Editing Hummingbird (section)

== Specialized characteristics and metabolism ==

===Humming===
[[File:Calliope hum(a)oga.ogg|thumb|A calliope hummingbird hovering near a [[Bird feeder#Hummingbird feeders|feeder]], creating the "humming" sound from its rapid wingbeats, while [[Bird vocalization|chirping by vocalization]]]]

Hummingbirds are named for the prominent humming sound their wingbeats make while flying and hovering to feed or interact with other hummingbirds.<ref name="hightower21">{{Cite journal|last1=Hightower |first1=Ben J. |last2=Wijnings |first2=Patrick W.A. |last3=Scholte |first3=Rick |last4=Ingersoll |first4=Rivers |last5=Chin |first5=Diana D. |last6=Nguyen |first6=Jade |last7=Shorr |first7=Daniel |last8=Lentink |first8=David |display-authors=3 |date=2021-03-16 |title=How oscillating aerodynamic forces explain the timbre of the hummingbird's hum and other animals in flapping flight |journal=eLife |volume=10 |page=e63107 |doi=10.7554/elife.63107 |issn=2050-084X |pmc=8055270 |pmid=33724182 |doi-access=free }}</ref> Humming serves communication purposes by alerting other birds of the arrival of a fellow forager or potential mate.<ref name=hightower21/> The humming sound derives from [[aerodynamic force]]s generated by the downstrokes and upstrokes of the rapid wingbeats, causing [[Harmonic oscillator|oscillations and harmonics]] that evoke an acoustic quality likened to that of a [[musical instrument]].<ref name=hightower21/><ref name="hum">{{Cite web |last=Eindhoven University of Technology |date=16 March 2021 |title=New measurement technique unravels what gives hummingbird wings their characteristic sound |url=https://phys.org/news/2021-03-technique-unravels-hummingbird-wings-characteristic.html |access-date=13 May 2021 |publisher=Phys.org}}</ref> The humming sound of hummingbirds is unique among flying animals, compared to the whine of [[mosquito]]es, buzz of [[bee]]s, and "whoosh" of larger birds.<ref name=hightower21/><ref name=hum/>

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).<ref name="inger">{{Cite journal |last1=Ingersoll |first1=Rivers |last2=Lentink |first2=David |date=2018-10-15 |title=How the hummingbird wingbeat is tuned for efficient hovering |journal=Journal of Experimental Biology |volume=221 |issue=20 |doi=10.1242/jeb.178228 |issn=1477-9145 |pmid=30323114 |doi-access=free|bibcode=2018JExpB.221B8228I }}</ref>

=== Vision ===
[[File:Rufous Hummingbird, male 01.jpg|thumb|left|Male rufous hummingbird (''Selasphorus rufus'') displaying a proportionally large eye in relation to its head]]

Although hummingbird eyes are small in diameter (5–6&nbsp;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.<ref name="ocampo">{{Cite journal |last1=Ocampo |first1=Diego |last2=Barrantes |first2=Gilbert |last3=Uy |first3=J. Albert C. |date=2018-09-27 |title=Morphological adaptations for relatively larger brains in hummingbird skulls |journal=Ecology and Evolution |volume=8 |issue=21 |pages=10482–10488 |doi=10.1002/ece3.4513 |issn=2045-7758 |pmc=6238128 |pmid=30464820|bibcode=2018EcoEv...810482O }}</ref>

Further, hummingbird eyes have large [[cornea]]s, which comprise about 50% of the total transverse eye diameter, combined with an extraordinary density of [[retinal ganglion cell]]s responsible for visual processing, containing some 45,000 [[neuron]]s per mm<sup>2</sup>.<ref name="lisney">{{Cite journal |author1=Lisney, T.J. |author2=Wylie, D.R. |author3=Kolominsky, J. |author4=Iwaniuk, A.N. |year=2015 |title=Eye morphology and retinal topography in hummingbirds (''Trochilidae Aves'') |url=https://www.karger.com/Article/FullText/441834 |journal=Brain, Behavior and Evolution |volume=86 |issue=3–4 |pages=176–190 |doi=10.1159/000441834 |pmid=26587582 |doi-access=free}}</ref> 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.<ref name="lisney" />

