Hummingbird

Hummingbirds exhibit sexual size dimorphism according to Rensch's rule,[13] in which males are smaller than females in small-bodied species, and males are larger than females in large-bodied species.[14] The extent of this sexual size difference varies among clades of hummingbirds.[14][15] For example, the Mellisugini clade (bees) exhibits a large size dimorphism, with females being larger than males.[15] Conversely, the Lesbiini clade (coquettes) displays very little size dimorphism; males and females are similar in size.[15] Sexual dimorphisms in bill size and shape are also present between male and female hummingbirds,[15] where in many clades, females have longer, more curved bills favored for accessing nectar from tall flowers.[16] For males and females of the same size, females will tend to have larger bills.[15]

Sexual size and bill differences likely evolved due to constraints imposed by courtship because mating displays of male hummingbirds require complex aerial maneuvers.[13] Males tend to be smaller than females, allowing conservation of energy to forage competitively and participate more frequently in courtship.[13] Thus, sexual selection will favor smaller male hummingbirds.[13]

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.[16] Thus females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size.[16] Directional selection will thus favor the larger hummingbirds in terms of acquiring food.[14]

Another evolutionary cause of this sexual bill dimorphism is that the selective forces from competition for nectar between the sexes of each species are what drive the sexual dimorphism.[15] Depending on which sex holds territory in the species, it is advantageous for the other sex to have a longer bill and be able to feed on a wide variety of flowers, decreasing intraspecific competition.[16] For example, in species of hummingbirds where males have longer bills, males do not hold a specific territory and have a lek mating system.[16] In species where males have shorter bills than females, males defend their resources and therefore females must have a longer bill in order to feed from a broader range of flowers.[16]

Purple-throated carib feeding at a flower

Hummingbirds are specialized nectarivores[17] 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 length, curvature, and volume.[18] 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. However, even in the most specialized hummingbird-plant mutualisms the number of food plant lineages of the individual hummingbird species increases with time.[19] 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.[20][21]

Many plants pollinated by hummingbirds produce flowers in shades of red, orange, and bright pink, though the birds will 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.[22][23] 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.[24]

Hummingbirds and the plants they visit for nectar have a tight co-evolutionary association, generally called a plant–bird mutualistic network.[25] 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.[25][26]

Upon maturity, males of a particular species, Phaethornis longirostris, the long-billed hermit, appear to be evolving a daggerlike weapon on the beak tip as a secondary sexual trait to defend mating areas.[27]

• List of hummingbird species, sortable alphabetically by common name, binomial name, or taxonomic sequence.

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.[33] The number of beats per second increases above "normal" while hovering during courtship displays (up to 90 beats per second for the calliope hummingbird, Stellula calliope), a wingbeat rate 40% higher than its typical hovering rate.[34]

During turbulent airflow conditions created experimentally in a wind tunnel, hummingbirds exhibit stable head positions and orientation when they hover at a feeder.[35] 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.[35] They also vary the orientation and enlarge the collective surface area of their tail feathers into the shape of a fan.[35] 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.[36] 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.[36]

During evolution, hummingbirds have adapted to the navigational needs of visual processing while in rapid flight or hovering by development of an exceptionally dense array of retinal neurons allowing for increased spatial resolution in the lateral and frontal visual fields.[37] Morphological studies showed that neuronal hypertrophy, relatively the largest in any bird, exists in a brain region called the pretectal nucleus lentiformis mesencephali (or nucleus of the optic tract in mammals) responsible for refining dynamic visual processing while hovering and during rapid flight.[38][39] The enlargement of this 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.[39][40] 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.[41] 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.[41]

Hummingbirds are highly sensitive to stimuli in their visual fields, responding to even minimal motion in any direction by reorienting themselves in midflight.[39][40][36] Their visual sensitivity allows them to precisely hover in place while in complex and dynamic natural environments,[36] functions enabled by the lentiform nucleus which is tuned to fast-pattern velocities, enabling highly tuned control and collision avoidance during forward flight.[39]

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.[3][42] Their heart rate can reach as high as 1,260 beats per minute, a rate once measured in a blue-throated hummingbird, with a breathing rate of 250 breaths per minute, even at rest.[43][44] During flight, oxygen consumption per gram of muscle tissue in a hummingbird is about 10 times higher than that measured in elite human athletes.[3]

Hummingbirds are rare among vertebrates in their ability to rapidly make use of ingested sugars to fuel energetically expensive hovering flight,[45] powering up to 100% of their metabolic needs with the sugars they drink (in comparison, human athletes max out at around 30%). Hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption.[46][47] 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.[48]

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.[46] 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.[44] 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.[44][49]

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.[50] 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.[51][52]

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.[50][53] 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.[50] 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.[50] In cold climates, hummingbirds retract their feet into breast feathers to eliminate skin exposure and minimize heat dissipation.[53]

The dynamic range of metabolic rates in hummingbirds[54] requires a parallel dynamic range in kidney function.[55] 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.[55][56] During brief periods of water deprivation, however, such as in nighttime torpor, GFR drops to zero, preserving body water.[55][56]

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.[57] 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.[56][58]

Consisting of chirps, squeaks, whistles and buzzes,[59] hummingbird songs originate from at least seven specialized nuclei in the forebrain.[60][61] In a genetic expression study, it was shown 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).[60] 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.[60][62]

