How any bird stays aloft seems miraculous, but how hummingbirds manoeuvre like insects is even more astonishing. Yet some species make their homes at altitudes that would make us gasp. The lack of oxygen must compromise the bird's high-level performance, and it's also harder to stay aloft in thin air. But somehow the tiny animals overcome both inconveniences to conquer some of the tallest mountain ranges in America; though `how' wasn't clear, until Douglas Altshuler began hiking through the Peruvian Andes. Filming over 40 species of wild hummingbird as they hovered at heights ranging from 400 m to 4,300 m, Altshuler discovered that at higher altitudes, the tiny birds sweep their wings through wider arcs to stay aloft as the air density drops (p.3139).
But working with the agile aeronauts in the Peruvian forest was far from straightforward. Altshuler had to capture the birds in mist nets as they darted through the trees. After six weeks alone in the jungle, Altshuler had only trapped 30. But when he returned a year later, he was accompanied by a team of enthusiastic Earthwatch volunteers; suddenly the project took off. Within a matter of weeks, the volunteers had trapped more birds than Altshuler had caught during the previous field season. Together they collected footage to find out how the birds modulated their wing beats at higher altitudes. Back home in Texas, after hours of painstaking analyses, Altshuler realised that as the height increased and the air quality fell, all of the birds increased their wing beat amplitude to stay aloft.
But the Peruvian jungle isn't the most controllable environment. Altshuler needed to systematically alter the atmosphere around a hovering hummingbird and monitor how its wing beats varied to find how they overcome the lack of air.
This time he travelled to the Rocky Mountains in Colorado, the transient home of Rufus and broad tailed hummingbirds. After gently trapping the birds,Altshuler placed them in an airtight cylinder, where he could control the atmosphere as the birds hovered. Would the birds increase their wing beat frequency, or wing sweep?
First Altshuler decreased the air density while maintaining the oxygen level. As the density fell, both species gained the extra lift they needed in thin air by widening their wing strokes. But how would they react to falling oxygen levels? Altshuler replaced the oxygen with nitrogen, but no matter how hard the birds tried to beat their wings, their wing beat frequencies fell until they `gently drifted down like a feather to the bottom of the chamber'he says. Without oxygen the birds could no longer beat their wings fast enough to hover.
But how would ruby-throated hummingbirds hover in thin air where the oxygen level was much higher; 35%? Surprisingly, even with the extra oxygen, the birds couldn't increase their wing beat frequency. They simply increased their wing beat amplitude, just as they had before. What was it that stopped the birds from beating their wings faster? Were the muscles working at their limit, or was something else preventing the birds from raising their performance?
Altshuler set the birds a weightlifting challenge to see whether they could ever increase their work output. Fitting them with a hummingbird-sized harness, he videoed the birds as they strained to lift a string of beads. For a fraction of a second, their wing beat frequency rocketed, but quickly fell again. The birds had switched from aerobic respiration to anaerobic respiration. So their muscles could work harder, but only briefly, which is why they resort to sweeping their wings wide while at altitude.