has been satisfied, he plans to turn his attention to what is going on inside the birds' beaks during feeding. Taken together, such findings could eventually lead to bioinspired technologies that capture things like toxic algae or microplastics, he said.

"What's at the heart of filter feeding in flamingos?" he said. "We as scientists want to understand both the form and function of these fascinating and mysterious birds as they interact with their fluid environment."