Hummingbirds: Living Jewels

By Jeremy Geist

Hummingbirds, the flying jewels of the Americas, have long captivated hearts and minds with their shimmering plumage and acrobatic maneuvers. Hovering flight, specialized feeding habits, and iridescent feathers make the members of the family Trochilidae some of the most unique and fascinating types of birds.

Flight

Rufous Hummingbird in flight. Public domain photo taken at Seedskadee National Wildlife Refuge.

If you’ve walked outside wearing bright clothing, you may have seen a confused hummingbird approach you closely. It moves up and down, left and right while hovering in the air to examine this strange flower that just moved into its territory. Hummingbirds’ incredible flying precision and ability to hover for an extended period of time without the aid of wind sets them apart from other birds. Robert Burton’s 2001 book The World of the Hummingbird describes hummingbirds as being able to fly in any direction, including while upside-down, and reorient themselves as they require.

Two centuries of scientific research have given us insights into the hows and whys of hummingbird anatomy and behavior. Hummingbirds’ wing movement, for example, is significantly different from most other birds. In her 2006 book Frequently Asked Questions About Hummingbirds, Rose Houk writes that their wings don’t flap, but rather move rapidly while fully extended, similar to a helicopter. Different types of flight require different wing movements; to hover for long periods in still air, hummingbirds move their wings in a figure-eight motion.

Dashing from flower to flower, sometimes feeding on insects in midair, hummingbirds spend much of their day airborne. Part of what keeps them aloft for so long is their low wing loading, described by Carrol L. Henderson in Birds in Flight (2008) as the ratio of a bird’s body weight to the surface area of its wings. Birds with low wing loading, such as eagles, owls, and pelicans, are generally able to spend more time in midair searching for food. In the case of hummingbirds, their low wing loading enables them to hover for longer periods of time.

In exchange for this remarkable flight ability, most hummingbirds are unable to move on the ground. Hummingbirds’ stubby feet are only used for perching, according to Henderson, as well as Esther Quesada Tyrrell in her 1985 book Hummingbirds: Their Life and Behavior. Even if they want to travel a short distance, hummingbirds generally must fly there. However, Burton points out that species that inhabit higher-elevation areas, such as helmetcrests, are able to walk on the ground to find insects.

This ability to hover gives hummingbirds an advantage in drinking the flower nectar that makes up a large part of their diet. Hummingbirds can precisely orient themselves in midair to feed at flowers, writes Burton, even when there aren’t any nearby perches to alight on or when the flowers are swaying in the wind. Tyrrell compares this versatility to other nectar-eating birds like Hawaiian Honeycreepers, which have to feed while perched.

Nectar and More

An Anna’s Hummingbird feeding at a flower. Photo licensed under Creative Commons 2.0 license from Matthew Field.

High-calorie flower nectar is important for all hummingbirds because they possess, according to both Burton and Tyrrell, the highest metabolic rate of any non-insect animal. The need for accelerated metabolism, writes Burton, is due both to hummingbirds’ small size, which gives them more surface area relative to their weight, and their hovering flight, which quickly uses up energy. This high metabolic rate leads to massive food requirements: A hummingbird has to drink half of its body weight in nectar per day.

Since plants don’t move around, hummingbirds generally don’t spend excess time searching for food, as long as they have access to enough nectar-bearing flowers.Burton and Tyrrell broadly divide hummingbirds’ feeding strategies into “residents,” which steadfastly defend an area possessing a variety of different flowers, and “trapliners,” hummingbirds that prefer a specific type of flower and visit their favorites on a wide, set route.

Many trapliners specialize in just one particular type of flower. Hermit hummingbirds, for example, have curved bills that allow them to sip nectar from equally curved flowers. Peter Feinsinger and Susan Budd Chaplin, in a 1975 paper in The American Naturalist, reported that trapliners have longer wings relative to their body size to support their increased flight requirements.

Nectar strategies will sometimes differ within a species, as well. Male Anna’s Hummingbirds, notes Burton, are territorial, while the females will often trapline.

Hummingbirds can’t subsist entirely on nectar; they also need to eat protein, usually provided by insects. This protein is needed especially for their feathers to grow, and Burton writes that hummingbirds will eat more insects during molting. Of course, eating solid food with a long bill is tricky, and different species of hummingbirds have worked around this in different ways. Burton provides a few examples: Some hummingbirds will fly through clouds of gnats with their mouths open, some will toss insects into the air and then fly forward to eat them, and others will use wind currents from their wings to direct insects into their mouths. Tyrrell writes that some hummingbirds will rob prey from spiders’ webs, in addition to eating the spiders themselves.

