**Hummingbirds: Experts in Extreme Adaptation**
People are captivated by hummingbirds—small, vividly colored flashes that zip around, hovering at blossoms and vigorously protecting their claim to a feeder. Yet, for the scientists who investigate them, hummingbirds represent far more than just a delightful display. Their diminutive size and rapid metabolism mean they exist on a precarious edge, sometimes needing to almost entirely suspend their bodily functions to conserve sufficient energy for overnight survival—or to migrate thousands of miles, occasionally across vast expanses of ocean.
Their nectar-heavy diet results in blood-sugar levels that could put a human in a coma. Furthermore, their rapid, darting flight can generate g-forces potent enough to cause a fighter pilot to black out. The deeper researchers delve, the more unexpected revelations they discover within these petite beings, the smallest in the bird kingdom.
“They’re the only birds capable of flying upside down and backward,” states Holly Ernest, a conservation ecologist at the University of Wyoming. “They consume pure sugar without suffering from diabetes.”
Ernest is among a select group of scientists examining how hummingbirds manage the intense challenges posed by their lifestyles. Here’s some of what researchers have uncovered about the remarkable adaptations of hummingbirds.
### Invest the Effort
For quite some time, most scientists believed that hummingbirds only spent about 30 percent of their day engaged in the energy-demanding task of moving from flower to flower and consuming nectar, while resting for the remainder. However, upon closer examination by physiological ecologist Anusha Shankar, she discovered that they often exert themselves much more than initially thought.
Now based at the Tata Institute of Fundamental Research in Hyderabad, India, Shankar attempted to determine how broad-billed hummingbirds in Southern Arizona allocate their time throughout the day. By employing a range of experimental techniques, she assessed the birds’ metabolic rates during different activities and estimated their total daily energy consumption. Incorporating previously published data, Shankar calculated the energy costs per minute for resting, flying, and hovering—three primary ways a bird spends its time.
From this, she inferred how much time the birds dedicated to feeding compared to perching throughout the day.
“We discovered that it’s incredibly variable,” Shankar explains. During the early summer when flowers are plentiful, some birds could satisfy their daily energy requirements with only a few hours of feeding, spending up to 70 percent of the day resting. However, as flowers became less abundant with the onset of summer monsoon rains, birds at one location perched just 20 percent of the time, devoting the rest of the day to feeding.
“That amounts to 13 hours a day!” Shankar remarks. “I don’t know how they manage to run for 13 hours a day.”
### Effectively Cool Down
Hummingbirds possess a strategy to help them stretch their energy supply: When a bird risks depleting its energy reserves, it can enter a state of torpor at night, lowering its body temperature nearly to that of the surrounding air—sometimes just slightly above freezing. While torpid, the bird appears almost lifeless, slow to react to stimuli, and breathing only sporadically. This tactic can conserve up to 95 percent of metabolic costs during chilly nights, according to Shankar’s calculations. This is particularly crucial after periods when a bird has fed less than usual, such as after a thunderstorm. It also aids in energy conservation for fat storage prior to migration.
Shankar is currently investigating which aspects of their physiology hummingbirds prioritize during torpor, focusing on which genetic products are indispensable. “If you’re a hummingbird operating at 10 percent of your normal metabolism, what is that critical 10 percent keeping you alive?” she inquires.
One category of genes that the birds appear to leave intact are those governing their internal clock. “It’s essential for them to perform activities at the right time while in torpor,” Shankar explains. In preparation for the day, for instance, the birds begin to awaken from their torpor roughly one hour before sunrise, well ahead of any noticeable light signals.
### Manage the Sugar
To sustain their high metabolic rate, hummingbirds consume around 80 percent of their body weight in nectar daily. This equates to a 150-pound person drinking nearly a hundred 20-ounce sodas each day—and nectar is frequently sweeter than soda.
The human digestive system cannot absorb sugar at such a rapid pace, which explains why overeating candy or soda can upset the stomach, notes Ken Welch, a comparative physiologist at the University of Toronto at Scarborough. Hummingbirds tackle this influx by having permeable gastrointestinal tracts that allow sugars to enter the bloodstream between gut cells rather than solely through them. This mechanism ensures sugars are efficiently transported out of
