Hummingbird

For other uses, see Hummingbird (disambiguation).
Hummingbird
Female black-chinned hummingbird
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Subclass: Neornithes
Infraclass: Neognathae
(unranked): Cypselomorphae
Order: Apodiformes
Family: Trochilidae
Vigors, 1825
Subfamilies

Phaethornithinae
Trochilinae

For a taxonomic list of genera, see:

For an alphabetic species list, see:

Hummingbirds are New World birds that constitute the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm (3–5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm bee hummingbird, weighing less than a U.S. penny.

They are known as hummingbirds because of the humming sound created by their beating wings which flap at high frequencies audible to humans. They hover in mid-air at rapid wing-flapping rates, typically around 50 times per second, allowing them also to fly at speeds exceeding 15 m/s (54 km/h; 34 mph),[1] backwards.[2]

Hummingbirds have the highest metabolism of any homeothermic animal.[3] To conserve energy when food is scarce, and nightly when not foraging, they go into torpor, a state similar to hibernation, slowing metabolic rate to 1/15th of its normal rate.[4]

Evolution

A map of the hummingbird family tree—reconstructed from analysis of 284 of the world's 338 known species—shows rapid diversification from 22 million years ago.[5] Hummingbirds fall into nine main clades defining their relationship to nectar-bearing flowering plants, and the birds' continued spread into new geographic areas.[5]

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 both hummingbirds and plants. Due to this exceptional evolutionary pattern, as many as 25 hummingbird species are able to coexist in specific regions because their food needs are the same.[5]

The hummingbird evolutionary tree shows that ancestral hummingbirds splitting from insectivorous swifts (family Apodidae) and treeswifts (family Hemiprocnidae) about 42 million years ago, probably in Eurasia.[5] One key evolutionary factor appears to be an altered taste receptor that enabled hummingbirds to seek nectar.[6] By 22 million years ago, the ancestral species of current hummingbirds became established in South America where environmental conditions stimulated further diversification.[5]

The Andes Mountains appear to be a particularly rich environment for hummingbird evolution (140 hummingbird species live in the Andes today) because diversification occurred simultaneously with mountain uplift over the past 10 million years.[5]

Hummingbirds remain in dynamic diversification inhabiting ecological regions across North America, South America, and the Caribbean, indicating an enlarging evolutionary radiation.[5]

Co-evolution with ornithophilous flowers

Purple-throated carib feeding at a flower

Hummingbirds are specialized nectarivores[7] and are tied to the ornithophilous flowers they feed upon. Some species, especially those with unusual bill shapes such as the sword-billed hummingbird and the sicklebills, are co-evolved with a small number of flower species.

Many plants pollinated by hummingbirds produce flowers in shades of red, orange, and bright pink, though the birds will take nectar from flowers of many colors. Hummingbirds can see wavelengths into the near-ultraviolet, but their 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.[8][9] Hummingbird-pollinated flowers also produce relatively weak nectar (averaging 25% sugars w/w) containing high concentrations of sucrose, whereas insect-pollinated flowers typically produce more concentrated nectars dominated by fructose and glucose.[10]

Systematics

A color plate illustration from Ernst Haeckel's Kunstformen der Natur (1899), showing a variety of hummingbirds.

In traditional taxonomy, hummingbirds are placed in the order Apodiformes, which also contains the swifts. However, some taxonomists have separated them into their own order, Trochiliformes. Hummingbirds' wing bones are hollow and fragile, making fossilization difficult and leaving their evolutionary history poorly documented. Though scientists theorize that hummingbirds originated in South America, where there is the greatest species diversity, possible ancestors of extant hummingbirds may have lived in parts of Europe to what is southern Russia today.[11]

There are between 325 and 340 species of hummingbird, depending on taxonomic viewpoint, divided into two subfamilies, the hermits (subfamily Phaethornithinae, 34 species in six genera), and the typical hummingbirds (subfamily Trochilinae, all the others). However, recent phylogenetic analyses suggest that this division is slightly inaccurate, and that there are nine major clades of hummingbirds: the topazes and jacobins, the hermits, the mangoes, the coquettes, the brilliants, the giant hummingbird (Patagona gigas), the mountain-gems, the bees, and the emeralds.[12] The topazes and jacobins combined have the oldest split with the rest of the hummingbirds. The hummingbird family has the second greatest number of species of any bird family on Earth (after the tyrant flycatchers).