Slowing Down

Hummingbirds have another method to stay warm besides eating a lot: not staying warm at all. Most hummingbirds can go into a torpor state at night, which, according to Scott Weidensaul’s 1989 book Hummingbirds, is used to lower their metabolism and body temperature. Torpor is similar to hibernation in animals like bears and squirrels, writes Burton, but it only lasts for a night.

Torpor can massively lower the pace of a hummingbird’s biorhythms: Wiedensaul writes that in blue-throated hummingbirds, their heartbeat will drop from between 480 and 1200 beats per minute to a mere 36 while in torpor. Tyrrell notes that hummingbirds don’t always go into torpor at night – it’s highly dependent on energy stores, external temperature, and the like, and torpor is only used when it’s necessary for survival. A hummingbird entering torpor will often fluff out its feathers, which, Tyrrell writes, vents trapped body heat and lowers its temperature.

The Splendor of Iridescence

Costa’s Hummingbird. Public domain photo taken at San Diego Zoo.

The shimmering, jewel-like plumage of hummingbirds is perhaps their most eye-catching feature. Many hummingbirds possess iridescent feathers; the male Anna’s Hummingbird, common to the Western United States, has an iridescent green back and a red throat “gorget,” as well as shimmering red patches on its crown.  Depending on the light and the hummingbirds’ positioning, these feathers can shine a bright, iridescent red or appear dull.

Iridescence in hummingbirds is created through their unique feather structure. The surface of an iridescent feather has tiny air bubbles on it, which creates an irregular surface mentions bird guide writer David Sibley on his blog Sibley Guides. Some light wavelengths are absorbed, depending on the thickness of the bubbles. Of the remaining light, some reflects off the outer surface, while some passes through and reflects off the inner surface. The result is the bright, metallic colors that can be seen with the naked eye.

Mating And Raising Young

Each hummingbird species, Tyrrell writes, has its own mating display, which frequently involves a spectacular display of aerobatics. The male Anna’s Hummingbird will climb to massive heights (75 to 150 feet) above the female, then dive at top speed. It ends its descent with a loud squeak noise that, Burton explains, is produced by air rushing rapidly through its tail feathers. On the other end of the spectrum, traplining hummingbirds, which have no home territory, will often display “lekking” behavior, where a large number of males gather in one place to display and call, writes Burton. Male hummingbirds only stay with the female to mate and do not help her raise the young; Burton notes that this behavior is unusually consistent across every hummingbird species.

When preparing to lay eggs, female hummingbirds create nests out of various substances. Spider webs, according to Houk, are one of the most common, since their stretchiness means the nest can expand as the chicks grow. The interior of the nest may be lined with down, feathers, moss, leaves, and various other plant and animal materials, writes Tyrrell. Nests aren’t always built from scratch; Burton writes that hummingbirds will sometimes reuse their old nests or refurbish those of other hummingbirds. Regardless of how they’re made, nests are almost always constructed near plentiful sources of nectar.

A mother hummingbird will incubate her eggs for two to three weeks after laying, which Tyrrell notes is a surprisingly long amount of time for a bird so tiny. Like other birds, hummingbirds use a bare patch of skin called a “brood patch” to better transmit body heat to the eggs; however, while other birds shed feathers to create the patch, hummingbirds will simply move their feathers out of the way of their skin to preserve their plumage. Nestlings generally stay in the nest for about three weeks.

Hummingbirds’ shimmering plumage and audacious behavior makes them one of the most beautiful and intriguing birds on Earth. A variety of garden flowers, along with the tiny insects they attract, provides excellent nutrition for hummingbirds. Look for tube-shaped flowers like fuschia, penstemon, and columbine, as well as others recommended by Sunset. In this setting, you can watch hummingbirds’ intricate feeding behavior, males dogfighting for territory, and maybe even a mating display.

References

Burton, Robert. 2001. The World of the Hummingbird. Firefly Books, Buffalo.

Feinsinger, Peter, and Chaplin, Susan B. 1975. “On the Relationship between Wing Disc Loading and Foraging Strategy in Hummingbirds.” The American Naturalist.

Henderson, Carrol L. 2008. Birds in Flight. Voyageur Press, Minneapolis.

Houk, Rose. 2006. Frequently Asked Questions About Hummingbirds. Western National Parks Association.

Sibley, David. 20 September 2011. “The Basics of Iridescence.” Sibley Guides. Web.

Tyrrell, Ester Q., and Tyrrell, Robert A. 1985.  Hummingbirds: Their Life and Behavior. Crown Publishers, New York.

Weidensaul, Scott. 1989. Hummingbirds. Portland House, New York.

Text copyright Anne M Rosenthal.