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 [[geometric terms of location|lateral and frontal]] [[visual field]]s.<ref name=lisney/> [[Morphology (biology)|Morphological]] studies of the hummingbird brain showed that neuronal [[hypertrophy]] {{Ndash}} relatively the largest in any bird {{Ndash}} exists in a region called the ''[[pretectal area|pretectal]] nucleus lentiformis [[Midbrain|mesencephali]]'' (called the ''nucleus of the [[optic tract]]'' in mammals) responsible for refining dynamic visual processing while hovering and during rapid flight.<ref>{{Cite journal |author1=Iwaniuk, A.N. |author2=Wylie, D.R. |year=2007 |title=Neural specialization for hovering in hummingbirds: hypertrophy of the pretectal nucleus Lentiformis mesencephali |url=http://www.psych.ualberta.ca/~dwylie/Iwaniuk%20and%20Wylie%20JCN%202007.pdf |journal=Journal of Comparative Neurology |volume=500 |issue=2 |pages=211–221 |doi=10.1002/cne.21098 |pmid=17111358 |s2cid=15678218}}</ref><ref name="gaede">{{Cite journal |last1=Gaede |first1=A.H. |last2=Goller |first2=B. |last3=Lam |first3=J.P. |last4=Wylie |first4=D.R. |last5=Altshuler |first5=D.L. |year=2017 |title=Neurons responsive to global visual motion have unique tuning properties in hummingbirds |journal=Current Biology |volume=27 |issue=2 |pages=279–285 |doi=10.1016/j.cub.2016.11.041 |pmid=28065606 |doi-access=free |bibcode=2017CBio...27..279G |s2cid=28314419}}</ref>

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.<ref name=gaede/><ref name="sd2017">{{Cite web |date=5 January 2017 |title=Hummingbirds see motion in an unexpected way |url=https://www.sciencedaily.com/releases/2017/01/170105123115.htm |access-date=24 April 2017 |website=ScienceDaily}}</ref> A study of broad-tailed hummingbirds indicated that hummingbirds have a fourth [[Photoreceptor cell#Difference between rods and cones|color-sensitive visual cone]] (humans have three) that detects [[Ultraviolet|ultraviolet light]] and enables discrimination of [[Color#Spectral colors|non-spectral colors]], possibly having a role in flower identity, courtship displays, territorial defense, and predator evasion.<ref name="stoddard">{{Cite journal |author1=Stoddard, M.C. |author2=Eyster, H.N. |author3=Hogan, B.G. |author4=Morris, D.H. |author5=Soucy, E.R. |author6=Inouye, D.W. |date=2020-06-15 |title=Wild hummingbirds discriminate nonspectral colors |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=26  |display-authors=3 |pages=15112–122 |doi=10.1073/pnas.1919377117 |issn=0027-8424 |pmc=7334476 |pmid=32541035 |bibcode=2020PNAS..11715112S |doi-access=free}}</ref> 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.<ref name=stoddard/>

Hummingbirds are highly sensitive to stimuli in their visual fields, responding to even minimal motion in any direction by reorienting themselves in midflight.<ref name=gaede/><ref name=sd2017/><ref name="goller">{{Cite journal |author1=Goller, B. |author2=Altshuler, D.L. |year=2014 |title=Hummingbirds control hovering flight by stabilizing visual motion |journal=Proceedings of the National Academy of Sciences |volume=111 |issue=51 |pages=18375–380 |bibcode=2014PNAS..11118375G |doi=10.1073/pnas.1415975111 |pmc=4280641 |pmid=25489117 |doi-access=free}}</ref> Their visual sensitivity allows them to precisely hover in place while in complex and dynamic natural environments,<ref name=goller/> functions enabled by the [[lentiform nucleus]] which is tuned to fast-pattern velocities, enabling highly tuned control and collision avoidance during forward flight.<ref name=gaede/>