Fossil hummingbirds are known from the Pleistocene of Brazil and the Bahamas; however, neither has yet been scientifically described, and there are fossils and subfossils of a few extant species known. Until recently, older fossils had not been securely identifiable as those of hummingbirds.

In 2004, Dr Gerald Mayr of the Senckenberg Museum in Frankfurt am Main identified two 30-million-year-old hummingbird fossils and published his results in the journal Science.[13] The fossils of this primitive hummingbird species, named Eurotrochilus inexpectatus ("unexpected European hummingbird"), had been sitting in a museum drawer in Stuttgart; they had been unearthed in a clay pit at Wiesloch–Frauenweiler, south of Heidelberg, Germany, and, because it was assumed that hummingbirds never occurred outside the Americas, were not recognized to be hummingbirds until Mayr took a closer look at them.

Fossils of birds not clearly assignable to either hummingbirds or a related, extinct family, the Jungornithidae, have been found at the Messel pit and in the Caucasus, dating from 40–35 mya; this indicates that the split between these two lineages indeed occurred at that date. The areas where these early fossils have been found had a climate quite similar to that of the northern Caribbean or southernmost China during that time. The biggest remaining mystery at the present time is what happened to hummingbirds in the roughly 25 million years between the primitive Eurotrochilus and the modern fossils. The astounding morphological adaptations, the decrease in size, and the dispersal to the Americas and extinction in Eurasia all occurred during this timespan. DNA-DNA hybridization results[14] suggest that the main radiation of South American hummingbirds took place at least partly in the Miocene, some 12 to 13 million years ago, during the uplifting of the northern Andes.

In 2013, a 50-million-year-old fossil bird unearthed in Wyoming was found to be a predecessor to both hummingbirds and swifts before the groups diverged.[15]

Lists of species and genera

Specialized characteristics and metabolism

With the exception of insects, hummingbirds while in flight have the highest metabolism of all animals, a necessity in order to support the rapid beating of their wings during hovering and fast forward flight.[3][16] Their heart rate can reach as high as 1,260 beats per minute, a rate once measured in a blue-throated hummingbird,[17] with a breathing rate of 250 breaths per minute, even at rest.[18] During flight, oxygen consumption per gram of muscle tissue in a hummingbird is approximately 10 times higher than that seen for elite human athletes.[3]

Hummingbirds consume more than their own weight in nectar each day, and to do so they must visit hundreds of flowers daily. Hummingbirds are continuously hours away from starving to death and are able to store just enough energy to survive overnight.[19]

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 (in comparison, human athletes max out at around 30%). One study[20] showed that hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption.[21] These data suggest that hummingbirds are able to oxidize sugar in flight muscles at rates high enough to satisfy their extreme metabolic demands. By relying on newly ingested sugars to fuel flight, hummingbirds can reserve their limited fat stores to sustain them overnight fasting or to power migratory flights.[20]

The dynamic range of metabolic rates in hummingbirds[22] requires a corresponding dynamic range in kidney function.[23] The glomerulus is a cluster of capillaries in each nephron of the kidney that removes certain substances from the blood, like a filtration mechanism. The rate at which blood is processed is called the glomerular filtration rate (GFR). Most often these fluids are reabsorbed by the kidneys. GFR also slows when a bird is undergoing water deprivation. The interruption of GFR is a survival and physiological mechanism unique to hummingbirds.[23]

Studies of hummingbirds' metabolisms are relevant to the question of how a migrating ruby-throated hummingbird can cross 800 km (500 mi) of the Gulf of Mexico on a nonstop flight.[18] This hummingbird, like other birds preparing to migrate, stores fat as a fuel reserve, thereby augmenting its weight by as much as 100% and hence increasing potential flying time over open water.[18][24]

Torpor

Hummingbirds are capable of slowing their metabolism at night or any time food is not readily available, entering a hibernation-like, deep sleep state known as torpor needed to prevent energy reserves from falling to a critical level. During nighttime torpor, body temperature falls from 40oC to 18oC,[25] with heart and breathing rates both slowed dramatically (heart rate to roughly 50 to 180 beats per minute from its daytime rate of higher than 1000).[26]

During torpor, to prevent dehydration, the kidney glomerular filtration rate ceases, preserving needed compounds like glucose, water and nutrients.[23] Further, body mass declines throughout nocturnal torpor at a rate of 0.04 g per hour, amounting to about 10% of weight loss each night.[23] The circulating hormone, corticosterone, is one signal that arouses a hummingbird from torpor.[27]

