Hummingbird

Morphology

Across the estimated 366 species, hummingbird weights range from as small as 2 grams (0.071 oz) to as large as 20 grams (0.71 oz).^[14][15] They have characteristic long, narrow beaks (bills) which may be straight (of varying lengths) or highly curved.^[14][15] The bee hummingbird – only 6 centimetres (2.4 in) long and weighing about 2 grams (0.071 oz) – is the world's smallest bird and smallest warm-blooded vertebrate.^[14][16]

Hummingbirds have compact bodies with relatively long, bladelike wings having anatomical structure enabling helicopter-like flight in any direction, including the ability to hover.^[14][15] Particularly while hovering, the wing beats produce the humming sounds, which function to alert other birds.^[14] In some species, the tail feathers produce sounds used by males during courtship flying.^[14][15] Hummingbirds have extremely rapid wing-beats as high as 80 per second, supported by a high metabolic rate dependent on foraging for sugars from flower nectar.^[7][15]

Hummingbird legs are short with feet having three toes pointing forward and one backward – the hallux.^[17][18] The toes of hummingbirds are formed as claws with ridged inner surfaces to aid gripping onto flower stems or petals.^[18] Hummingbirds do not walk on the ground or hop like most birds, but rather shuffle laterally and use their feet to grip while perching, preening feathers, or nest-building (by females), and during fights to grab feathers of opponents.^[17][18]

Hummingbirds apply their legs as pistons for generating thrust upon taking flight, although the shortness of their legs provides about 20% less propulsion than assessed in other birds.^[19] During flight, hummingbird feet are tucked up under the body, enabling optimal aerodynamics and maneuverability.^[18]

Of those species that have been measured during flight, the top flight speeds of hummingbirds exceed 15 m/s (54 km/h; 34 mph).^[16] During courtship, some male species dive from 30 metres (100 ft) of height above a female at speeds around 23 m/s (83 km/h; 51 mph).^[20][21]

The sexes differ in feather coloration, with males having distinct brilliance and ornamentation of head, neck, wing, and breast feathers.^[14][15] The most typical feather ornament in males is the gorget – a bib-like iridescent neck-feather patch that changes brilliance with the viewing angle to attract females and warn male competitors away from territory.^[14]

Life cycle

Hummingbirds begin mating when they are a year old.^[22] Sex occurs over 3–5 seconds when the male joins its cloaca with the female's, passing sperm to fertilize the female's eggs.^[22]

Hummingbird females build a nest resembling a small cup about 1.5 inches (3.8 cm) in diameter, commonly attached to a tree branch using spider webs, lichens, moss, and loose strings of plant fibers (image).^[14][15] Typically, two pea-shaped white eggs (image) – the smallest of any bird – are incubated over 2–3 weeks in breeding season.^[14][15] Fed by regurgitation only from the mother, the chicks fledge about 3 weeks after hatching.^[15][23]

The average lifespan of a ruby-throated hummingbird is estimated to be 3–5 years, with most deaths occurring in yearlings,^[23] although one banded ruby-throated hummingbird lived for 9 years and 2 months.^[24] Bee hummingbirds live 7–10 years.^[16]

Population estimates and threatened species

Although most hummingbird species live in remote habitats where their population numbers are difficult to assess, population studies in the United States and Canada indicate that the ruby-throated hummingbird numbers are around 34 million, rufous hummingbirds are around 19 million, black-chinned, Anna's, and broad-tailed hummingbirds are about 8 million each, calliopes at 4 million, and Costa's and Allen's hummingbirds are around 2 million each.^[2] Several species exist only in the thousands or hundreds.^[2]

According to the International Union for Conservation of Nature Red List of Threatened Species in 2024, 8 hummingbird species are classified as critically endangered, 13 are endangered, 13 are vulnerable, and 20 species are near-threatened.^[3] Two species – the Brace's emerald (Riccordia bracei) and Caribbean emerald (Riccordia elegans) – have been declared extinct.^[3]

Of the 15 species of North American hummingbirds that inhabit the United States and Canada,^[2] several have changed their range of distribution, while others showed declines in numbers since the 1970s,^[2][4] including in 2023 with dozens of hummingbird species in decline. As of the 21st century, 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 at nearly double the rate of population loss over the previous 50 years.^[2][4][25] The ruby-throated hummingbird population – the most populous North American hummingbird – decreased by 17% over the early 21st century.^[4] Habitat loss, glass collisions, cat predation, pesticides, and possibly climate change affecting food availability, migration signals, and breeding are factors that may contribute to declining hummingbird numbers.^[2][25] By contrast, Anna's hummingbirds had large population growth at an accelerating rate since 2010,^[4] and expanded their range northward to reside year-round in cold winter climates.^[26]

