The Thing with Feathers (12 page)

Being small carries great advantages. Hummingbirds are nearly immune to predators. Because bite-sized hummers are so quick and light, they don’t have to worry much about hawks and other attackers. One study in 1985 could find only thirteen confirmed instances of predation on adult hummingbirds in North America ever, of which several events were classified as “bizarre”—praying mantises, spiders, fish, frogs—and
concluded that North American hummingbirds “do not have natural predators in the usual sense.”

With this in mind, the authors analyzed hummingbird life spans. Using an equation of body mass and scaling factors to predict life expectancy, they suggested that a hypothetical hummingbird weighing three or four grams should live between 5.5 and 6.1 years, barring disease, predation, or accident. Though nobody really knows how long hummingbirds live, one study in Colorado documented several wild broad-tailed hummingbirds surviving at least eight years, past their theoretical physiological limits, and a broad-tailed currently holds the hummingbird longevity record at about twelve years. Without significant predators, hummingbirds may expect to live into old age as long as they stay healthy. They’re just too small and fast for predators to bother with.

The only published account of any animal consistently attacking hummingbirds involves a pair of bat falcons in Venezuela. While watching the small, nimble falcons for 164 days, one researcher observed them catch all kinds of fast-flying prey, including ten species of hummingbirds, two species of swallows, eight species of swifts, and four species of bats. He estimated that the pair of falcons had consumed roughly 600 birds and bats during his study, of which 100 were hummingbirds. Because bat falcons are relatively common and widespread in the tropics of Central and South America, they may have a real effect on hummingbird populations in those areas, but this appears to be the only exception to the no-predators rule.

Another benefit of small size is increased agility. Hummingbirds are the only birds that fly backward, and they have no trouble tracing lines better suited for helicopters than airplanes. Their dexterity means that they can access food sources usually unavailable to other birds, such as hanging flowers, and
zip around at 30 miles per hour without poking one another’s eyes out.

This agility has lately inspired researchers at the Pentagon, who recently announced the hummingbird-like Nano Drone, a miniature flying robot with two wings that looks impressively like the real thing. The tiny drone, operated by remote control, can hover, maneuver backward and sideways, and dart in and out of buildings while transmitting a live video feed. The military envisions it as a spy device capable of reconnaissance missions and perching near targets without arousing suspicion, though one hopes that birders would be able to spot the difference. Prototypes have even been modeled and painted to closely resemble hummingbirds.

At the moment, the main issue with the drone is its battery life. Early versions could fly for twenty seconds, which has since increased to eight minutes—still not quite long enough to be useful for real-world applications. One of the project’s managers has enthusiastically explained that the Nano uses biomimicry to copy natural flight, but he didn’t mention the basic problem with living, breathing hummingbirds: They burn energy like fighter jets. To accurately mimic nature, the high-performance drone would have to spend most of its day refueling instead of spying.

In terms of energy, hummingbirds live at the edge of physical possibility. Birds, and other warm-blooded animals, constantly lose energy as heat is transferred through the skin (touch your face and you can feel the warmth leaking out of you). This energy, of course, is supplied by consumed calories. Small animals lose heat much more quickly because their surface area has a greater ratio to their volume—the same reason that small ice cubes melt faster than big ones—so hummers have to eat more relative to their body weight than other birds just to stay
warm. If they were any smaller, it would not be possible to eat enough each day to make up for the lost energy.

Large animals have the opposite problem: They can’t lose heat fast enough. That’s why desert rabbits have big, floppy ears and why camels have long, spindly legs—to increase their surface area, and dump extra heat. Cold-blooded creatures aren’t as constrained by heat loss, which is why many of the world’s animals smaller than a hummingbird, from butterflies to frogs, do not maintain a constant body temperature.