=== Song, vocal learning, and hearing ===
[[File:Acoustic-Divergence-with-Gene-Flow-in-a-Lekking-Hummingbird-with-Complex-Songs-pone.0109241.s010.oga|thumb|Complex songs of male [[wedge-tailed sabrewing]] hummingbirds (''Campylopterus curvipennis'') in [[lek mating|mating leks]] of eastern Mexico<ref name="Gonz">{{cite journal | last1=González | first1=Clementina | last2=Ornelas | first2=Juan Francisco | title=Acoustic divergence with gene flow in a lekking hummingbird with complex songs | journal=PLOS ONE| volume=9 | issue=10 | date=2014-10-01 | issn=1932-6203 | doi=10.1371/journal.pone.0109241 | page=e109241|pmid=25271429|pmc=4182805 | bibcode=2014PLoSO...9j9241G | doi-access=free }}</ref>]]

Many hummingbird species exhibit a diverse vocal repertoire of chirps, squeaks, whistles and buzzes.<ref name="duque">{{cite journal |author1=Duque, F.G. |author2=Carruth, L.L. |title=Vocal communication in hummingbirds |journal=Brain, Behavior and Evolution |volume=97 |issue=3–4 |pages=241–252 |date=2022 |pmid=35073546 |doi=10.1159/000522148 |s2cid=246278322 |url=https://www.karger.com/Article/FullText/522148|type=Review|doi-access=free }}</ref><ref name="clo">{{Cite web |date=2015 |title=Song sounds of various hummingbird species |url=https://www.allaboutbirds.org/guide/browse.aspx?shape=37,11 |access-date=25 June 2016 |website=All About Birds |publisher=The Cornell Lab of Ornithology, Cornell University, Ithaca, New York}}</ref> Vocalizations vary in complexity and spectral content during social interactions, foraging, territorial defense, courtship, and mother-nestling communication.<ref name=duque/> Territorial vocal signals may be produced in rapid succession to discourage aggressive encounters, with the chirping rate and loudness increasing when intruders persist.<ref name=duque/> During the breeding season, male and female hummingbirds vocalize as part of courtship.<ref name=duque/>

Hummingbirds exhibit vocal production learning to enable song variation {{ndash}} "dialects" that exist across the same species.<ref name=duque/> For example, the blue-throated hummingbird's song differs from typical oscine songs in its wide frequency range, extending from 1.8&nbsp;kHz to about 30&nbsp;kHz.<ref name="pytte">{{Cite journal |last1=Pytte |first1=C.L. |last2=Ficken |first2=M.S. |last3=Moiseff |first3=A. |year=2004 |title=Ultrasonic singing by the blue-throated hummingbird: A comparison between production and perception |url=https://www.researchgate.net/publication/8542654 |journal=Journal of Comparative Physiology A |volume=190 |issue=8 |pages=665–673 |doi=10.1007/s00359-004-0525-4 |pmid=15164219 |s2cid=7231117}}</ref> It also produces [[ultrasound|ultrasonic]] vocalizations which do not function in communication.<ref name="pytte"/> 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.<ref name="pytte"/> Anna's, Costa's, long-billed hermits, and Andean hummingbirds have song dialects that vary across habitat locations and phylogenetic clades.<ref name=duque/><ref name="duque2">{{cite journal |author1=Duque, F.G. |author2=Rodríguez-Saltos, C.A. |author3=Wilczynsk, W. |title=High-frequency vocalizations in Andean hummingbirds |journal=Current Biology |volume=28 |issue=17 |pages=R927–R928 |date=September 2018 |pmid=30205060 |doi=10.1016/j.cub.2018.07.058 |s2cid=52188456 |doi-access=free |bibcode=2018CBio...28.R927D }}</ref>
[[File:Calypte anna - Anna's Hummingbird XC109651.mp3|thumb|Song of male Anna's hummingbird (''Calypte anna'')]]