Superficially similar species

Some species of sunbirds — an Old World group restricted in distribution to Eurasia, Africa, and Australia — resemble hummingbirds in appearance and behavior,^[27] but are not related to hummingbirds, as their resemblance is due to convergent evolution.^[28]

The hummingbird moth has flying and feeding characteristics similar to those of a hummingbird.^[29] Hummingbirds may be mistaken for hummingbird hawk-moths, which are large, flying insects with hovering capabilities, and exist only in Eurasia.^[27]

Phylogeny

A phylogenetic tree unequivocally indicates that modern hummingbirds originated in South America, with the last common ancestor of all living hummingbirds living around 22 million years ago.^[9]

A map of the hummingbird family tree – reconstructed from analysis of 284 species – shows rapid diversification from 22 million years ago.^[42] Hummingbirds fall into nine main clades – the topazes, hermits, mangoes, brilliants, coquettes, the giant hummingbird, mountaingems, bees, and emeralds – defining their relationship to nectar-bearing flowering plants which attract hummingbirds into new geographic areas.^[9][43][44]

Molecular phylogenetic studies of the hummingbirds have shown that the family is composed of nine major clades.^[43][9] When Edward Dickinson and James Van Remsen Jr. updated the Howard and Moore Complete Checklist of the Birds of the World for the 4th edition in 2013, they divided the hummingbirds into six subfamilies.^[45]

Molecular phylogenetic studies determined the relationships between the major groups of hummingbirds.^[9][44] In the cladogram below, the English names are those introduced in 1997.^[46] The scientific names are those introduced in 2013.^[47]

While all hummingbirds depend on flower nectar to fuel their high metabolisms and hovering flight, coordinated changes in flower and bill shape stimulated the formation of new species of hummingbirds and plants. Due to this exceptional evolutionary pattern, as many as 140 hummingbird species can coexist in a specific region, such as the Andes range.^[42]

The hummingbird evolutionary tree shows that one key evolutionary factor appears to have been an altered taste receptor that enabled hummingbirds to seek nectar.^[48]

Upon maturity, males of a particular species, Phaethornis longirostris, the long-billed hermit, appear to be evolving a dagger-like weapon on the beak tip as a secondary sexual trait to defend mating areas.^[49]

Geographic diversification

The Andes Mountains appear to be a particularly rich environment for hummingbird evolution because diversification occurred simultaneously with mountain uplift over the past 10 million years.^[42] Hummingbirds remain in dynamic diversification inhabiting ecological regions across South America, North America, and the Caribbean, indicating an enlarging evolutionary radiation.^[42]

Within the same geographic region, hummingbird clades coevolved with nectar-bearing plant clades, affecting mechanisms of pollination.^[50][51] The same is true for the sword-billed hummingbird (Ensifera ensifera), one of the morphologically most extreme species, and one of its main food plant clades (Passiflora section Tacsonia).^[52]

Coevolution with ornithophilous flowers

Hummingbirds are specialized nectarivores tied to the ornithophilous flowers upon which they feed.^[53] This coevolution implies that morphological traits of hummingbirds, such as bill length, bill curvature, and body mass, are correlated with morphological traits of plants, such as corolla length, curvature, and volume.^[54] Some species, especially those with unusual bill shapes, such as the sword-billed hummingbird and the sicklebills, are coevolved with a small number of flower species. Even in the most specialized hummingbird–plant mutualisms, the number of food plant lineages of the individual hummingbird species increases with time.^[55] The bee hummingbird (Mellisuga helenae) – the world's smallest bird – evolved to dwarfism likely because it had to compete with long-billed hummingbirds having an advantage for nectar foraging from specialized flowers, consequently leading the bee hummingbird to more successfully compete for flower foraging against insects.^[56][57]

Many plants pollinated by hummingbirds produce flowers in shades of red, orange, and bright pink, although the birds take nectar from flowers of other colors. Hummingbirds can see wavelengths into the near-ultraviolet, but hummingbird-pollinated flowers do not reflect these wavelengths as many insect-pollinated flowers do. This narrow color spectrum may render hummingbird-pollinated flowers relatively inconspicuous to most insects, thereby reducing nectar robbing.^[58][59] Hummingbird-pollinated flowers also produce relatively weak nectar (averaging 25% sugars) containing a high proportion of sucrose, whereas insect-pollinated flowers typically produce more concentrated nectars dominated by fructose and glucose.^[60]

