Hovering flight is one of the most strenuous activities imaginable. It's so hard that one might imagine that it would be impossible in thin mountain air,which provides less lift and has less oxygen. And yet hummingbirds live,thrive – and hover – high in the forests of the Andes, betraying no sign that flying is any harder for them than for their cousins in the valleys. Not only that, but high-altitude hummingbirds tend to weigh more– possibly to compensate for the cold air – making hovering seem even more difficult. But despite all these challenges at altitude,hummingbirds don't seem to care – their cost of hovering is the same,whether in valleys or on mountaintops.
The tiny birds can't have their mountain habitat for free, thought Doug Altshuler of the California Institute of Technology. He already knew they had a few tricks to make high-altitude hovering easier. Mountain hummingbirds have larger wings relative to their weight than valley birds, and they beat their wings farther back and forth than their low-flying cousins. But these tricks just make power requirements the same for high- and low-altitude hovering– where do mountain birds pay the price?
To answer this question, Altshuler and his colleagues hiked into the Peruvian Andes, capturing 76 species of hummingbirds over a 4000 metre change in elevation and examining wing shape and hovering ability. They took DNA samples to construct a detailed phylogeny, so they could adjust their comparisons to take account of the similarity of closely related species. And they estimated how strong the birds were, above the demands of hovering, by making them lift weights. They tied strings of weighted beads to the birds,then scared them. This usually made the birds fly straight up – until the weight of the beads overloaded them.
It turned out that mountain hummingbirds can lift fewer beads than valley birds, suggesting that the mountain-dwellers do pay a price for their high-altitude homes. As Altshuler's team had seen before, the power required to hover is the same for all hummingbirds, regardless of elevation, because of changes in wing size and increases in wing motion. But the key was that the high-altitude hummers have less energy to spare for other tricks, like lifting weights attached to them by keen researchers, or more biologically relevant tasks like flying quickly and performing the acrobatic manoeuvres they're famous for.
And this lack of extra power has potentially interesting consequences for the tiny birds' ecology. They use fast, acrobatic flight while competing to catch insects and feed on flower nectar, both of which are more scarce in the mountains. But, possibly more importantly, male hummingbirds use complex aerial display flights to attract females, rapidly flying up and down, and side to side. It may be that competition, both for food and for mates, is less fierce in the mountains – simply because the hummingbirds lack the extra energy. No one knows.