Life in the Fast Lane
Scientists are finally learning how tiny hummingbirds survive in their world of incredible extremes
No bird can maneuver in mid-air as deftly as a hummingbird. Biologist Gayle Brown learned that fact firsthand while doing field work a few years ago on Vancouver Island. She was trying to band some of the tiny rufous hummingbirds that breed on the island during summer. Unfortunately, Brown´s intended study animals quickly learned to evade her nets. Then she discovered the hummingbird´s Achilles heel, so to speak: its stomach.
A biologist at the Northwest Biological Science Center in Seattle, Brown took to luring the animals with a feeder dispensing sugar water that resembles the flower nectar the birds crave. Day by day, she moved the feeder closer and closer to an open bird cage. Finally, she placed the feeder inside the cage, which greatly simplified banding. By the end of summer, Brown sat in her tent and let the birds come to her. "Hummingbirds are so anxious for food," she says, "they don´t seem to be afraid of things that ought to make them afraid."
Food anxiety is an apt, if anthropomorphic, way to describe the overarching feature of hummingbird life. For the world´s smallest birds, life is spent balanced precariously on a metabolic knife edge between gorging and starvation. Though by day a hummer takes in half its body weight in sugar, by night it approaches the point of starvation while it sleeps. "A forgetful lab assistant can easily kill 30 hummingbirds in a day, just by not putting sugar in their water," says Brown´s one-time academic mentor, zoologist Lee Gass of the University of British Columbia. "They are obviously burning energy very fast just to stay alive."
Hummingbirds do indeed live life in the fast lane. And directly or indirectly, almost everything about hummingbirds--not just their fast-forward metabolism but their body structures, their mode of flying, their diet, even their feistiness--can be tied to their improbable size. Moreover, by living a life of extremes, hummingbirds provide biologists with one of nature´s most stripped-down "model systems" for revealing how birds in general manage to do what they do.
Found only in the Western Hemisphere, hummingbirds survive through the extremes of climate as well. Their range extends from Alaska to Tierra del Fuego, from sea-level rain forests to the edges of Andean snow and ice fields at an altitude of 15,000 feet. Most of the 320 or so species of the birds, however, are found year-round in the tropical forests of Ecuador and Colombia.
Of the species that migrate, barely a dozen fly as far north as the United States and Canada to breed in spring and summer (compared to the 54 species that breed in tiny Costa Rica alone). Some of these, like the Anna´s hummingbird, are year-round residents of this country. Others, including the broad-tailed and calliope hummingbirds, venture north to breed in the Rocky Mountains. The smallest bird found in the United States is the calliope hummingbird. With feathers, it´s the size of your thumb; without, it´s about the size of a bumblebee. Either way, it weighs little more than a penny.
East of the Mississippi River, the ruby-throated hummingbird is the most commonly seen species. Weighing just one-seventh of an ounce, it flies 500 miles nonstop each year across the Gulf of Mexico.
The most "adventurous" species is the rufous, which travels more than 2,000 miles from Central America to western Canada and Alaska--and then back every year. How do such tiny birds accomplish such aerial feats? The answer is found in the creature´s anatomy.
Nearly one-third of a hummingbird´s weight is muscle, giving it the most powerful build of any bird; its heart is proportionally the largest of any animal´s. As for its wings, the wrist and elbow joints are nearly rigid, while the shoulders are extremely flexible. In its hovering mode of flight, a hummingbird holds its body partially upright and pivots its wings at the shoulder. Its wings beat invisibly fast--as much as 80 times per second in some species--producing the characteristic buzz. The bird produces lift on the upstroke as well as the downstroke. Without this economy of power, hovering would be too taxing to maintain for long.
Even so, hovering is a gas-guzzling way to fly, metabolically speaking. Ounce for ounce, hummingbirds burn energy faster than any other creatures except flying insects. A 170-pound human burns between 2,500 and 3,000 calories a day. If there were 170-pound hummingbirds, they would burn more than 150,000 calories a day. Hummingbird authority Crawford Greenewalt once calculated that if human bodies used energy at that rate, our skin, if dry, would heat up to 750 degrees F. Hummingbirds don´t overheat largely because, in relative terms, they have so much exterior and so little interior; no part of them is far enough from the surface to trap much heat.
Male hummingbirds use their flying prowess, along with their iridescent plumage, to impress the opposite sex. As a female watches, a male Allen´s hummingbird will dive and pull up repeatedly, pausing at the top of his arc to quiver alluringly. After such a big buildup, the act of mating itself lasts only a few seconds. This brief tryst is the only time that males and females of most species spend together.
