Hummingbirds: Frantic and Fascinating
by Terry Dunn
“A glittering fragment of the rainbow...a lovely little creature moving on humming winglets through the air, suspended as if by magic in it, flitting from one flower to another....” Thus did John James Audubon describe hummingbirds in the early 19th century. Other scientists fascinated by these mesmerizing birds have bestowed upon them names such as purple-crowned fairy, green-breasted mango, glittering-throated emerald, sapphire-vented puffleg, fiery topaz, peacock coquette, and shining sunbeam.
No doubt our admiration began the first time a hummingbird bolted out of the blue and hovered in front of a person’s startled eyes. Hummingbirds certainly didn't escape the notice of early human residents of the Americas. Some of the most intriguing myths and legends come from the Maya, who believed that hummingbirds were the sun in disguise. Huitzilopochtli, the most powerful Aztec god, was thought to come from a ball of hummingbird feathers that fell from the sky. Today, in some parts of Central and South America, dead hummingbirds are dried and ground into magical powders used to attract money, power, or romance.
Wild hummingbirds grace the New World solely. Seeing them for the first time, Spanish explorer Gonzalo Fernandez de Oviedo y Valdes speculated that they were large insects or a cross between insects and birds. Christopher Columbus may have been referring to them when he described, “Little birds...so different from our own it is a marvel” in his journal entry for October 21, 1492. It didn't take long for news of these flashy birds to reach the Old World. Only a few years after the arrival of Columbus, a hummingbird skin was sent to Pope Leo X in Rome.
By the late 19th century, hummingbirds were well-known in Europe, but with fame came exploitation. A growing market in London and other European cities for their skins, bodies, and feathers fueled the killing of hundreds of thousands of hummingbirds. Feathers were used for hat decorations. The skins were used in collections and to make artificial flowers and dust catchers. Preserved hummingbird bodies were paired with flowers and arranged like museum dioramas on top of women's hats. The trade in hummingbirds escalated to a point where, in a single year, one London dealer imported 400,000 hummingbird skins from the West Indies.
Not until the passage of the Migratory Bird Treaty Act of 1918, which prohibited commercial trade in any migratory bird species or its feathers, were conservationists in the United States able to change the tide of fashion and slow the decline in hummingbird populations. The number of species already driven extinct by that time remains a mystery.
The Bold and The Beautiful
While the majority of the 328 species of hummingbird inhabit
the tropics, hummingbirds can be found from Argentina to Alaska,
from sea level to 15,000 feet, and from humid jungles to deserts,
temperate forests, grasslands, coastlines, and urban areas.
In some species, individuals cover very little territory in
their daily activities, often staying close to a single flowering
plant all day. In others, the birds range farther, yet sometimes
travel a similar route each day.
Hummingbirds from across the spectrum of species are known for their aggressive personalities. They will defend their breeding and feeding turf by dive-bombing competitors and occasionally stabbing them with their needle-like bills. Then, when chicks are fledged or flowering is over, they may abandon their fiercely protected territory and move on.
Scientists and bird-watchers have spent lifetimes trying to unlock the mysteries of the hummingbird family, called the Trochilidae. Early observers were convinced that no bird could fly backward. George Campbell, Duke of Argylle, declared that hummingbirds just “fell backward” out of a flower when they were finished feeding. Charles Darwin was among the first credible scientists who tried to figure out how hummingbirds fly. He concluded that hummingbirds expand and contract their tail feathers to stay aloft in a vertical position. However, the real answer lies in their wings rather than their tails. Unlike those of other birds, hummingbirds' elbows and wrist bones are fused and virtually immobile. Yet the range of motion at the shoulder is a full 180 degrees. Most birds are capable of creating lift only on the wing's downstroke. For a hummingbird, every wing motion is a power stroke, as lift is created on both the downstroke and the upstroke.
Hummingbird flight is often compared to that of a helicopter. According to H. Ross Hawkins, founder of the Hummingbird Society—and a man who admits to bird-watching from his window before getting out of bed—there are some important distinctions. “The difference is that the hummer's wings go back and forth, switching the angle with each stroke, while the helicopter [propeller] moves around continuously,” says Hawkins. “Both oscillatory and rotating wing motion create lift [in a helicopter], but the hummer's lift is balanced and has no tendency to make the bird rotate,” he explains. The pattern of the wing-beat is more of a figure-eight than a circle like a helicopter or an up and down motion like other birds. With this motion, hummingbirds can use their unusual wings to hover, fly forward, fly backwards, and even fly upside down.