Hummingbirds and the plants they visit for nectar have a tight coevolutionary association, generally called a plant–bird mutualistic network.^[61] These birds show high specialization and modularity, especially in communities with high species richness. These associations are also observed when closely related hummingbirds, such as two species of the same genus, visit distinct sets of flowering species.^[61][62]

Feather colors

The hummingbird plumage coloration gamut, particularly for blue, green, and purple colors in the gorget and crown of males, occupies 34% of the total color space for bird feathers.^[5] White (unpigmented) feathers have the lowest incidence in the hummingbird color gamut.^[5] Hummingbird plumage color diversity evolved from sexual and social selection on plumage coloration, which correlates with the rate of hummingbird species development over millions of years.^[5] Bright plumage colors in males are part of aggressive competition for flower resources and mating.^[5][67] The bright colors result from pigmentation in the feathers and from prismal cells within the top layers of feathers of the head, gorget, breast, back and wings.^[5][68] When sunlight hits these cells, it is split into wavelengths that reflect to the observer in varying degrees of intensity,^[68] with the feather structure acting as a diffraction grating.^[68] Iridescent hummingbird colors result from a combination of refraction and pigmentation, since the diffraction structures themselves are made of melanin, a pigment,^[5][67] and may also be colored by carotenoid pigmentation and more subdued black, brown or gray colors dependent on melanin.^[68]

By merely shifting position, feather regions of a muted-looking bird can instantly become fiery red or vivid green.^[68] In courtship displays for one example, males of the colorful Anna's hummingbird orient their bodies and feathers toward the sun to enhance the display value of iridescent plumage toward a female of interest.^[69]

One study of Anna's hummingbirds found that dietary protein was an influential factor in feather color, as birds receiving more protein grew significantly more colorful crown feathers than those fed a low-protein diet.^[70] Additionally, birds on a high-protein diet grew yellower (higher hue) green tail feathers than birds on a low-protein diet.^[70]

Humming

Hummingbirds are named for the prominent humming sound their wingbeats make while flying and hovering to feed or interact with other hummingbirds.^[71] Humming serves communication purposes by alerting other birds of the arrival of a fellow forager or potential mate.^[71] The humming sound derives from aerodynamic forces generated by the downstrokes and upstrokes of the rapid wingbeats, causing oscillations and harmonics that evoke an acoustic quality likened to that of a musical instrument.^[71][72] The humming sound of hummingbirds is unique among flying animals, compared to the whine of mosquitoes, buzz of bees, and "whoosh" of larger birds.^[71][72]

The wingbeats causing the hum of hummingbirds during hovering are achieved by elastic recoil of wing strokes produced by the main flight muscles: the pectoralis major (the main downstroke muscle) and supracoracoideus (the main upstroke muscle).^[73]

Vision

Although hummingbird eyes are small in diameter (5–6 mm), they are accommodated in the skull by reduced skull ossification, and occupy a larger proportion of the skull compared to other birds and animals.^[74]

Further, hummingbird eyes have large corneas, which comprise about 50% of the total transverse eye diameter, combined with an extraordinary density of retinal ganglion cells responsible for visual processing, containing some 45,000 neurons per mm².^[75] The enlarged cornea relative to total eye diameter serves to increase the amount of light perception by the eye when the pupil is dilated maximally, enabling nocturnal flight.^[75]

During evolution, hummingbirds adapted to the navigational needs of visual processing while in rapid flight or hovering by development of the exceptionally dense array of retinal neurons, allowing for increased spatial resolution in the lateral and frontal visual fields.^[75] Morphological studies of the hummingbird brain showed that neuronal hypertrophy – relatively the largest in any bird – exists in a region called the pretectal nucleus lentiformis mesencephali (called the nucleus of the optic tract in mammals) responsible for refining dynamic visual processing while hovering and during rapid flight.^[76][77]

The enlargement of the brain region responsible for visual processing indicates an enhanced ability for perception and processing of fast-moving visual stimuli encountered during rapid forward flight, insect foraging, competitive interactions, and high-speed courtship.^[77][78] A study of broad-tailed hummingbirds indicated that hummingbirds have a fourth color-sensitive visual cone (humans have three) that detects ultraviolet light and enables discrimination of non-spectral colors, possibly having a role in flower identity, courtship displays, territorial defense, and predator evasion.^[79] The fourth color cone would extend the range of visible colors for hummingbirds to perceive ultraviolet light and color combinations of feathers and gorgets, colorful plants, and other objects in their environment, enabling detection of as many as five non-spectral colors, including purple, ultraviolet-red, ultraviolet-green, ultraviolet-yellow, and ultraviolet-purple.^[79]

