When hummingbirds fly unfriendly skies
Hovering behind a fat obstacle like a tree trunk on a windy day may have costs
HOVER MASTER An Anna’s hummingbird can sip nectar in fairly bumpy air but may have to calculate when a meal isn’t worth the energy expenditure of fighting turbulence.
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
