Hummingbirds

Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics around the equator. They are small birds, with most species measuring in length. The smallest extant hummingbird species is the bee hummingbird, which weighs less than . The largest hummingbird species is the giant hummingbird, weighing . They are specialized for feeding on flower nectar, but all species also consume flying insects or spiders.

Hummingbirds split from their sister group, the swifts and treeswifts, around 42 million years ago. The common ancestor of extant hummingbirds is estimated to have lived 22 million years ago in South America. 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, which vary from around 12 beats per second in the largest species to around 80 per second in small hummingbirds. Of those species that have been measured during flying in wind tunnels, their top speeds exceed . During courtship, some male species dive from of height above a female at speeds around .

Hummingbirds have the highest mass-specific metabolic rate of any homeothermic animal. To conserve energy when food is scarce and at night when not foraging, they can enter torpor, a state similar to hibernation, and slow their metabolic rate to 1/15 of its normal rate.

Taxonomy and systematics

The family Trochilidae was introduced in 1825 by Irish zoologist Nicholas Aylward Vigors with ''Trochilus'' as the type genus. Molecular phylogenetic studies of the hummingbirds have shown that the family is composed of nine major clades. 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.

Molecular phylogenetic studies determined the relationships between the major groups of hummingbirds. In the cladogram below, the English names are those introduced in 1997. The Latin names are those introduced in 2013.

In traditional taxonomy, hummingbirds are placed in the order Apodiformes, which also contains the swifts, but some taxonomists have separated them into their own order, the 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 species diversity is greatest, possible ancestors of extant hummingbirds may have lived in parts of Europe and what is southern Russia today.

Around 360 hummingbirds have been described. They have been traditionally divided into two subfamilies: the hermits (subfamily Phaethornithinae) and the typical hummingbirds (subfamily Trochilinae, all the others). Molecular phylogenetic studies have shown, though, that the hermits are sister to the topazes, making the former definition of the Trochilinae not monophyletic. The hummingbirds form nine major clades: the topazes and jacobins, the hermits, the mangoes, the coquettes, the brilliants, the giant hummingbird (''Patagona gigas''), the mountaingems, the bees, and the emeralds. The topazes and jacobins combined have the oldest split with the rest of the hummingbirds. The hummingbird family has the third-greatest number of species of any bird family (after the tyrant flycatchers and the tanagers).

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

In 2004, Gerald Mayr identified two 30-million-year-old hummingbird fossils. 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 hummingbirds were assumed to have 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 35 to 40 million years ago; this indicates that the split between these two lineages indeed occurred around that time. 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 present 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 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 bird fossil unearthed in Wyoming was found to be a predecessor to both hummingbirds and swifts before the groups diverged.

Evolution

Hummingbirds are thought to have split from other members of Apodiformes, the insectivorous swifts (family Apodidae) and treeswifts (family Hemiprocnidae), about 42 million years ago, probably in Eurasia. Despite their current New World distribution, the earliest known species of hummingbird are known from the early Oligocene (Rupelian ~34–28 million years ago) of Europe, belonging to the genus '' Eurotrochilus,'' which is very similar in its morphology to modern hummingbirds. 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.

A map of the hummingbird family treereconstructed from analysis of 284 of the world's 338 known speciesshows rapid diversification from 22 million years ago. 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 and the birds' continued spread into new geographic areas.

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.

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

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

Within the same geographic region, hummingbird clades co-evolved with nectar-bearing plant clades, affecting mechanisms of pollination. 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'').

Sexual dimorphisms

Hummingbirds exhibit sexual size dimorphism according to Rensch's rule, in which males are smaller than females in small-bodied species, and males are larger than females in large-bodied species. The extent of this sexual size difference varies among clades of hummingbirds. For example, the Mellisugini clade (bees) exhibits a large size dimorphism, with females being larger than males. Conversely, the Lesbiini clade (coquettes) displays very little size dimorphism; males and females are similar in size.
Sexual dimorphisms in bill size and shape are also present between male and female hummingbirds, where in many clades, females have longer, more curved bills favored for accessing nectar from tall flowers. For males and females of the same size, females tend to have larger bills.

Sexual size and bill differences likely evolved due to constraints imposed by courtship, because mating displays of male hummingbirds require complex aerial maneuvers. Males tend to be smaller than females, allowing conservation of energy to forage competitively and participate more frequently in courtship. Thus, sexual selection favors smaller male hummingbirds.

Female hummingbirds tend to be larger, requiring more energy, with longer beaks that allow for more effective reach into crevices of tall flowers for nectar. Thus, females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size. Directional selection thus favors the larger hummingbirds in terms of acquiring food.

Another evolutionary cause of this sexual bill dimorphism is that the selective forces from competition for nectar between the sexes of each species drives sexual dimorphism. Depending on which sex holds territory in the species, the other sex having a longer bill and being able to feed on a wide variety of flowers is advantageous, decreasing intraspecific competition. For example, in species of hummingbirds where males have longer bills, males do not hold a specific territory and have a lek mating system. In species where males have shorter bills than females, males defend their resources, so females benefit from a longer bill to feed from a broader range of flowers.