Use and duration of torpor vary among hummingbird species and are affected by whether a dominant bird defends territory, with non-territorial subordinate birds having longer periods of torpor.[28]

Lifespan

Hummingbirds have 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 leaving the nest (fledging), those that survive may occasionally live a decade or more. Among the better-known North American species, the average lifespan is probably 3 to 5 years. By comparison, the smaller shrews, among the smallest of all mammals, seldom live more than 2 years.[29] The longest recorded lifespan in the wild is that of a female broad-tailed hummingbird that was banded (ringed) as an adult at least one year old, then recaptured 11 years later, making her at least 12 years old. Other longevity records for banded hummingbirds include an estimated minimum age of 10 years 1 month for a female black-chinned similar in size to the broad-tailed, and at least 11 years 2 months for a much larger buff-bellied hummingbird.[30]

Reproduction

As far as is known, male hummingbirds do not take part in nesting.[31] Most species build a cup-shaped nest on the branch of a tree or shrub; although a few tropical species normally attach their nests to leaves. The nest varies in size relative to the particular species—from smaller than half a walnut shell to several centimeters (1 centimeter = 0.39 inches) in diameter.[31]

Many hummingbird species use spider silk and lichen to bind the nest material together and secure the structure.[32] The unique properties of the silk allow the nest to expand as the young hummingbirds grow. Two white eggs are laid,[32] which, despite being the smallest of all bird eggs, are in fact large relative to the adult hummingbird's size. Incubation lasts 14 to 23 days,[32] depending on the species, ambient temperature and female attentiveness to the nest.[31] The mother feeds her nestlings on small arthropods and nectar by inserting her bill into the open mouth of a nestling, and then regurgitating the food into its crop.[31]

Wing structure and colors

Many of the hummingbird species have bright plumage with exotic coloration. In many species, the coloring does not come from pigmentation in the feather structure, but instead from prism-like cells within the top layers of the feathers. When light hits these cells, it is split into wavelengths that reflect to the observer in varying degrees of intensity. The hummingbird feather structure acts as a diffraction grating. The result is that, merely by shifting position, a muted-looking bird will suddenly become fiery red or vivid green. However, not all hummingbird colors are due to the prism feather structure. The rusty browns of Allen's and rufous hummingbirds come from pigmentation. Iridescent hummingbird colors actually result from a combination of refraction and pigmentation, since the diffraction structures themselves are made of melanin, a pigment.[33]

Aerodynamics of flight

A female ruby-throated hummingbird hovering in mid-air
A trail of wake vortices generated by a hummingbird's flight. Discovered after training a bird to fly through a cloud of neutrally buoyant helium-filled soap bubbles and recording airflows in the wake with stereo photography.[34]

Hummingbird flight has been studied intensively from an aerodynamic perspective using wind tunnels and high-speed video cameras.

Two studies of rufous or Anna's hummingbirds in a wind tunnel used particle image velocimetry techniques to investigate the lift generated on the bird's upstroke and downstroke.[35][36] The authors concluded that the birds produced 75% of their weight support during the downstroke and 25% during the upstroke. Many earlier studies had assumed that lift was generated equally during the two phases of the wingbeat cycle, as is the case of insects of a similar size. This finding shows that hummingbird hovering is similar to, but distinct from, that of hovering insects such as the hawk moths. Further studies using electromyography in hovering rufous hummingbirds showed that muscle strain in the pectoralis major (principal downstroke muscle) was the lowest yet recorded in a flying bird, and the primary upstroke muscle (supracoracoideus) is proportionately larger than in other bird species.[37] Hummingbird hovering has been estimated to be 20% more efficient than performed by a helicopter drone.[38]

The giant hummingbird's wings beat is as low as 12 beats per second[39] and the wings of typical hummingbirds beat up to 80 times per second.[38]

A slow motion video has shown how the hummingbirds deal with rain when they are flying. To remove the water from their heads, they shake their heads and body, similar to a dog shaking to shed water.[40] Further, when raindrops collectively may weigh as much as 38% of the bird's body weight, hummingbirds shift their bodies and tails horizontally, beat their wings faster, and reduce their wings' angle of motion when flying in heavy rain.[41]

