Hummingbird

• 1. Introduction

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  The hummingbird family, Trochilidae, includes the bee hummingbird, a bird that weighs less than 2 grams. In a blue-throated hummingbird, researchers measured a heart rate as high as 1,260 beats per minute. Even at rest, that bird took 250 breaths per minute. Hummingbirds live only in the Americas, from south central Alaska to Tierra del Fuego. Roughly 375 species in 113 genera are known today. How does a body this small keep itself running at such speed? What do its eyes, its voice and its tongue do that other birds cannot? How did a family now confined to the New World leave fossils in a German clay pit? And why are so many species losing ground?

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• 2. Wings That Hum

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  The highest wingbeat rate recorded for a hovering hummingbird is 99.1 per second, measured in male woodstars of the genus Chaetocercus. The giant hummingbird sits at the other extreme, beating its wings as few as 12 times per second. These beats make the hum that gave the family its name. The sound is unlike the whine of a mosquito, the buzz of a bee or the whoosh of a larger bird. It tells other birds that a fellow forager or a possible mate has arrived.

  Wind tunnels and a technique called particle image velocimetry showed how the wings hold the bird in the air. In studies of rufous and Anna's hummingbirds, the wings traced a figure 8. The downstroke supplied 75% of the weight support and the upstroke supplied 25%. Earlier work had assumed an even split, as in insects of similar size. Hummingbird hovering therefore resembles that of a hawk moth, but it is not the same.

  Side gusts in the tunnel could not shake a hummingbird's head from its steady position at a feeder. The birds answered by widening their wing strokes, changing the angle of the stroke plane and fanning out their tail feathers. Slow-motion video of flight in rain showed another trick. When raindrops together may weigh as much as 38% of the bird's body weight, hummingbirds shift body and tail horizontally, beat faster and shorten the arc of the wings. They also shake water from their heads and bodies, much as a dog does.

  Each hummingbird foot has three toes pointing forward and one pointing back, and the claws have ridged inner surfaces for gripping. The birds do not hop or walk. They shuffle sideways along a perch. At takeoff the legs work like pistons, although their shortness gives about 20% less thrust than in other birds. In the air the feet tuck under the body, and the top speeds of measured species exceed 15 m/s.

  About 35 m above a female, a male Anna's hummingbird turns and dives at 27 m/s, or 385 body lengths per second. Measured against body length, that speed is the highest known for any vertebrate, and about twice the diving speed of a peregrine falcon in pursuit of prey. The bird pulls out of the dive at roughly 10 g. At the bottom of the dive, near the female, a high-pitched chirp sounds. It comes from the outer tail feathers, which flutter as air rushes past them. A male missing those feathers cannot make the sound, and the same feathers chirped on their own in a wind tunnel. The bird's small syrinx can sing at that pitch, but never as loudly.

  Male rufous and broad-tailed hummingbirds, of the genus Selasphorus, carry a second instrument in the ninth and tenth primary feathers of each wing. Air rushing through slots formed by the tapered tips of those feathers makes a jingling trill during ordinary flight. The trill announces a male's presence, defends a feeding territory and helps draw a mate. Researchers can hear it from as far as 100 m away.

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• 3. Engine at Full Burn

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  Per gram of flight muscle, a hovering hummingbird uses about 10 times the oxygen measured in elite human athletes. The muscles achieve this with an unusual density of capillaries and mitochondria packed close together. To power its wings, the bird spends the human equivalent of 150,000 calories per day, an estimated 10 times the energy use of a marathon runner in competition.

  A 2017 review described how hummingbirds can power up to 100% of their metabolic needs with the sugars they drink. Newly swallowed sugar can fuel hovering within 30 to 45 minutes. The review pointed to the loss of a gene called fructose-bisphosphatase 2, or FBP2, which coincided with the start of hovering some 35 million years ago according to fossil evidence. Without that gene, the flight muscles break down sugar more efficiently. Because drinking covers the cost of flight, the bird keeps its small fat stores for overnight fasting and for migration.