Using spider cobwebs as a glue, females build nests that could be mistaken for a slight thickening of a tree´s branch. Hummingbirds typically lay two small eggs, which in fact are larger than most other bird eggs in proportion to the mother´s size.
Because hummingbirds fly solo rather than in flocks, their movements are hard to track. Tracing the precise route of a migrating hummingbird is especially hard--to date, in fact, it´s been impossible. "The birds are too small to carry a radio collar and too small to follow," says Gayle Brown. As a result, researchers catch and band hummingbirds, and then hope for the best.
"One bird we banded during the breeding season on Vancouver Island was recaptured a few weeks later in southern New Mexico," she says. A year later almost to the day, the same bird reappeared on Vancouver Island. "I knew its leg-band number by heart," she says. "When I saw the number on that bird and realized it had been to New Mexico and then Mexico and back since the last time I´d held it in my hand--I just couldn´t believe it."
Hummingbirds live largely on sugary nectar that their grooved tongues extract from the corollas of flowers. Nectar is an almost explosively digestible fuel. We humans equate sugar with junk food because it gives us a quick jolt of energy and then is gone. But that´s exactly what hummingbirds need: a quick hit of energy, then another a few minutes later, then another and another.
To add some protein and other nutrients to their diet, hummingbirds also feed occasionally on insects and spiders. And sometimes, they even pick up salt from the faces of startled birdwatchers. "They do have a salt problem," says long-time hummingbird researcher Bill Calder, professor of ecology at the University of Arizona. "I´ve had one fly up to me and French-kiss me."
Recently, Calder has been studying how hummingbirds maintain their weight so precisely. By using an electronic balance as a perch, he has measured how much nectar captive birds sip each time they feed. "Breeding males seem to keep their weight just shy of starvation," he says. "They have energy reserves for only about an hour of running time." The advantage, he surmises, is that this leaves them light enough to accelerate quickly on the wing.
But if the birds maintain only about an hour´s worth of energy reserves, how do they rest? Hummingbirds do not starve overnight, when they are inactive, mainly because sleep requires much less energy than do flying and foraging. Thanks to a lower pulse and body temperature, an hour of the birds´ "running time" can stretch to a night´s worth of sleeping time. As a sort of insurance, the creatures tend to "top off" their energy reserves by indulging in a burst of late-afternoon feeding.
Hummingbirds preparing to migrate handle food and rest in a different way, however. They have to, says Raul K. Suarez, a zoologist at the University of California, Santa Barbara: "The Gulf of Mexico is a habitat that doesn´t have many flowering plants." Through evolution, the birds´ metabolic machinery has become fine-tuned to cope with such temporary food shortages.
To prepare for what can be a nonstop, nearly 24-hour flight, a hummingbird begins rapidly converting the carbohydrates it takes in--the sugars in nectar--to fat, which is an animal´s most efficient form of energy storage. Hummingbirds have evolved the ability to "regulate the rate at which they gain fat and the rate at which they burn it, which is something we humans have a hard time doing," says Suarez. In order to accomplish this, the birds may rely on an inherent ability: They may automatically slow their metabolism during sleep beyond their normal slowdown into a condition called torpor--a nocturnal version of hibernation.
While in such a state of torpor, a blue-throated hummingbird´s heart rate, for example, may drop from a fluttery resting pulse of 21 beats per second to little more than one lethargic beat every two seconds. Its body temperature plummets, and its breathing almost stops. The payoff for the hummingbird is that it may use just one-fiftieth as much energy as it would otherwise. "Torpor is a way for small animals with a high body temperature and metabolic rate to save energy," says Sara Hiebert, a biologist at Swarthmore College.
The conventional wisdom among experts is that hummingbirds rely on torpor mainly during emergencies--when food has been scarce and they´re running low on body fat. But while studying migratory rufous hummingbirds recently, Hiebert found that fat birds can use torpor as a quick way to get fatter. By slowing down their nightly metabolism, they gain body mass more quickly--sometimes doubling their weight--in preparation for their long flights south.
In another study, Hiebert cut back the rations she gave to captive hummingbirds for a week or two. "All of a sudden, I noticed, they started using torpor, and they´d be putting on body mass," she says. Somehow, the researcher suspects, a physiological switch is thrown that directs the animal´s body to bulk up in anticipation of continued hard times, such as during spring after the hummingbird molts. "It was Jenny Craig´s nightmare," adds Hiebert. "You´re putting them on a diet, and they´re gaining weight!"
Writer Doug Stewart shares his Massachusetts habitat with migrating ruby-throated hummingbirds.