Hummingbirds are highly sensitive to stimuli in their visual fields, responding to even minimal motion in any direction by reorienting themselves in midflight.^[77][78][80] Their visual sensitivity allows them to precisely hover in place while in complex and dynamic natural environments,^[80] functions enabled by the lentiform nucleus which is tuned to fast-pattern velocities, enabling highly tuned control and collision avoidance during forward flight.^[77]

Song, vocal learning, and hearing

Many hummingbird species exhibit a diverse vocal repertoire of chirps, squeaks, whistles and buzzes.^[82][83] Vocalizations vary in complexity and spectral content during social interactions, foraging, territorial defense, courtship, and mother-nestling communication.^[82] Territorial vocal signals may be produced in rapid succession to discourage aggressive encounters, with the chirping rate and loudness increasing when intruders persist.^[82] During the breeding season, male and female hummingbirds vocalize as part of courtship.^[82]

Hummingbirds exhibit vocal production learning to enable song variation – "dialects" that exist across the same species.^[82] For example, the blue-throated hummingbird's song differs from typical oscine songs in its wide frequency range, extending from 1.8 kHz to about 30 kHz.^[84] It also produces ultrasonic vocalizations which do not function in communication.^[84] As blue-throated hummingbirds often alternate singing with catching small flying insects, it is possible the ultrasonic clicks produced during singing disrupt insect flight patterns, making insects more vulnerable to predation.^[84] Anna's, Costa's, long-billed hermits, and Andean hummingbirds have song dialects that vary across habitat locations and phylogenetic clades.^[82][85]

The avian vocal organ, the syrinx, plays an important role in understanding hummingbird song production.^[86] What makes the hummingbird's syrinx different from that of other birds in the Apodiformes order is the presence of internal muscle structure, accessory cartilages, and a large tympanum that serves as an attachment point for external muscles, all of which are adaptations thought to be responsible for the hummingbird's increased ability in pitch control and large frequency range.^[86][87]

Hummingbird songs originate from at least seven specialized nuclei in the forebrain.^[88][89] A genetic expression study showed that these nuclei enable vocal learning (ability to acquire vocalizations through imitation), a rare trait known to occur in only two other groups of birds (parrots and songbirds) and a few groups of mammals (including humans, whales and dolphins, and bats).^[88] Within the past 66 million years, only hummingbirds, parrots, and songbirds out of 23 bird orders may have independently evolved seven similar forebrain structures for singing and vocal learning, indicating that evolution of these structures is under strong epigenetic constraints possibly derived from a common ancestor.^[88][90]

Generally, birds have been assessed to vocalize and hear in the range of 2–5 kHz, with hearing sensitivity falling with higher frequencies.^[85] In the Ecuadorian hillstar (Oreotrochilus chimborazo), vocalizations were recorded in the wild to be at a frequency above 10 kHz, well outside of the known hearing ability of most birds.^[85] Song system nuclei in the hummingbird brain are similar to those songbird brains, but the hummingbird brain has specialized regions involved for song processing.^[82]

Metabolism

Hummingbirds have the highest metabolism of all vertebrate animals – a necessity to support the rapid beating of their wings during hovering and fast forward flight.^[7][91] During flight and hovering, oxygen consumption per gram of muscle tissue in a hummingbird is about 10 times higher than that measured in elite human athletes.^[6] Hummingbirds achieve this extraordinary capacity for oxygen consumption by an exceptional density and proximity of capillaries and mitochondria in their flight muscles.^[92]

Hummingbirds are rare among vertebrates in their ability to rapidly make use of ingested sugars to fuel energetically expensive hovering flight, powering up to 100% of their metabolic needs with the sugars they drink.^[93] Hummingbird flight muscles have extremely high capacities for oxidizing carbohydrates and fatty acids via hexokinase, carnitine palmitoyltransferase, and citrate synthase enzymes at rates that are the highest known for vertebrate skeletal muscle.^[94] To sustain rapid wingbeats during flight and hovering, hummingbirds expend the human equivalent of 150,000 calories per day,^[95] an amount estimated to be 10 times the energy consumption by a marathon runner in competition.^[96]

Hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption.^[97][98] These data suggest that hummingbirds are able to oxidize sugar in flight muscles at rates rapid enough to satisfy their extreme metabolic demands – as indicated by a 2017 review showing that hummingbirds have in their flight muscles a mechanism for "direct oxidation" of sugars into maximal ATP yield to support a high metabolic rate for hovering, foraging at altitude, and migrating.^[99] This adaptation occurred through the evolutionary loss of a key gene, fructose-bisphosphatase 2 (FBP2), coinciding with the onset of hovering by hummingbirds estimated by fossil evidence to be some 35 million years ago.^[100][101] Without FBP2, glycolysis and mitochondrial respiration in flight muscles are enhanced, enabling hummingbirds to metabolize sugar more efficiently for energy.^[100][101]

By relying on newly ingested sugars to fuel flight, hummingbirds reserve their limited fat stores to sustain their overnight fasting during torpor or to power migratory flights.^[97] Studies of hummingbird metabolism address how a migrating ruby-throated hummingbird can cross 800 km (500 mi) of the Gulf of Mexico on a nonstop flight.^[7] This hummingbird, like other long-distance migrating birds, stores fat as a fuel reserve, augmenting its weight by as much as 100%, then enabling metabolic fuel for flying over open water.^[7][102] The amount of fat (1–2 g) used by a migrating hummingbird to cross the Gulf of Mexico in a single flight is similar to that used by a human climbing about 50 feet (15 m).^[7]

The heart rate of hummingbirds can reach as high as 1,260 beats per minute, a rate measured in a blue-throated hummingbird with a breathing rate of 250 breaths per minute at rest.^[7][103]

Heat dissipation

The high metabolic rate of hummingbirds – especially during rapid forward flight and hovering – produces increased body heat that requires specialized mechanisms of thermoregulation for heat dissipation, which becomes an even greater challenge in hot, humid climates.^[104] Hummingbirds dissipate heat partially by evaporation through exhaled air, and from body structures with thin or no feather covering, such as around the eyes, shoulders, under the wings (patagia), and feet.^[105][106]

While hovering, hummingbirds do not benefit from the heat loss by air convection during forward flight, except for air movement generated by their rapid wing-beat, possibly aiding convective heat loss from the extended feet.^[104][107] Smaller hummingbird species, such as the calliope, appear to adapt their relatively higher surface-to-volume ratio to improve convective cooling from air movement by the wings.^[104] When air temperatures rise above 36 °C (97 °F), thermal gradients driving heat passively by convective dissipation from around the eyes, shoulders, and feet are reduced or eliminated, requiring heat dissipation mainly by evaporation and exhalation.^[104] In cold climates, hummingbirds retract their feet into breast feathers to eliminate skin exposure and minimize heat dissipation.^[107]

Kidney function

The dynamic range of metabolic rates in hummingbirds^[108] requires a parallel dynamic range in kidney function.^[109] During a day of nectar consumption with a corresponding high water intake that may total five times the body weight per day, hummingbird kidneys process water via glomerular filtration rates (GFR) in amounts proportional to water consumption, thereby avoiding overhydration.^[109][110] During brief periods of water deprivation, however, such as in nighttime torpor, GFR drops to zero, preserving body water.^[109][110]

Hummingbird kidneys also have a unique ability to control the levels of electrolytes after consuming nectars with high amounts of sodium and chloride or none, indicating that kidney and glomerular structures must be highly specialized for variations in nectar mineral quality.^[111] Morphological studies on Anna's hummingbird kidneys showed adaptations of high capillary density in close proximity to nephrons, allowing for precise regulation of water and electrolytes.^[110][112]

Hemoglobin adaptation to altitude

Dozens of hummingbird species live year-round in tropical mountain habitats at high altitudes, such as in the Andes over ranges of 1,500 metres (4,900 ft) to 5,200 metres (17,100 ft) where the partial pressure of oxygen in the air is reduced, a condition of hypoxic challenge for the high metabolic demands of hummingbirds.^{[113][114][115]} In Andean hummingbirds living at high elevations, researchers found that the oxygen-carrying protein in blood – hemoglobin – had increased oxygen-binding affinity, and that this adaptive effect likely resulted from evolutionary mutations within the hemoglobin molecule via specific amino acid changes due to natural selection.^{[113][114][116]}

Adaptation to winter

Anna's hummingbirds are the northernmost year-round residents of any hummingbird. Anna's hummingbirds were recorded in Alaska as early as 1971, and resident in the Pacific Northwest since the 1960s, particularly increasing as a year-round population during the early 21st century.^[26][117] Scientists estimate that some Anna's hummingbirds overwinter and presumably breed at northern latitudes where food and shelter are available throughout winter, tolerating moderately cold winter temperatures.^[26][117]