But wings alone don't tell the whole story. A hummingbird's powerful chest muscles account for a third of its overall body mass. Those muscles help the bird achieve wing-beats up to 90 beats per second when hovering, and double that during a power dive. Perhaps to save weight, hummingbirds lack down feathers—the fluffy feathers closest to a bird's skin that help it stay warm. This and other weight-saving tactics enable the birds to reach speeds of up to 30 miles per hour in normal flight. Males can dive at twice that speed when trying to impress a female.
In the late 1950s, Crawford H. Greenewalt, then president of E.I. du Pont de Nemours & Company, blended his twin hobbies of bird watching and photography to design a system for capturing hummingbirds' wing-beats on film. At the time, the typical motion picture camera took about half a second to go from zero frames per second to full speed—too long for a human finger to trip the switch and catch a hummingbird image, much less a full wing-beat. The solution was to develop a camera that started instantly when the subject flew into a beam of light shining on a photocell, in a sense allowing the bird to take its own picture. Using this setup, Greenewalt was able to get rolls of images that not only captured the motion of hummingbird wings but could aid in the calculation of wing-beat speed.
Calculating
flight speed was another matter. To tackle this challenge,
Greenewalt developed a homemade wind tunnel with a fan at
one end and feeder at the other. He placed hummingbirds in
the wind tunnel and revved the fan at various speeds. Hungry
hummers could reach the feeder in the face of winds just under
30 m.p.h.—the apparent upper limit of their speed in
the wind tunnel. Retreating from the feeder in a strong wind,
the birds wound up flying on their backs with their tails
toward the feeder, ending their getaways with a semi-roll.
Hummingbird
feathers have offered another fertile source for scientific
investigation. Surprisingly, these feathers come in only two
pigment colors: a reddish brown and black. However, some hummingbird
feathers contain granules of melanin and microscopic air bubbles
that refract light and help create a metallic sheen. Depending
on the arrangement of the melanin granules and the air bubbles,
different colors of the spectrum appear. When light is not
shining on an iridescent feather at the correct angle, only
the pigment colors can be seen, and the bird appears dull.
Gaze at the feathers at just the right angle though, and you
will be treated to a show of spectacular, but fleeting, color.
The Need to Feed
Perhaps nowhere else is the interdependence of fauna and flora
so obvious as in hummingbirds and flowers. A hummingbird's
long bill is the perfect probe for reaching nectar at the
base of tubular-shaped flowers. Differences in bill length
and shape between species often appear to correlate with variations
in the types of flower they visit, suggesting that hummingbirds
and certain flower species co-evolved. For example, the Andean
sword-billed hummingbird (Ensifera ensifera), with
a bill length up to 4.1 inches, prefers to feed from a passion
flower species with a corolla length of about 4.5 inches,
making the hummer's bill and extended tongue just long enough
to sip the nectar from this large flower. Meanwhile, the white-tipped
sicklebill (Eutoxeres aquila) has a curved bill that
enables it to take advantage of the curved corollas of wild
plantains and heliconia flowers.
A hummer that feeds mainly on one particular species is more likely to transport pollen among flowers of the same species rather than transporting it to another species—for example, fuchsia pollen to a heliconia. Flowers have other strategies for making sure their pollen is going to the right place. For instance, in one Arizona location where Indian paintbrush (Castilleja integra), beardtongue penstemon (Penstemon barbatus), and other flower species are pollinated by rufous (Selasphorus rufus), broad-tailed (Selasphorus platycercus), and black-chinned (Archilochus alexandri) hummingbirds, the placement of the anther and stigma are oriented differently in each flower species, ensuring that different parts of the hummingbird carry the pollen to the right target on the next flower.
Inside a hummingbird's lengthy bill, its tongue is divided into two fringe-edged tubes. The tongue extends into a flower, picks up some nectar with the fringy tips, and draws the nectar in. The nectar is scraped off when the tongue is extended out of the bill as the bird reaches for more nectar. Like so much in hummingbird life, the whole process happens fast: The birds can take as many as 12 slurps of nectar per second.
For hummingbirds, frenetic foraging rules their daylight hours. It's no wonder. Hummingbirds have the greatest relative energy output of any warm-blooded animal. Their high metabolic rate fuels an internal furnace that keeps their tiny bodies warm and creates a resting heart rate of around 500 beats per minute and a wing-beat that is blindingly fast. It's a vicious cycle: This supercharged strategy, which requires them to consume more than their body weight in food each day, is necessary in part because they hover while they feed. As calories are quickly burned, the search for food is a near-constant activity. Calories most often come in the form of high-energy nectar—although, in some tropical species, insects may account for nearly 100 percent of a hummer's diet.