The extreme lifestyle of hummingbirds is dictated by energy. Hummingbirds have made every possible physical concession to maintain their supercharged existence. To drive such a high metabolism, they have the largest hearts, relative to body mass, of any bird, and the fastest heartbeat of any animal—a hummingbird heart has been measured at more than 1,200 beats per minute in flight, about six times faster than a human’s maximum. Their circulatory and respiratory systems are incredibly efficient, from lungs that can process more than 250 breaths per minute to a remarkably high concentration of red blood cells to carry oxygen to their muscles. Because of the metabolic demands of hovering flight, hummingbirds use more oxygen than almost any other animal.

All of this requires a lot of fuel. Hummingbirds routinely ingest more than their own weight in nectar each day, equal to somewhere between three and seven calories—the equivalent of a human eating a couple hundred pounds of hamburger between breakfast and dinner—and pass about 80 percent of it through their kidneys, as if you or I urinated twenty gallons a day!

But that’s still not enough. To keep running at normal speed, a hummingbird would have to refuel so often that it would never be able to sleep. So hummers have adapted to shutting off
their engines at night, powering down to a state of torpor near death. A sleeping hummingbird lowers its internal body temperature so far that signs of life barely register; its heart slows to a lethargic pace, its breath is difficult to detect, and its metabolism is reduced by as much as 95 percent. While in this state of suspended animation, the hummingbird can’t be awakened. A couple of hours before dawn, something flickers in its brain, sending the signal to begin a bout of shivering. Within about twenty minutes, the hummingbird revs back to life, and after a quiet period near sunrise, it zooms off in search of breakfast.

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AS ANYONE WHO FEEDS THEM KNOWS,
hummingbirds are not the least bit cuddly. Backyard birders are sometimes horrified to watch their hummers body-slamming, clawing, and tearing at one another in knockdown fights around the yard.

“I thought they were just cute little interesting birds and had this sweet image of them in my mind,” a concerned blogger once wrote. “It was a shock for me to witness such unexpected violence.”

A birder in Alabama has complained, “Sitting on my back porch is like sitting in a Lilliputian battle field with miniature helicopters humming overhead,” while a gardener in Maine observed, “Everyone seems to love hummingbirds except other hummingbirds. One has to wonder why feeders have multiple perches. Their very nature keeps them at war with one another.”

The Aztecs, who knew a bit about violence, figured this out long ago and named a hummingbird as their god of war: Huitzilopochtli, roughly translated as the “hummingbird on the left,” who demanded occasional human sacrifices to stave
off the end of the world. He was usually depicted with a feathered head and was said to be so bright that soldiers could see him only by peering through the arrow slits in their shields. When Aztec warriors died in battle, they were believed to return to earth as hummingbirds.

I experienced the true ferocity of hummingbirds one afternoon in the highlands of Costa Rica. While walking along a remote gravel road above San José, I noticed something wriggling in the ditch next to my feet, bent down, and casually closed the fingers of my right hand over two fiery-throated hummingbirds locked together.

They didn’t even notice me until I had scooped them up. Apparently, the birds had been so preoccupied with fighting each other, struggling in a shimmery heap in the bottom of the ditch, that they were oblivious to the rest of the world. Now I regarded the heads of two tiny Aztec warriors scowling out from between the fingers of my fist, and wondered what to do with them.

Hummingbirds don’t have the weaponry to inflict serious damage on each other. Their beaks are softer and more sensitive than they appear, so stories of hummers spearing one another through the gut are probably exaggerated. Besides, long beaks are ineffective at close range—any wrestler could tell you not to bring a javelin to hand-to-hand combat.

Those beaks are surprisingly bendy, more like a springy diving board than a javelin. This flexibility wasn’t recognized until 2004, when researcher Margaret Rubega analyzed ultra-high-speed videos of hummingbirds catching fruit flies in midair, and demonstrated that their lower jaw can flex as much as 25 degrees even though it is made of solid bone and lacks a hinging joint present in other bird beaks. This setup allows hummers to open their beaks wider while they pursue tiny
flying insects, and, when the bill re-straightens, to snap it shut in less than a hundredth of a second—the same physical mechanism used by Venus flytraps.