The avian vocal organ, the [[Syrinx (bird anatomy)|syrinx]], plays an important role in understanding hummingbird song production.<ref name="Monte2020">{{Cite journal |last1=Monte |first1=Amanda |last2=Cerwenka |first2=Alexander F. |last3=Ruthensteiner |first3=Bernhard |last4=Gahr |first4=Manfred |last5=Düring |first5=Daniel N. |date=2020-07-06 |title=The hummingbird syrinx morphome: a detailed three-dimensional description of the black jacobin's vocal organ |journal=BMC Zoology |volume=5 |issue=1 |pages=7 |doi=10.1186/s40850-020-00057-3 |issn=2056-3132 |doi-access=free |s2cid=220509046|hdl=20.500.11850/429165 |hdl-access=free }}</ref> 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 [[Eardrum|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.<ref name="Monte2020"/><ref name="Riede2020">{{Cite journal |last1=Riede |first1=Tobias |last2=Olson |first2=Christopher R. |date=2020-02-06 |title=The vocal organ of hummingbirds shows convergence with songbirds |journal=Scientific Reports |language=en |volume=10 |issue=1 |pages=2007 |bibcode=2020NatSR..10.2007R |doi=10.1038/s41598-020-58843-5 |issn=2045-2322 |pmc=7005288 |pmid=32029812}}</ref>

Hummingbird songs originate from at least seven specialized [[nucleus (neuroanatomy)|nuclei]] in the [[forebrain]].<ref name="jarvis">{{Cite journal |last1=Jarvis |first1=Erich D. |last2=Ribeiro |first2=Sidarta |last3=da Silva |first3=Maria Luisa |last4=Ventura |first4=Dora |last5=Vielliard |first5=Jacques |last6=Mello |first6=Claudio V. |year=2000 |title=Behaviourally driven gene expression reveals song nuclei in hummingbird brain |journal=Nature |volume=406 |issue=6796 |pages=628–632 |bibcode=2000Natur.406..628J |doi=10.1038/35020570 |pmc=2531203 |pmid=10949303}}</ref><ref>{{Cite journal |last=Gahr M. |year=2000 |title=Neural song control system of hummingbirds: comparison to swifts, vocal learning (Songbirds) and nonlearning (Suboscines) passerines, and vocal learning (Budgerigars) and nonlearning (Dove, owl, gull, quail, chicken) nonpasserines |journal=J Comp Neurol |volume=486 |issue=2 |pages=182–196 |doi=10.1002/1096-9861(20001016)426:2<182::AID-CNE2>3.0.CO;2-M |pmid=10982462 |s2cid=10763166}}</ref> 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 ([[parrot]]s and [[songbird]]s) and a few groups of mammals (including humans, [[cetacea|whales and dolphins]], and [[bat]]s).<ref name="jarvis"/> Within the past 66 million years, only hummingbirds, parrots, and songbirds out of 23 bird [[order (biology)|orders]] may have independently evolved seven similar forebrain structures for singing and vocal learning, indicating that evolution of these structures is under strong [[epigenetics|epigenetic]] constraints possibly derived from a common ancestor.<ref name="jarvis"/><ref>{{Cite journal |last1=Renne |first1=Paul R. |last2=Deino |first2=Alan L. |last3=Hilgen |first3=Frederik J. |last4=Kuiper |first4=Klaudia F. |last5=Mark |first5=Darren F. |last6=Mitchell |first6=William S. |last7=Morgan |first7=Leah E. |last8=Mundil |first8=Roland |last9=Smit |first9=Jan |display-authors=3 |date=7 February 2013 |title=Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary |url=http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf |url-status=dead |journal=Science |volume=339 |issue=6120 |pages=684–687 |bibcode=2013Sci...339..684R |doi=10.1126/science.1230492 |pmid=23393261 |archive-url=https://web.archive.org/web/20170207164818/http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf |archive-date=7 February 2017 |access-date=1 April 2018 |s2cid=6112274}}</ref>

