Kolibris: Ein Leben am Limit
Their nectar-rich diet leads to blood-sugar levels that would put a person in a coma. And their zipping, zooming flight sometimes generates g-forces high enough to make a fighter pilot black out. The more researchers look, the more surprises lurk within those tiny bodies, the smallest in the avian world.
“They’re the only bird in the world that can fly upside down and backwards,” says Holly Ernest, a conservation ecologist with the University of Wyoming. “They drink pure sugar and don’t die of diabetes.”
Wie viel muss ein Kolibri am Tag arbeiten?
Adding in previous published data, Shankar was able to calculate the per-minute energy cost of perching, flying and hovering — basically a bird’s three options for spending time.
She then inferred how much time the birds must have spent feeding versus perching over the course of a day.
“We ended up finding that it’s super variable,” Shankar says. During the early part of the summer when flowers are abundant, birds could meet their daily energy needs with as little as a few hours of feeding, spending as much as 70 percent of the day just perching, she found. But when flowers became scarcer after the arrival of the summer monsoon rains, birds at one site perched just 20 percent of the time and used the rest of the day for feeding.
“That’s 13 hours a day!” Shankar says. “There’s no way I can spend 13 hours a day running. I don’t know how they do it.”
Hummingbirds have a trick to help them eke out their energy reserves: When a bird is in danger of running out of energy, it may go torpid at night, dropping its body temperature nearly to that of the surrounding air — sometimes just a few degrees above freezing. While in torpor, the bird appears almost comatose, unable to respond quickly to stimuli, and breathing only intermittently. The strategy can save up to 95 percent of hourly metabolic costs during cold nights, Shankar has calculated. That can be essential after days when a bird has fed less than usual, such as after a thunderstorm. It also helps birds save energy to pack on fat before migration.
To fuel their sky-high metabolic rate, hummingbirds suck down about 80 percent of their body weight in nectar each day. That’s the equivalent of a 150-pound person drinking nearly a hundred 20-ounce Cokes daily — and nectar is often much sweeter than a soda.
Sugar isn’t the only challenge posed by a nectar-rich diet. After all, nectar is mostly water — and birds that drink in so much liquid must get rid of most of it, without losing electrolytes. As a result, hummingbird kidneys are highly adapted to recapture electrolytes before they are excreted. “They pee almost distilled water,” says Carlos Martinez del Rio, an ecophysiologist now retired from the University of Wyoming.
But that brings a further problem: If a hummingbird kept producing dilute urine overnight, it would die of dehydration before morning. To avoid that, hummingbirds shut down their kidneys every night. “They go into what, in a human, would be considered acute renal failure,” says Martinez del Rio. “Hummingbirds have to do this, or they would piss themselves to death.”