During cold temperatures, Anna's hummingbirds gradually gain weight during the day as they convert sugar to fat.^[118][119] In addition, hummingbirds with inadequate stores of body fat or insufficient plumage are able to survive periods of subfreezing weather by lowering their metabolic rate and entering a state of torpor.^[120]

While their range was originally limited to the chaparral of California and Baja California, it expanded northward to Oregon, Washington, and British Columbia, and east to Arizona over the 1960s to 1970s.^[117] This rapid expansion is attributed to the widespread planting of non-native species, such as eucalyptus, as well as the use of urban bird feeders, in combination with the species' natural tendency for extensive postbreeding dispersal.^[121] In the Pacific Northwest, the fastest growing populations occur in regions with breeding-season cold temperatures similar to those of its native range.^[117] Northward expansion of the Anna's hummingbird represents an ecological release associated with introduced plants, year-round nectar availability from feeders supplied by humans, milder winter temperatures associated with climate change, and acclimation of the species to a winter climate cooler than its native region.^[26][117] Although quantitative data are absent, it is likely that a sizable percentage of Anna's hummingbirds in the Pacific Northwest still do migrate south for winter, as of 2017.^[26]

Anna's hummingbird is the official city bird of Vancouver, British Columbia, Canada,^[122] and is a non-migrating resident of Seattle where it lives year-round through winter enduring extended periods of subfreezing temperatures, snow, and high winds.^[123]

Torpor

The metabolism of hummingbirds can slow at night or at any time when food is not readily available; the birds enter a deep-sleep state (known as torpor) to prevent energy reserves from falling to a critical level. One study of broad-tailed hummingbirds found that body weight decreased linearly throughout torpor at a rate of 0.04 g per hour.^[109]

During nighttime torpor, body temperature in a Caribbean hummingbird was shown to fall from 40 to 18 °C,^[124] with heart and breathing rates slowing dramatically (heart rate of roughly 50 to 180 bpm from its daytime rate of higher than 1000 bpm).^[125] Recordings from a Metallura phoebe hummingbird in noctural torpor at around 3,800 metres (12,500 ft) in the Andes mountains showed that body temperature fell to 3.3 °C (38 °F), the lowest known level for a bird or non-hibernating mammal.^[126][127] During cold nights at altitude, hummingbirds were in torpor for 2–13 hours depending on species, with cooling occurring at the rate of 0.6 °C per minute and rewarming at 1–1.5 °C per minute.^[126] High-altitude Andean hummingbirds also lost body weight in negative proportion to how long the birds were in torpor, losing about 6% of weight each night.^[126]

During torpor, to prevent dehydration, the kidney function declines, preserving needed compounds, such as glucose, water, and nutrients.^[109] The circulating hormone, corticosterone, is one signal that arouses a hummingbird from torpor.^[128]

Use and duration of torpor vary among hummingbird species and are affected by whether a dominant bird defends territory, with nonterritorial subordinate birds having longer periods of torpor.^[129] A hummingbird with a higher fat percentage will be less likely to enter a state of torpor compared to one with less fat, as a bird can use the energy from its fat stores.^[120] Torpor in hummingbirds appears to be unrelated to nighttime temperature, as it occurs across a wide temperature range, with energy savings of such deep sleep being more related to the photoperiod and duration of torpor.^[120]

Lifespan

Hummingbirds have unusually long lifespans for organisms with such rapid metabolisms. Though many die during their first year of life, especially in the vulnerable period between hatching and fledging, those that survive may occasionally live a decade or more.^[130] Among the better-known North American species, the typical lifespan is probably 3 to 5 years.^[130] For comparison, the smaller shrews, among the smallest of all mammals, seldom live longer than 2 years.^[131] The longest recorded lifespan in the wild relates to a female broad-tailed hummingbird that was banded as an adult at least one year old, then recaptured 11 years later, making her at least 12 years old.^[132] Other longevity records for banded hummingbirds include an estimated minimum age of 10 years 1 month for a female black-chinned hummingbird similar in size to the broad-tailed hummingbird, and at least 11 years 2 months for a much larger buff-bellied hummingbird.^[133]

Predators

Praying mantises have been observed as predators of hummingbirds.^{[134][135][136]} Other predators include domestic cats, dragonflies, frogs, orb-weaver spiders, and other birds, such as the roadrunner.^[2][137]

Parasites

Hummingbirds host a highly specialized lice fauna. Two genera of Ricinid lice, Trochiloecetes and Trochiliphagus, are specialized on them, often infesting 5–15% of their populations. In contrast, two genera of Menoponid lice, Myrsidea and Leremenopon^{[clarification needed]}, are extremely rare on them.^[138][139]

