Hummingbirds have long intrigued scientists. Their wings can beat 80 times a second. Their hearts can beat more than 1,000 times a minute. They live on nectar and can pack on 40 percent of their body weight in fat for migration.

But sometimes they are so lean that they live close to caloric bankruptcy. At such times, some hummingbirds could starve to death while they sleep because they're not getting to eat every half-hour or so. Instead they enter a state of torpor, with heartbeat and body temperature turned way down to diminish the need for food.

Kenneth C. Welch Jr. at the University of Toronto, Scarborough has studied the metabolisms of hummingbirds for more than a decade. His most recent research with Derrick J. E. Groom and other colleagues found that during strenuous hovering flight, bigger hummingbirds are more efficient energy users than smaller ones. The research was published in Proceedings of the Royal Society B.

Welch said that unlike us, hummingbirds can use the glucose that they're ingesting in nectar and can move it through their guts, through their circulatory system, and to their muscle cells so fast that they can keep that pipeline going in real time.

"You and I can't do that. We can support some small portion of exercise with newly ingested glucose, about 30 percent. But what's just as remarkable is that the diet of hummingbirds is nectar and that's half glucose and half fructose," he said.

"Hummingbirds can apparently take up fructose in their cells, and we're trying to figure out what enables that. We do know that there is a different form of glucose transporter that is a specialist at taking up fructose. In our muscle cells that transporter is barely present. But hummingbird muscle fibers tend to have a lot of this transporter."

Larger hummingbirds are more efficient than smaller ones.

New York Times