Feather sonation

Courtship dives

The outer tail-feathers of male Anna's hummingbird (Calypte anna) vibrate during courtship display dives and produce a loud chirp.[42] When courting, the male ascends some 35 meters before diving over an interested female at a speed of 27 m/s, equal to 385 body lengths/second, producing a high-pitched sound.[43] This downward acceleration during a dive is the highest reported body length displacement for any vertebrate undergoing a voluntary aerial maneuver; by comparison, it is about twice the diving speed of peregrine falcons in pursuit of prey.[43] At maximum descent speed, approximately 10 g of gravitational force occurs in the courting hummingbird during a dive.[43] By comparison to humans, this is a g-force acceleration causing near loss of consciousness in fighter pilots during flight of fixed-wing aircraft in a high-speed banked turn.[43][44]

Experiments showed that hummingbirds could not make the courtship dive sound when missing their outer tail-feathers, and that those same feathers could produce the dive-sound in a wind tunnel.[42] The bird can sing at the same frequency as the tail-feather chirp, but its small syrinx is not capable of the same volume.[45] Further studies showed that 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.[42][46]

Many other species of hummingbirds also produce sounds with their wings or tail, including the wings of broad-tailed hummingbird, rufous hummingbird, Allen's hummingbird, streamertail, as well as the tail of the Costa's hummingbird and the black-chinned hummingbird, and a number of related species.[47] However, the harmonics of sounds during courtship dives vary across species of hummingbirds.[48]

Wing 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.[49] Studies showed that 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.[49]

Behaviorally, the whistle serves several purposes[49]

Range

Hummingbirds are restricted to the Americas from southern Alaska to Tierra del Fuego, including the Caribbean. The majority of species occur in tropical and subtropical Central and South America, but several species also breed in temperate climates and some hillstars occur even in alpine Andean highlands at altitudes of up to 5,200 metres (17,100 ft).[50] The greatest species richness is in humid tropical and subtropical forests of the northern Andes and adjacent foothills, but the number of species found in the Atlantic Forest, Central America or southern Mexico also far exceeds the number found in southern South America, the Caribbean islands, the United States and Canada. While fewer than 25 different species of hummingbirds have been recorded from the United States and fewer than 10 from Canada and Chile each,[51] Colombia alone has more than 160[52] and the comparably small Ecuador has about 130 species.[53] The migratory ruby-throated hummingbird breeds in the southeastern United States, while the black-chinned hummingbird, its close relative and another migrant, is the most widespread and common species in the southwestern United States. The rufous hummingbird is the most widespread species in western North America.[54]

Migration

Most hummingbirds of the U.S. and Canada migrate south in fall to spend winter in Mexico or Central America. A few southern South American species also move north to the tropics during the southern winter. A few species are year-round residents of California and far southern desert regions of the USA. Among these are Anna's hummingbird, a common resident from southern Arizona and inland California, and buff-bellied hummingbird, an uncommon resident in subtropical woodlands of southern Texas east through the Gulf coast to the Atlantic coast of Florida. Ruby-throated hummingbirds migrate from as far north as Ontario, Canada in summer, returning to Mexico, South America, southern Texas, and Florida to winter.[55]

The rufous hummingbird breeds further north than any other species of Hummingbird, often breeding in large numbers in temperate western North America and winter in increasing numbers in the subtropical Gulf and Florida, rather than in tropical Mexico. By migrating in spring as far north as the Yukon or southern Alaska,[56] the rufous hummingbird nests farther north than any other hummingbird species and must tolerate occasional temperatures below freezing in its breeding territory. This cold hardiness enables it to survive temperatures below freezing, provided that adequate shelter and food are available.

The northward migration of rufous hummingbirds occurs along the Pacific flyway[56] and may be time-coordinated with flower and tree leaf emergence in spring in early March, and also with availability of insects as food.[57] Arrival at breeding grounds before nectar availability from mature flowers may jeopardize breeding opportunities, a factor of phenology possibly determining future migratory patterns linked to climate change.[58]

Diet and specializations for food gathering

Green violetear at a flower.
Hummingbird in Copiapó, Chile. The apparent slow movement of its wings are as a result of the stroboscopic effect.

Hummingbirds drink nectar, a sweet liquid inside certain flowers. Like bees, they are able to assess the amount of sugar in the nectar they eat; they normally reject flower types that produce nectar that is less than 10% sugar and prefer those whose sugar content is higher. Nectar is a mixture of glucose, fructose, and sucrose, and is a poor source of nutrients, so hummingbirds meet their needs for protein, amino acids, vitamins, minerals, etc. by preying on insects and spiders.[59]

Hummingbird bill shapes vary dramatically, as an adaptation for specialized feeding. Some species, such as hermits (Phaethornis spp.) have bills that are long allowing them to probe deep into flowers that have a long corolla. Thornbills have short, sharp bills adapted for feeding from flowers with short corollas and piercing the bases of longer ones. The sicklebills' extremely decurved bills are adapted to extracting nectar from the curved corollas of flowers in the family Gesneriaceae. The bill of the fiery-tailed awlbill has an upturned tip, as in the avocets. The male tooth-billed hummingbird has barracuda-like spikes at the tip of its long, straight bill.