Wingbeats and flight stability

The highest recorded wingbeat rate for hummingbirds during hovering is 99.1 per second, as measured for male woodstars (Chaetocercus sp.).^[156] Males in the genus Chaetocercus have been recorded above 100 beats per second during courtship displays.^[156] The number of beats per second increases above "normal" hovering while flying during courtship displays (up to 90 per second for the calliope hummingbird, Selasphorus calliope); a wingbeat rate 40% higher than its typical hovering rate.^[157]

During turbulent airflow conditions created experimentally in a wind tunnel, hummingbirds exhibit stable head positions and orientation when they hover at a feeder.^[158] When wind gusts from the side, hummingbirds compensate by increasing wing-stroke amplitude and stroke plane angle and by varying these parameters asymmetrically between the wings and from one stroke to the next.^[158] They also vary the orientation and enlarge the collective surface area of their tail feathers into the shape of a fan.^[158] While hovering, the visual system of a hummingbird is able to separate apparent motion caused by the movement of the hummingbird itself from motions caused by external sources, such as an approaching predator.^[80] In natural settings full of highly complex background motion, hummingbirds are able to precisely hover in place by rapid coordination of vision with body position.^[80]

Courtship dives

When courting, the male Anna's hummingbird ascends some 35 m (115 ft) above a female, before diving at a speed of 27 m/s (89 ft/s), equal to 385 body lengths/sec – producing a high-pitched sound near the female at the nadir of the dive.^[159] This downward acceleration during a dive is the highest reported for any vertebrate undergoing a voluntary aerial maneuver; in addition to acceleration, the speed relative to body length is the highest known for any vertebrate. For instance, it is about twice the diving speed of peregrine falcons in pursuit of prey.^[159] At maximum descent speed, about 10 g of gravitational force occur in the courting hummingbird during a dive (Note: G-force is generated as the bird pulls out of the dive).^[159][a]

The outer tail feathers of male Anna's (Calypte anna) and Selasphorus hummingbirds (e.g., Allen's, calliope) vibrate during courtship display dives and produce an audible chirp caused by aeroelastic flutter.^[161][162] Hummingbirds cannot make the courtship dive sound when missing their outer tail feathers, and those same feathers could produce the dive sound in a wind tunnel.^[161] The bird can sing at the same frequency as the tail-feather chirp, but its small syrinx is not capable of the same volume.^[163] The sound is caused by the aerodynamics of rapid air flow past tail feathers, causing them to flutter in a vibration, which produces the high-pitched sound of a courtship dive.^[161][164]

Many other species of hummingbirds also produce sounds with their wings or tails while flying, hovering, or diving, including the wings of the calliope hummingbird,^[165] broad-tailed hummingbird, rufous hummingbird, Allen's hummingbird, and the streamertail species, as well as the tail of the Costa's hummingbird and the black-chinned hummingbird, and a number of related species.^[166] The harmonics of sounds during courtship dives vary across species of hummingbirds.^[162]

Wing feather trill

Male rufous and broad-tailed hummingbirds (genus Selasphorus) have a distinctive wing feature during normal flight that sounds like jingling or a buzzing shrill whistle – a trill.^[167] The trill arises from air rushing through slots created by the tapered tips of the ninth and tenth primary wing feathers, creating a sound loud enough to be detected by female or competitive male hummingbirds and researchers up to 100 m away.^[167]

Behaviorally, the trill serves several purposes: It announces the sex and presence of a male bird; it provides audible aggressive defense of a feeding territory and an intrusion tactic; it enhances communication of a threat; and it favors mate attraction and courtship.^[167]

Beak specializations

Curved beak (approx. 90^o) of the white-tipped sicklebill

Centropogon flowers

Coevolution of the sicklebill beak curve facilitates both nectar feeding and pollination of long tubular Centropogon flowers.^[193]

The shapes of hummingbird beaks (also called bills) vary widely as an adaptation for specialized feeding,^[65][66] with some 7000 flowering plants pollinated by hummingbird nectar feeding.^[194] Hummingbird beak lengths range from about 6 millimetres (0.24 in) to as long as 110 millimetres (4.3 in).^[195] When catching insects in flight, a hummingbird's jaw flexes downward to widen the beak for successful capture.^[182]

The extreme curved beaks of sicklebills are adapted for extracting nectar from the curved corolla tubes of Centropogon flowers.^[193] Some species, such as hermits (Phaethornis spp.), have long beaks that enable insertion deeply into flowers with long corolla tubes.^[194][196] Thornbills have short, sharp beaks adapted for feeding from flowers with short corolla tubes and piercing the bases of longer ones. The beak of the fiery-tailed awlbill has an upturned tip adapted for feeding on nectar from tubular flowers while hovering.^[197]

