Watch the hummingbirds ‘dance’ across the waterfalls | Science

Starlings, swifts and small birds called divers build nests in the most unlikely places: behind the mighty curtains of waterfalls. But how they penetrate the beating streams to reach their safe havens has long been a mystery. Now, research is revealing just how good little aviators can get through these splashy obstacles, and how waterfalls could protect themselves against another threat: bloodsucking bugs.

To understand the physics of crossing waterfalls, scientists turned to hummingbirds. While small buzzers usually don’t get in and out of falls, they are close relatives of cascading swifts and easier to use in a lab due to their smaller size.

First, the researchers built a 54-liter flight chamber, with a feeder on one side and a pole on the other. Separating them was an artificial waterfall 3 millimeters thick, created with a small jet of water and a pump. It was much smaller than what birds might encounter in the wild, the researchers note, but its flow was stronger than the extreme rains. Scientists then filmed four of Anna’s hummingbirds (Calgypt anna) as they tried to cross the waterfall to reach the pole.

All crossed without too many problems, reports the team today in Royal Society Open Science. But what surprised scientists was the unexpected approach of birds, says Victor Ortega-Jimenez, a biologist at Kennesaw State University, Kennesaw, and lead author of the study. The birds did not accelerate, fold their wings and cross the water like a ball, as he expected. Aside from a bird that entered the waterfall head-on, the others slid to the side, parting the falling water with a wing before passing the rest of their body (as seen above). “Nothing in the literature can predict this,” says Ortega-Jimenez.

By entering the water laterally rather than from the front, the birds can separate from the waterfall while continuing to propel themselves side to side, the scientists suggest. “One wing is always generating thrust, while the other is in the water,” adds David Hu, a mechanical engineer and biologist at the Georgia Institute of Technology who was not involved in the study.

To see how even smaller aviators were doing, scientists caught and tested house flies, green flies, and a crane fly. For the bugs, the trip through the waterfall (this time to an insect light) was much more treacherous. The sheet of water quickly swept the eight tiny fruit flies to the bottom of the cage until they died. The man-made fall also overpowered the crane fly, blocked by its long legs and slow flight. Although seven bottle flies and house flies managed to make it to the other side, all but two crashed immediately after their feat.

For insects, the waterfall poses several challenges, says Hu. The first is simply to break the surface tension to pass. “If you’re small enough, you’ll bounce like a trampoline,” he says. And once a bug has passed, it still needs enough momentum to fly forward, despite the downward force of the water. Finally, water droplets can weigh as much as a bottle fly, says Ortega-Jimenez, which can easily drag the virus into a watery grave.

This means that the waterfalls could serve as a protective barrier for nesting birds, protecting their chicks not only from large birds like raptors, but also blood-sucking insects and other pests. Ortega-Jimenez then plans to analyze this waterfall-crossing behavior in swifts, using drones to track these agile birds in the wild. In other words, assuming the drones survive.