Victor Ortega-Jimenez
By Lucy Hicks
Starlings, swifts, and small birds called dippers build nests in the most unlikely places: behind the mighty curtains of waterfalls. But how they traverse the pounding streams to reach their safe havens has long been a mystery. Now, research shows just how small floors can penetrate these splashing obstacles – and how the waterfalls could protect against another threat: blood-sucking bugs.
To understand the physics of crossing waterfalls, scientists turned to hummingbirds. Although the small buzzers simply do not pop in and out of waterfalls, they are close relatives of waterfall nest nests and easier to work with in a lab due to their smaller size.
First, the researchers built a 54-liter flight room, with a feeder on one side and a perch on the other. She separated was a 3-millimeter thick artificial waterfall, made with a small water jet and pump. It was much smaller than what birds in the wild might encounter, the researchers note, but its stream was stronger than extreme rain. The scientists then filmed four hummingbirds of Anna (Calypte anna) when they tried to traverse the cascade to reach the perch.
All crossed without much effort, the team reports today Royal Society Open Science. But what surprised the scientists was the birds’ unexpected approach, says Victor Ortega-Jimenez, a biologist at Kennesaw State University, Kennesaw, and lead author of the study. The birds did not set up, folded their wings and went through the water like a bullet, as he expected. Except for one bird entering the waterfall ahead, the rest glide to the south, dividing the falling water by one wing before the rest of its body continues (as shown above). “Nothing in the literature could predict that,” says Ortega-Jimenez.
By entering the water laterally as forward, the birds can share the cascade as they move from one side to the other, the scientists suggest. “One wing always generates pressure while the other wing is in water,” adds David Hu, a Georgia Institute of Technology mechanical engineer and biologist who was not involved in the study.
To see how even smaller floors decayed, the scientists caught and examined houseflies, green bottles and one crane fly. For the insects, the journey across the waterfall (this time to a bow light) was much more treacherous. The water leaf soared almost all eight of the small fruit flies to the death of the cow. The artificial fall also overwhelmed the crane fly, stymied by its long legs and slow flight. Although seven bottles and houseflies managed to make it to the other side, all but two crashed directly to their feat.
For the bugs, the waterfall poses several challenges, Hu says. The first is to simply break the surface tension to get through. “If you’re small enough, you will break off like a trampoline,” he says. And once a bug is through, it still needs enough momentum to fly forward, despite the declining force of the water. Eventually, water droplets can weigh as much as a bottle of fly, says Ortega-Jimenez, who can easily drag the bug to a watery grave.
This means that cascades can serve as a protective barrier for nesting birds, thus keeping their chicks safe from not only large birds such as predators, but also blood-sucking bugs and other parasites. Ortega-Jimenez plans next to analyze this behavior in waterfalls in swifts, using drones to track these rough birds in the wild. That is, assuming the drones survive.
