Hovering behind a fat obstacle like a tree trunk on a windy day may have costs
Matthew Field/Wikimedia Commons
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.
TOUGH AIR An Anna’s hummingbird can cope with modest breezes and turbulence but reaches a point where vortices spinning off an obstacle require strenuous work to hover. Credit: V.M. Ortega.
V.M. Ortega-Jimenez et al. Into turbulent air: size-dependent effects of von Kármán vortex streets on hummingbird flight kinematics and energetic. Proceedings of The Royal Society B. Published online March 26, 2014. doi: 10.1098/rspb.2014.0180.