At night, many hummingbird species lower their metabolism, going into torpor. Whether or not an individual hummer enters torpor depends on the temperature and how well nourished the bird is as night falls. If a bird’s body temperature drops 30 to 50 degrees below daytime levels, its heartbeat will slow to as low as 160 beats a minute, and its breathing will nearly stop. Like a person who can't function before their morning cup of coffee, a hummingbird coming out of torpor may need 10 to 15 minutes to recover before it can fly again. Dramatic shivering increases the bird's internal temperature, raises its heartbeat, and stimulates breathing until it is ready for another frantic day in the air.
Migrating Over the Miles
Despite their high caloric needs, some species manage to migrate
thousands of miles each year. Fifteen hummingbird species
breed in the United States and Canada, almost all of which
migrate from winter range in Mexico and Central America. The
ruby-throated hummingbird (Archilochus colubris), however,
stands out as the overachiever of the group. Ruby-throats
winter between southern Mexico and Panama. By February, they
are spending their days fattening up on the Yucatn coast,
preparing for their flight across the Gulf of Mexico. They
double their weight from just over three grams (or one-tenth
of an ounce) to more than six grams. After leaving at dusk
for a nonstop, 18-to-22-hour flight across 500 miles of water,
the birds arrive on the U.S. Gulf Coast weighing only 2.5
grams. This journey seems so improbable that a persistent
rumor circulates that the ruby-throats piggyback on bigger
birds.
Once the ruby-throats have had anywhere from a day to a week's rest in Texas and Florida, they move northward at a rate of about 20 miles a day. According to Bob Sargent, a long-time bird bander and hummingbird enthusiast, "The birds come back to the same sites year after year. They are consistently caught at the same feeders." However, the birds stay at a feeder only for a day or two before continuing their whirlwind tour up through the eastern U.S.
The banding work of Sargent and others is slowly contributing to our knowledge of hummingbird movements. Until recently it was believed that ruby-throated hummingbirds were the sole species east of the Mississippi. Sargent knows otherwise. “There are a lot more hummingbirds in the eastern U.S. than anyone dreamed of,” says Sargent. There are now records of 14 different species visiting the eastern U.S., although many of these are rarely seen.
If tracking hummingbird migration to and from the U.S. is difficult, it's even harder to keep tabs on the movements of hummingbirds within the tropics. John Rappole, research biologist at the Smithsonian National Zoo's Conservation and Research Center, and Karl Schuchmann, curator of birds at the Alexander Koenig Museum in Bonn, Germany, have recently combed through the available data on hummingbird movement in the tropics.
They found that 87 species were altitudinal migrants—moving between lower and higher elevations with the changing seasons—while 42 species were latitudinal migrants that traveled between 10 and 1,000 kilometers (6.2 and 621 miles), and 29 species were long-distance migrants that moved greater than 1,000 kilometers. That's not to say that other hummingbird species aren’t migrating, but the information on their movements remains sketchy. Still, it's surprising how much tropical hummingbirds are getting around. “The conclusion,” says Rappole, “is that migration is a very widespread phenomenon in the tropics.”
For hummingbird species that migrate, healthy patches of several habitat types are needed for survival. Birdlife International lists 19 hummingbird species that are endangered, nine of which have a fifty-fifty chance of going extinct in the next five years. The World Conservation Union "Red List" contains dozens of endangered and threatened hummingbird species throughout the Americas, including the Honduran emerald (Amazilia luciae), the mangrove hummingbird (Amazilia boucardi) of Costa Rica, and the short-crested coquette (Lophornis brachylopha) of Mexico. They are threatened primarily by habitat alteration in all its incarnations: Urban development, expansion of agriculture, logging, road building, and even pipelines through tropical forests nibble away at the habitats hummingbirds live in and fly through. The Honduran emerald, for instance, has already lost much of its habitat, and more road building threatens what's left.
Too small to track with radio transmitters and elusive in their habits, hummingbirds provide many further challenges for scientists and concerned bird-watchers. As Bob Sargent confesses, "The face of ignorance is the one I'm shaving. But ignorance is the great motivator."
MORE! Hummers At the Zoo!
—Terry Dunn is a freelance writer, environmental educator,
and artist living in Albuquerque, New Mexico.
ZooGoer 31(2) 2002. Copyright 2002 Friends of the National Zoo. All rights reserved.