  Heat is the other side of the bargain. While hovering, a hummingbird gets almost no cooling from forward airflow, except for the breeze of its own wings. It sheds heat through exhaled air and through thinly feathered areas around the eyes, shoulders, under the wings and on the feet. When the air climbs above 36 C, passive loss from those areas fades away, and evaporation does most of the work. In cold weather, the birds pull their feet into breast feathers to hide bare skin.

  Water poses its own problem. In a day of feeding on nectar, a hummingbird may take in as much as five times its body weight in water. Its kidneys filter in proportion to that intake, which prevents overhydration. Studies of Anna's hummingbird kidneys found a high density of capillaries close to the nephrons, allowing fine control of water and electrolytes.

  Dozens of species live year-round in the Andes between 1500 m and 5200 m, where the oxygen in the air is thin. In Andean hummingbirds, researchers found hemoglobin that binds oxygen more tightly, most likely through amino acid changes shaped by natural selection. High-altitude species also have larger wings, because lower air density raises the power needed to hover.

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• 4. Cold and Distance

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  At about 3800 m in the Andes, a Metallura phoebe hummingbird in nighttime torpor let its body temperature fall to 3.3 °C. That is the lowest known for a bird or a non-hibernating mammal. In a Caribbean hummingbird, torpor lowered body temperature from 40 to 18 °C, and the heart slowed to roughly 50 to 180 beats per minute. Among cold-night Andean species, torpor lasted 2 to 13 hours. The birds cooled by 0.6 °C per minute, rewarmed by 1 to 1.5 °C per minute and lost about 6% of their weight each night.

  The kidneys slow down during torpor to hold on to water, glucose and nutrients. A hormone called corticosterone is one signal that wakes the bird. A hummingbird with more body fat is less likely to enter torpor. In species where a dominant bird defends a territory, the nonterritorial subordinates spend longer in torpor.

  Anna's hummingbirds are the northernmost year-round residents of any hummingbird. They were recorded in Alaska as early as 1971 and have lived in the Pacific Northwest since the 1960s. Their original range was the chaparral of California and Baja California. Over the 1960s and 1970s it spread north to Oregon, Washington and British Columbia, and east to Arizona. The spread is linked to planted non-native species such as eucalyptus, to backyard feeders and to a natural habit of wandering widely after breeding. Anna's hummingbird is now the official city bird of Vancouver, and in Seattle it endures snow, high winds and long spells below freezing.

  Most hummingbirds never need to leave home. Of roughly 375 species, only 12 to 15 migrate each year, mostly in North America. The rest live in the tropical rainforest belt between Amazonia and Central America, where temperature and food barely change. A ruby-throated hummingbird, however, crosses 800 km of the Gulf of Mexico in a single nonstop flight. It first doubles its weight with fat, and it burns 1 to 2 g of that fat on the crossing, about what a human uses to climb 50 ft.

  The rufous hummingbird breeds farther north than any other species, reaching the Yukon and southern Alaska. At just over 3 inches long, it flies 3900 miles one way between Alaska and Mexico. That distance equals 78,470,000 body lengths. The 13-inch Arctic tern, flying about 18,000 km one way, covers 51,430,000 body lengths, or 65% as many. In spring the northward trip follows the Pacific flyway, and it may be timed to the emergence of flowers, tree leaves and insects.

  In South America, the southern giant hummingbird moves between sea level and 4400 m in the Andes. Satellite transmitters and geolocators traced a seasonal loop of 8335 km between Chile and Ecuador. In 1834, Charles Darwin noted the birds arriving in spring from the "parched deserts of the north."

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• 5. Seeing and Singing

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  A hummingbird eye is only 5 to 6 mm across, yet it takes up a larger share of the skull than the eyes of other birds. The retina holds about 45,000 neurons per square millimeter, an extraordinary density. An enlarged cornea gathers more light when the pupil opens fully, which makes nocturnal flight possible. In the brain, a region called the lentiform nucleus is relatively the largest of any bird. It refines the processing of moving images, so a hovering bird can reorient at the smallest motion and hold its place near a flower.