Co-evolution with ornithophilous flowers

Hummingbirds are specialized nectarivores and are tied to the ornithophilous flowers upon which they feed. This coevolution implies that morphological traits of hummingbirds, such as bill length, bill curvature, and body mass are correlated with morphological traits of plants, for example corolla and the sicklebills, are co-evolved with a small number of flower species. Even in the most specialized hummingbird–plant mutualisms, though, the number of food plant lineages of the individual hummingbird species increases with time. 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.

Many plants pollinated by hummingbirds produce flowers in shades of red, orange, and bright pink, though the birds take nectar from flowers of other colors as well. 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. Hummingbird-pollinated flowers also produce relatively weak nectar (averaging 25% sugars w/w) containing a high proportion of sucrose, whereas insect-pollinated flowers typically produce more concentrated nectars dominated by fructose and glucose.

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

Bill evolution

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.

Specialized characteristics and metabolism

Humming

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

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).

Wingbeats and flight stability

The highest recorded wingbeats for wild hummingbirds during hovering is 88 per second, as measured for the purple-throated woodstar (''Calliphlox mitchellii'') weighing 3.2 g. The number of beats per second increases above "normal" while hovering during courtship displays (up to 90 per second for the calliope hummingbird, ''Selasphorus calliope''), a wingbeat rate 40% higher than its typical hovering rate.

During turbulent airflow conditions created experimentally in a wind tunnel, hummingbirds exhibit stable head positions and orientation when they hover at a feeder. 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. They also vary the orientation and enlarge the collective surface area of their tail feathers into the shape of a fan. 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. 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.

Vision

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

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. 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.

The enlargement of the brain region responsible for visual processing indicates an enhanced ability for perception and processing of fast-moving visual stimuli which hummingbirds encounter during rapid forward flight, insect foraging, competitive interactions, and high-speed courtship. 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 courtship displays, territorial defense, and predator evasion. 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.

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

Metabolism

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

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 maximize around 30%). Hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption. These data suggest that hummingbirds are able to oxidize sugar in flight muscles at rates high enough to satisfy their extreme metabolic demands. A 2017 review indicated that hummingbirds have in their flight muscles a mechanism for "direct oxidation" of sugars into maximal ATP yield to support their high metabolic rate for hovering, foraging at altitude, and migrating.

By relying on newly ingested sugars to fuel flight, hummingbirds can reserve their limited fat stores to sustain their overnight fasting or to power migratory flights. Studies of hummingbird metabolism address how a migrating ruby-throated hummingbird can cross of the Gulf of Mexico on a nonstop flight. 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.

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 to where the oxygen content of air is reduced, a condition of hypoxic challenge for the high metabolic demands of hummingbirds. 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.

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. 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.

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. 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. When air temperatures rise above , 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. In cold climates, hummingbirds retract their feet into breast feathers to eliminate skin exposure and minimize heat dissipation.

Kidney function

The dynamic range of metabolic rates in hummingbirds requires a parallel dynamic range in kidney function. 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. During brief periods of water deprivation, however, such as in nighttime torpor, GFR drops to zero, preserving body water.

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. 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.

Song and vocal learning

Consisting of chirps, squeaks, whistles and buzzes, hummingbird songs originate from at least seven specialized nuclei in the forebrain. 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). 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.

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. It also produces ultrasonic vocalizations which do not function in communication. 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.

The avian vocal organ, the syrinx, plays an important role in understanding hummingbird song production. 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.

Torpor

The metabolism of hummingbirds can slow at night or at any time when food is not readily available; the birds enter a hibernatory, 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.

During nighttime torpor, body temperature in a Caribbean hummingbird was shown to fall from 40 to 18 °C, with heart and breathing rates both slowed dramatically (heart rate of roughly 50 to 180 bpm from its daytime rate of higher than 1000 bpm). Recordings from a '' Metallura phoebe'' hummingbird in noctural torpor at around 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. 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. 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.

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

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. 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. 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.

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. Among the better-known North American species, the typical lifespan is probably 3 to 5 years. For comparison, the smaller shrews, among the smallest of all mammals, seldom live longer than 2 years. The longest recorded lifespan in the wild relates to 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 hummingbird similar in size to the broad-tailed hummingbird, and at least 11 years 2 months for a much larger buff-bellied hummingbird.Patuxent Wildlife Research Center, Bird Banding Laboratory
Longevity Records AOU Numbers 3930–4920
2009-08-31. Retrieved 2009-09-27.

Predators

Praying mantises have been observed as predators of hummingbirds. Other predators include dragonflies, frogs, orb-weaver spiders, and other birds, such as the roadrunner.

Reproduction

Male hummingbirds do not take part in nesting. Most species build a cup-shaped nest on the branch of a tree or shrub. The nest varies in size relative to the particular species – from smaller than half a walnut shell to several centimeters in diameter.

Many hummingbird species use spider silk and lichen

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