The two halves of a hummingbird's bill have a pronounced overlap, with the lower half (mandible) fitting tightly inside the upper half (maxilla). When hummingbirds feed on nectar, the bill is usually opened only slightly, allowing the tongue to dart out and into the interior of flowers. Hummingbird bill sizes range from about 5 millimeters to as long as 100 millimeters (about 4 inches).[60]

Hummingbirds drink with their tongue by rapidly lapping nectar. Their tongues have tubes which run down their lengths and help the hummingbirds drink the nectar. While it had been believed that capillary action was what drew nectar into these tubes, high-speed photography has 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.[61][62] Consequently, tongue flexibility enables accessing, transporting and unloading nectar.[63]

Hummingbirds do not spend all day flying, as the energy cost would be prohibitive; the majority of their activity consists simply of sitting or perching. Hummingbirds eat many small meals and consume approximately half their weight in pure sugar (twice their weight in nectar, if the nectar is 25% sugar) each day.[64] Hummingbirds digest their food rapidly due to their small size and high metabolism; a mean retention time (MRT) of less than an hour has been reported.[65] Hummingbirds spend an average of 10–15% of their time feeding and 75–80% sitting and digesting.

Because they starve so easily, hummingbirds are highly attuned to food sources. Some species, including many found in North America, are territorial and will try to guard food sources (such as a feeder) against other hummingbirds, attempting to ensure a future food supply for itself.

Feeders and artificial nectar

Hummingbirds will either hover or perch to feed; red feeders are preferred, but colored liquid is not necessary.

In the wild, hummingbirds visit flowers for food, extracting nectar, which is 55% sucrose, 24% glucose and 21% fructose.[66] Hummingbirds will also take sugar-water from bird feeders. Such feeders 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 reduce the amount of pollination their feeding naturally provides.[67]

White granulated sugar is the best sweetener to use in hummingbird feeders. A ratio of 1 part sugar to 4 parts water (20% sugar) is a common recipe,[68] although hummingbirds will defend feeders more aggressively when sugar content is at 35%, indicating preference for nectar with higher sweetness and sugar content.[69] Boiling and then cooling this mixture before use has been recommended to help deter the growth of bacteria and fungi. Powdered sugars contain corn starch as an anti-caking agent which can contribute to premature fermentation of the solution. Brown, turbinado, and "raw" sugars contain iron, which can be deadly to hummingbirds if consumed over long periods.[70] 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.[71][72][73]

Red food dye is often added to homemade solutions, however is not necessary. Commercial products sold as "instant nectar" or "hummingbird food" may also contain preservatives and/or artificial flavors as well as dyes, and are not necessary. The long-term effects of these additives on hummingbirds have not been studied.[74] Although some commercial products contain small amounts of nutritional additives, hummingbirds obtain all necessary nutrients from the insects they eat. This renders the added nutrients unnecessary in most situations.[54]

Other animals also visit hummingbird feeders. Bees, wasps, and ants are attracted to the sugar-water and may crawl into the feeder, where they may become trapped and drown. Orioles, woodpeckers, bananaquits, and other larger animals are known to drink from hummingbird feeders, sometimes tipping them and draining the liquid.[75] In the southwestern United States, two species of nectar-drinking bats (Leptonycteris yerbabuenae and Choeronycteris mexicana) visit hummingbird feeders to supplement their natural diet of nectar and pollen from saguaro cacti and agaves.[76]

Superficially similar birds

Some species of sunbirds of Africa, southern and southeastern Asia, and Australia resemble hummingbirds in appearance and behavior, as do perhaps also the honeyeaters of Australia and Pacific islands. These two groups, however, are not related to hummingbirds, as their resemblance is due to convergent evolution.[77]

The hummingbird moth is often mistaken for a hummingbird.

In myth and culture

Huitzilopochtli wielding the xiuhcoatl, as depicted in the Tovar Codex.

Gallery

See also

References

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External links

Wikiquote has quotations related to: Hummingbirds
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Look up hummingbird in Wiktionary, the free dictionary.
Wikisource has the text of the 1920 Encyclopedia Americana article Humming-birds.