  A study of broad-tailed hummingbirds found a fourth color-sensitive cone, which humans lack. It detects ultraviolet light, and it may let the birds tell apart mixed colors such as ultraviolet-green and ultraviolet-purple. Such sight may help with flower identity, courtship, territorial defense and escaping predators.

  The feathers give that vision plenty to see. The blue, green and purple colors in the gorget and crown of males cover 34% of the total color space of bird feathers. The shimmer comes from diffraction structures made of melanin, sometimes tinted further by carotenoid pigments. By merely shifting position, a muted-looking bird can flash fiery red or vivid green. A male Anna's hummingbird turns his body toward the sun to show off his plumage to a female. One study found that Anna's hummingbirds fed more protein grew more colorful crown feathers than birds on a low-protein diet.

  Hummingbirds also sing. Their repertoire includes chirps, squeaks, whistles and buzzes, which change with foraging, courtship, territorial defense and mother-nestling contact. Anna's, Costa's, long-billed hermit and Andean hummingbirds have song dialects that differ by habitat. Producing such dialects requires vocal learning, a rare ability found among birds only in parrots and songbirds. A genetic expression study traced it in hummingbirds to at least seven specialized nuclei in the forebrain.

  The blue-throated hummingbird sings across a range from 1.8 kHz to about 30 kHz. It also makes ultrasonic clicks that serve no communication purpose. Since the bird alternates singing with catching small flying insects, the clicks may disrupt insect flight and leave prey more vulnerable. Birds generally vocalize and hear within 2 to 5 kHz. The Ecuadorian hillstar was recorded calling above 10 kHz, far beyond the known hearing of most birds.

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• 6. Nectar and Flowers

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  A hummingbird may have to visit one or two thousand flowers in a single day. Each lick of its forked tongue lasts 14 milliseconds, at a rate of up to 20 licks per second. High-speed photography overturned the old idea of capillary action. The two grooves of the tongue open along their sides in the nectar, then close around it, trapping the liquid like a micropump. The gut then extracts over 99% of the glucose within minutes.

  Tasting the sweetness is its own puzzle. Birds lack the known sweet receptor, T1R2. Experiments showed that hummingbirds repurposed a carbohydrate receptor, the same T1R1-T1R3 type that humans perceive as umami, so that it detects sweet nectar. This change arose as they diverged from the swifts, and it opened the way to flower-rich regions across the Americas.

  Some 7,000 plant species coevolved with hummingbirds. The first plant group to do so in the Americas was probably Heliconia, over 16 to 23 million years ago. Bill lengths run from about 6 mm to as long as 110 mm. The sicklebills have strongly curved bills for the curved tubes of Centropogon flowers. The sword-billed hummingbird, Ensifera ensifera, is one of the most extreme species, tied to flowers of the Passiflora section Tacsonia. Hummingbird-pollinated flowers tend to be red, yellow or purple, scentless and rich in nectar, with long tubes and no landing platform. Hummingbirds see near-ultraviolet light, but such flowers do not reflect it, which may keep them less visible to insects.

  The nectar itself averages 25% sugar and is high in sucrose. The pairings are looser than scientists once thought. Hummingbirds regularly visit flowers poorly suited to their bills, and the plants rely on several species. Even so, feeding is most efficient when bill and flower fit.

  Nectar cannot supply everything. It is low in lipids and lacks some essential amino acids, so nearly all hummingbirds also eat arthropods. By mass, the diet is generally about 90% nectar and 10% arthropods. They snatch small flying insects in the air and glean spiders from webs. In a study of 140 species, about 95% of individuals showed signs of arthropod eating. Nestlings eat nothing but arthropods, so breeding females spend three to four times as long as males hunting them.

  Feeding takes up an average of 20% of a hummingbird's time. The other 75 to 80% goes to sitting and digesting. An enlarged hippocampus helps the bird remember which flowers it has already visited. At backyard feeders, the common recipe is white granulated sugar at 20% concentration, but hummingbirds defend feeders more fiercely at 35%. Honey spoils quickly once diluted. Red dye was once thought to help, yet it is unnecessary.