Perception of sweet nectar

Perception of sweetness in nectar evolved in hummingbirds during their genetic divergence from insectivorous swifts, their closest bird relatives.^[198] Although the only known sweet sensory receptor, called T1R2,^[199] is absent in birds, receptor expression studies showed that hummingbirds adapted a carbohydrate receptor from the T1R1-T1R3 receptor, identical to the one perceived as umami in humans, essentially repurposing it to function as a nectar sweetness receptor.^[198] This adaptation for taste enabled hummingbirds to detect and exploit sweet nectar as an energy source, facilitating their distribution across geographical regions where nectar-bearing flowers are available.^[198]

Tongue as a micropump

Hummingbirds drink with their long tongues by rapidly lapping nectar. Their tongues have semicircular tubes which run down their lengths to facilitate nectar consumption via rapid pumping in and out of the nectar.^[200][201] While capillary action was believed to be what drew nectar into these tubes,^[202] high-speed photography revealed that the tubes open down their sides as the tongue goes into the nectar, and then close around the nectar, trapping it so it can be pulled back into the beak over a period of 14 milliseconds per lick at a rate of up to 20 licks per second.^[203][204] The tongue, which is forked, is compressed until it reaches nectar, then the tongue springs open, the rapid action traps the nectar which moves up the grooves, like a pump action, with capillary action not involved.^{[200][201][204][205]} Consequently, tongue flexibility enables accessing, transporting and unloading nectar via pump action,^[200][201] not by a capillary syphon as once believed.^[202]

Feeders and artificial nectar

In the wild, hummingbirds visit flowers for food, extracting nectar, which is 55% sucrose, 24% glucose, and 21% fructose on a dry-matter basis.^[206] Hummingbirds also take sugar-water from bird feeders, which allow people to observe and enjoy hummingbirds up close while providing the birds with a reliable source of energy, especially when flower blossoms are less abundant. A negative aspect of artificial feeders, however, is that the birds may seek less flower nectar for food, and so may reduce the amount of pollination their feeding naturally provides.^[207]

White granulated sugar is used in hummingbird feeders in a 20% concentration as a common recipe,^[208] although hummingbirds will defend feeders more aggressively when sugar content is at 35%, indicating preference for nectar with higher sugar content.^[209] Organic and "raw" sugars contain iron, which can be harmful,^[210] and brown sugar, agave syrup, molasses, and artificial sweeteners also should not be used.^[211] Honey is made by bees from the nectar of flowers, but it is not good to use in feeders because when it is diluted with water, microorganisms easily grow in it, causing it to spoil rapidly.^{[212][213][214]}

Red food dye was once thought to be a favorable ingredient for the nectar in home feeders, but it is unnecessary.^[215] Commercial products sold as "instant nectar" or "hummingbird food" may also contain preservatives or artificial flavors, as well as dyes, which are unnecessary and potentially harmful.^[215][216] Although some commercial products contain small amounts of nutritional additives, hummingbirds obtain all necessary nutrients from the insects they eat, rendering added nutrients unnecessary.^[217]

Visual cues of foraging

Hummingbirds have exceptional visual acuity providing them with discrimination of food sources while foraging.^[75] Although hummingbirds are thought to be attracted to color while seeking food, such as red flowers or artificial feeders, experiments indicate that location and flower nectar quality are the most important "beacons" for foraging.^[218][219] Hummingbirds depend little on visual cues of flower color to beacon to nectar-rich locations, but rather they use surrounding landmarks to find the nectar reward.^{[220][221][222]}

In at least one hummingbird species – the green-backed firecrown (Sephanoides sephaniodes) – flower colors preferred are in the red-green wavelength for the bird's visual system, providing a higher contrast than for other flower colors.^[223] Further, the crown plumage of firecrown males is highly iridescent in the red wavelength range (peak at 650 nanometers), possibly providing a competitive advantage of dominance when foraging among other hummingbird species with less colorful plumage.^[223] The ability to discriminate colors of flowers and plumage is enabled by a visual system having four single cone cells and a double cone screened by photoreceptor oil droplets which enhance color discrimination.^[218][223]

Olfaction

While hummingbirds rely primarily on vision and hearing to assess competition from bird and insect foragers near food sources, they may also be able to detect by smell the presence in nectar of insect defensive chemicals (such as formic acid) and aggregation pheromones of foraging ants, which discourage feeding.^[224]