Hummer feet are likewise too weak to cause injury. They can barely grasp the branch they sit on, and a perched hummer can’t even turn 180 degrees without lifting off to change position, much less walk across flat ground. But that doesn’t stop the birds from getting into constant spats.

The only way to defuse a fight is to separate the fighters, so I carefully untangled my two fiery-throated hummingbirds, held one in each hand as they glared at each other from behind my respective thumbnails, and then released them in opposite directions, hoping they’d settled their differences. At least my sudden appearance had interrupted their scuffle.

Hummingbirds generally lead solitary lives, coming into contact with one another only near food sources. Mating is dispensed with quickly; females aren’t allowed in male territories except during a brief period in spring, and males of most species don’t help build nests, incubate eggs, or care for the young. That’s why you’ll never see a flock of hummingbirds. Males of a few tropical hummers congregate during the mating season, perching near one another in the dense forest understory to sing for the attention of females, but, for the most part, the birds are stubbornly unsociable.

A study of migrant rufous hummingbirds in California’s Sierra Nevada once tried to determine just how selfish the birds were. Researchers wondered whether the hummers would stop defending flowers after eating their fill of nectar or would aggressively keep eating as much as they could cram in all day long. Were hummingbirds “time minimizers,” who would consume just as much nectar as they needed, or “energy maximizers,” who would eat as much as possible?

At first, the results seemed to show that migrant rufous hummingbirds were time minimizers, because the birds spent about 75 percent of the day perched. But after some analysis, the researchers concluded that the opposite was true: Hummingbirds could fit only so much nectar in their crops before they became too heavy to fly, so most of their sitting time was necessary for digestion. Meanwhile, the hummers continued to hotly defend their flower patches so that they could keep tanking up whenever their stomachs had enough space. They were energy maximizers.

The researchers decided to experiment. They covered half of the flowers in each hummingbird’s territory with clear plastic bags, halving the available nectar. In response, the hummingbirds almost universally doubled the size of their territories to include neighboring patches of unclaimed flowers. The birds had to spend more time defending the bigger territories, and it took longer for them to commute from flower to flower while feeding. When the bags were removed, the territories shrank back to normal size.

This supports the idea that hummingbird territories are based on energy—like most things in a hummer’s life—rather than physical area, as is true of many animals. The birds know how many flowers they need to control to stay full all the time.

But evidently that logic goes only so far. Give a hummingbird a nice, compact feeder with a continuous supply of nectar, like Liz did at her lodge in Costa Rica, and he’ll maximize the opportunity by defending it at all costs. And who can blame him? A hummingbird who finds a feeder must feel like baby Zeus did after he accidentally broke off the horn of his goat nursemaid, unleashing the divine power of an unlimited cornucopia.

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TAKE ALMOST ANY ANIMAL
in the world, multiply its metabolic rate by its life expectancy, and you come up with an interesting result: Most creatures spend about 1 billion heartbeats on this earth, no matter what size they are or where they live. Humans, perhaps because of advances in medicine in the past couple hundred years, are a bit above average; we can expect 2 or 3 billion. Hummingbirds probably clock out between 1 and 2 billion.

If this seems too simple to be true, consider a quick calculation.

Assuming that a human’s heart rate is 70 beats per minute at rest and 60 beats per minute while asleep, that we sleep roughly a third of our lives, and that we live about seventy years, we can expect our hearts to beat 2.45 billion times.

Hummingbirds are trickier, because their heart rate is so variable. Let’s assume that a typical hummer’s heart rate is 1,200 beats per minute in flight, 250 beats per minute at rest, and 50 beats per minute while asleep. It might be flying a quarter of the time it is awake, and might sleep through a third of its seven-year life. In that case, a hummingbird’s heart would beat 1.26 billion times.

The rule applies to mice, elephants, and most animals in between, and works because larger, longer-lived creatures tend to have slower metabolisms. It doesn’t account for disease or sudden death before old age, and it’s obviously a generalization. But it does indicate that the hearts of most animals are wired to put in about the same effort, no matter how fast they run.