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• 7. The Unexpected European

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  In 2004, Gerald Mayr took a close look at two 30-million-year-old fossils that had been sitting in a museum drawer in Stuttgart. They had come from a clay pit at Wiesloch-Frauenweiler, south of Heidelberg. Nobody had recognized them as hummingbirds, because hummingbirds were assumed never to have lived outside the Americas. Mayr named the species Eurotrochilus inexpectatus, the "unexpected European hummingbird." Its climate resembled that of the northern Caribbean or southernmost China today.

  Hummingbirds split from the swifts and treeswifts about 42 million years ago, probably in Eurasia. Bird fossils from the Messel pit and the Caucasus, 35 to 40 million years old, hint at the same divide from a related extinct family. Hollow, fragile wing bones rarely fossilize, so the record is thin. The largest gap is the roughly 25 million years between Eurotrochilus and the modern fossils. During that time the family evolved its flight adaptations, shrank, spread to the Americas and died out in Eurasia. In 2013, a 50-million-year-old fossil from Wyoming was identified as a predecessor of both hummingbirds and swifts.

  DNA analysis indicates that the last common ancestor of all living hummingbirds lived in South America about 22 million years ago. Diversification was rapid, and it ran alongside the uplift of the northern Andes. A map of the family tree, built from 284 species, shows nine major groups. They include the hermits, the mangoes, the brilliants, the bees and the emeralds. Nicholas Aylward Vigors, an Irish zoologist, introduced the family Trochilidae in 1825.

  Only the tyrant flycatchers and the tanagers include more species than hummingbirds. Diversity peaks in the humid forests of the northern Andes. Colombia alone has more than 160 species, and much smaller Ecuador has about 130. In one Andean region, as many as 140 species can live side by side. Fewer than 25 species have been recorded in the United States, and fewer than 10 each in Canada and Chile. Evolution is still running. Male long-billed hermits appear to be growing a dagger-like point on the bill tip, a weapon for defending mating areas.

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• 8. Small Lives, Shrinking Numbers

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  A female hummingbird builds a cup-shaped nest about 1.5 inches across, usually on a branch and bound with spider webs, lichens, moss and plant fibers. The silk lets the nest stretch as the chicks grow. Males take no part. She lays two white eggs, the smallest of any bird, and incubates them for 14 to 23 days depending on the species and the temperature. She feeds the chicks by regurgitation. They leave the nest after 18 to 22 days, and she may keep feeding them for another 25. Nestlings raise their rear ends to shoot waste liquid beyond the rim.

  A typical lifespan for well-known North American species is probably 3 to 5 years, and most deaths come in the first year. One banded ruby-throated hummingbird lived 9 years and 2 months. A female broad-tailed hummingbird, banded as an adult and recaptured 11 years later, was at least 12 years old. By comparison, the smaller shrews seldom live beyond 2 years.

  Domestic cats are probably the most common predators of hummingbirds that are away from the nest. Praying mantises, dragonflies, frogs, orb-weaver spiders, snakes, roadrunners, Sharp-shinned Hawks and American Kestrels have also taken them. Two genera of Ricinid lice, Trochiloecetes and Trochiliphagus, specialize on the family. One study found them on about 14% of 22 species.

  Population studies in the United States and Canada put ruby-throated hummingbirds at around 34 million and rufous hummingbirds at around 19 million. Black-chinned, Anna's and broad-tailed hummingbirds number about 8 million each. In 2026 the International Union for Conservation of Nature listed 8 species as critically endangered, 13 as endangered, 13 as vulnerable and 22 as near-threatened. Two, Brace's emerald and the Caribbean emerald, have been declared extinct. Of the species assessed, 316 are of least concern, 87 are stable and 8 are increasing. A total of 255 species are declining.

  Rufous, Costa's, calliope, broad-tailed and Allen's hummingbirds are in significant decline, some losing as much as 67% of their numbers since 1970. The ruby-throated hummingbird, the most numerous in North America, fell by 17% in the early 21st century. Habitat loss, glass collisions, cat predation and pesticides may all contribute. Climate change may also play a part through food supply, migration signals and breeding. Several species now number only in the thousands or hundreds, and most live in remote places where counting them is difficult.

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