The blue-throated hummingbird’s song differs from typical oscine songs in its wide frequency range, extending from 1.8 kHz to approximately 30 kHz.[63] It also produces ultrasonic vocalizations which do not function in communication.[63] 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.[63]

The avian vocal organ, the syrinx, plays an important role in understanding hummingbird song production.[64] What makes the hummingbird's syrinx different from 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.[64][65]

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. During nighttime torpor, body temperature falls from 40 to 18 °C,[66] 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).[67]

During torpor, to prevent dehydration, the GFR (Glomerular Filtration Rate) ceases, preserving needed compounds such as glucose, water, and nutrients.[55] 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.[55] The circulating hormone, corticosterone, is one signal that arouses a hummingbird from torpor.[68]

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.[69] The hummingbirds of the Andes in South America are known for entering exceptionally deep torpor and dropping their body temperature.[70]

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.[71] Among the better-known North American species, the average lifespan is probably 3 to 5 years.[71] For comparison, the smaller shrews, among the smallest of all mammals, seldom live longer than 2 years.[72] 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.[73] 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.[74] Due to their small size they are occasionally prey of spiders and insects, particularly praying mantises.[75][76][77]

When courting, the male Anna's hummingbird ascends some 35 metres (115 ft) above a female, before diving at a speed of 27 metres (89 ft) per second – equal to 385 body lengths per second – producing a high-pitched sound near the female at the nadir of the dive.[98] 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.[98] At maximum descent speed, about 10 g of gravitational force occurs in the courting hummingbird during a dive (Note: G-force is generated as the bird pulls out of the dive).[98] By comparison to humans, this is a G-force acceleration well beyond the threshold of causing near loss of consciousness in fighter pilots (occurring at about +5 Gz) during flight of fixed-wing aircraft in a high-speed banked turn.[98][99]

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.[100][101] 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.[100] The bird can sing at the same frequency as the tail feather chirp, but its small syrinx is not capable of the same volume.[102] 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.[100][103]

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,[104] broad-tailed hummingbird, rufous hummingbird, Allen's hummingbird, and streamertail, as well as the tail of the Costa's hummingbird and the black-chinned hummingbird, and a number of related species.[105] The harmonics of sounds during courtship dives vary across species of hummingbirds.[101]

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.[106] 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.[106]

Behaviorally, the trill serves several purposes:[106]

• Announces the sex and presence of a male bird
• Provides audible aggressive defense of a feeding territory and an intrusion tactic
• Enhances communication of a threat
• Favors mate attraction and courtship

Perception of sweetness in nectar evolved in hummingbirds during their genetic divergence from insectivorous swifts, their closest bird relatives.[127] Although the only known sweet sensory receptor, called T1R2,[128] 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.[127] 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.[127]

Hummingbirds drink with their long tongues by rapidly lapping nectar. Their tongues have tubes which run down their lengths and help the hummingbirds drink the nectar.[129] While capillary action was believed to be 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.[130][131] The tongue, which is forked, is compressed until it reaches nectar, then the tongue springs open, the rapid action traps the nectar and the nectar moves up the grooves, like a pump action, with capillary action not involved.[129][132] Consequently, tongue flexibility enables accessing, transporting and unloading nectar.[133][134]

Male ruby-throated hummingbird displaying its tongue

In the wild, hummingbirds visit flowers for food, extracting nectar, which is 55% sucrose, 24% glucose and 21% fructose on a dry-matter basis.[135] Hummingbirds 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, so they reduce the amount of pollination their feeding naturally provides.[136]

White granulated sugar is the best sweetener to use in hummingbird feeders. A ratio of 1 part sugar to 4 parts water, or 25% concentration, is a common recipe,[137] although hummingbirds will defend feeders more aggressively when sugar content is at 35%, indicating preference for nectar with higher sweetness and sugar content.[138] Organic and "raw" sugars contain iron, which can be harmful,[139] and brown sugar, agave syrup, molasses, and artificial sweeteners also should not be used.[140] 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.[141][142][143]

Red food dye was once thought to be a favorable ingredient for homemade solutions, but it is unnecessary, and there is no point in adding it to the nectar. Some people speculate red dye could be bad for the birds, although this claim has not received scientific attention.[144] Commercial products sold as "instant nectar" or "hummingbird food" may also contain preservatives and/or artificial flavors as well as dyes which are unnecessary,[144] although the long-term effects of these additives on hummingbirds have not been systematically studied.[145] Although some commercial products contain small amounts of nutritional additives, hummingbirds obtain all necessary nutrients from the insects they eat, rendering added nutrients unnecessary.[112]

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, raccoons, and other larger animals are known to drink from hummingbird feeders, sometimes tipping them and draining the liquid.[146] 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.[147]

Hummingbirds hovering at an artificial nectar feeder

Hummingbirds have exceptional visual acuity providing them with discrimination of food sources while foraging.[37] Although it is commonly believed that hummingbirds are 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.[148][149] Hummingbirds depend little on visual cues of flower color to beacon to nectar-rich locations, but rather they used surrounding landmarks to find the nectar reward.[150][151]

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.[152] 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.[152] 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.[148][152]