"Swirling air can make hummingbirds work harder to hover, but only when the air’s vortices open wider than a bird’s wing.
The first measurements of how much a flying animal’s metabolism revs up when coping with turbulent air come from five Anna’s hummingbirds (Calypte anna) that Victor M. Ortega-Jimenez of the University of California, Berkeley and his colleagues tested. In a wind tunnel, the hummingbirds hovered at a feeder downwind from a cylinder of varying size. Buffeted by vortices of air whipping off slim cylinders (2 or 4 centimeters in diameter), the birds held their position without needing extra oxygen even with wind speeds of 9 meters a second, or about 20 miles per hour.
But when researchers used a 9-centimeter-wide cylinder, vortices widened to 173 percent of wing length. This time hummingbird metabolisms increased some 25 percent on average — even at gentler wind speeds of 3 and 6 meters per second. The hummingbirds relied on asymmetric tail and wing motions to hover in place, the researchers report March 26 in Proceedings of the Royal Society B.”