Generally, birds have been assessed to vocalize and hear in the range of 2–5&nbsp;kHz, with hearing sensitivity falling with higher frequencies.<ref name=duque2/> In the [[Ecuadorian hillstar]] (''Oreotrochilus chimborazo''), vocalizations were recorded in the wild to be at a frequency above 10&nbsp;kHz, well outside of the known hearing ability of most birds.<ref name=duque2/> Song system nuclei in the hummingbird brain are similar to those songbird brains, but the hummingbird brain has specialized regions involved for song processing.<ref name=duque/>

=== 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.<ref name=Hargrove/><ref>{{Cite journal |last1=Altshuler |first1=D.L. |last2=Dudley |first2=R. |year=2002 |title=The ecological and evolutionary interface of hummingbird flight physiology |journal=The Journal of Experimental Biology |volume=205 |issue=Pt 16 |pages=2325–336 |doi=10.1242/jeb.205.16.2325 |pmid=12124359|bibcode=2002JExpB.205.2325A |url=https://journals.biologists.com/jeb/article/205/16/2325/9117/The-ecological-and-evolutionary-interface-of|url-access=subscription }}</ref> 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.<ref name="suarez"/> Hummingbirds achieve this extraordinary capacity for oxygen consumption by an exceptional density and proximity of capillaries and [[mitochondrion|mitochondria]] in their flight muscles.<ref name="suarez91">{{cite journal |author1=Suarez R.K. |author2=Lighton J.R. |author3=Brown G.S. |author4=Mathieu-Costello O. |title=Mitochondrial respiration in hummingbird flight muscles |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=88 |issue=11 |pages=4870–3 |date=June 1991 |pmid=2052568 |pmc=51768 |doi=10.1073/pnas.88.11.4870|bibcode=1991PNAS...88.4870S |doi-access=free }}</ref>

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.<ref>{{Cite journal |last1=Welch |first1=K.C. Jr. |last2=Chen |first2=C.C. |year=2014 |title=Sugar flux through the flight muscles of hovering vertebrate nectarivores: A review |journal=Journal of Comparative Physiology B |volume=184 |issue=8 |pages=945–959 |doi=10.1007/s00360-014-0843-y |pmid=25031038 |s2cid=11109453}}</ref> Hummingbird flight muscles have extremely high capacities for [[Redox|oxidizing]] [[carbohydrate]]s and [[fatty acid]]s via [[hexokinase]], [[carnitine palmitoyltransferase]], and [[citrate synthase]] [[enzyme]]s at rates that are the highest known for vertebrate [[skeletal muscle]].<ref name="fuel">{{cite journal | last1=Suarez |first1=R.K. |last2=Lighton |first2=J.R. |last3=Moyes |first3=C.D.|last4=Brown|first4=G.S.|last5=Gass|first5=C.L.|last6=Hochachka |first6=P.W. |display-authors=3 | title = Fuel selection in rufous hummingbirds: ecological implications of metabolic biochemistry | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 87 | issue = 23 | pages = 9207–10 | date = 1 December 1990 | pmid = 2251266 | pmc = 55133 | doi = 10.1073/pnas.87.23.9207|bibcode=1990PNAS...87.9207S |doi-access=free }}</ref> To sustain rapid wingbeats during flight and hovering, hummingbirds expend the [[human equivalent]] of 150,000 [[calorie]]s per day,<ref name="bartlett">{{cite web |last1=Barlett |first1=Paige |title=Fueling the hummingbird's extreme biology |url=https://www.hopkinsmedicine.org/research/advancements-in-research/fundamentals/in-depth/fueling-the-hummingbirds-extreme-biology |publisher=Johns Hopkins Medicine |access-date=27 March 2023 |date=2018 |archive-date=22 March 2023 |archive-url=https://web.archive.org/web/20230322213148/https://www.hopkinsmedicine.org/research/advancements-in-research/fundamentals/in-depth/fueling-the-hummingbirds-extreme-biology |url-status=dead }}</ref> an amount estimated to be 10 times the energy consumption by a [[marathon]] runner in competition.<ref name="campbell">{{cite web |last1=Campbell|first1=Don |title=Hummingbird metabolism unique in burning glucose and fructose equally |url=https://utsc.utoronto.ca/news-events/archived/hummingbird-metabolism-unique-burning-glucose-and-fructose-equally |publisher=University of Toronto - Scarborough |access-date=27 March 2023 |date=3 December 2013}}</ref>

Hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption.<ref name="chen">{{Cite journal |last1=Chen|first1=Chris Chin Wah |last2=Welch|first2=Kenneth Collins |year=2014 |title=Hummingbirds can fuel expensive hovering flight completely with either exogenous glucose or fructose |journal=Functional Ecology |volume=28 |issue=3 |pages=589–600 |doi=10.1111/1365-2435.12202 |doi-access=free|bibcode=2014FuEco..28..589C }}</ref><ref>{{Cite journal |last1=Welch |first1=K.C. Jr. |last2=Suarez |first2=R.K. |year=2007 |title=Oxidation rate and turnover of ingested sugar in hovering Anna's (''Calypte anna'') and rufous (''Selasphorus rufus'') hummingbirds |journal=Journal of Experimental Biology |volume=210 |issue=Pt 12 |pages=2154–162 |doi=10.1242/jeb.005363 |pmid=17562889 |doi-access=free|bibcode=2007JExpB.210.2154W }}</ref> These data suggest that hummingbirds are able to oxidize sugar in flight muscles at rates rapid enough to satisfy their extreme metabolic demands {{ndash}} as indicated by a 2017 review showing that hummingbirds have in their flight muscles a mechanism for "direct oxidation" of sugars into maximal [[Adenosine triphosphate|ATP]] yield to support a high metabolic rate for hovering, foraging at altitude, and migrating.<ref name="Suarez">{{Cite journal |last1=Suarez |first1=Raul |last2=Welch |first2=Kenneth |date=12 July 2017 |title=Sugar metabolism in hummingbirds and nectar bats |journal=Nutrients |volume=9 |issue=7 |page=743 |doi=10.3390/nu9070743 |issn=2072-6643 |pmc=5537857 |pmid=28704953 |doi-access=free}}</ref> This adaptation occurred through the [[natural selection|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.<ref name="callier">{{cite web |first=Viviane |last=Callier |title=Evolution Turns These Knobs to Make a Hummingbird Hyperquick and a Cavefish Sluggishly Slow|url=https://www.scientificamerican.com/article/evolution-turns-these-knobs-to-make-a-hummingbird-hyperquick-and-a-cavefish-sluggishly-slow/|publisher=Scientific American |date=24 February 2023 |access-date=27 February 2023}}</ref><ref name="osipova">{{cite journal|display-authors=3 |last1=Osipova |first1=Ekaterina |last2=Barsacchi |first2=Rico |last3=Brown |first3=Tom |last4=Sadanandan |first4=Keren |last5=Gaede |first5=Andrea H. |last6=Monte |first6=Amanda |last7=Jarrells |first7=Julia |last8=Moebius |first8=Claudia |last9=Pippel |first9=Martin |last10=Altshuler |first10=Douglas L. |last11=Winkler |first11=Sylke |last12=Bickle |first12=Marc |last13=Baldwin |first13=Maude W. |last14=Hiller |first14=Michael |title=Loss of a gluconeogenic muscle enzyme contributed to adaptive metabolic traits in hummingbirds |journal=Science|volume=379 |issue=6628 |date=2023-01-13 |issn=0036-8075 |doi=10.1126/science.abn7050 |pages=185–190|pmid=36634192 |bibcode=2023Sci...379..185O |s2cid=255749672 |url=https://www.science.org/doi/10.1126/science.abn7050|url-access=subscription }}</ref> Without ''FBP2'', [[glycolysis]] and mitochondrial respiration in flight muscles are enhanced, enabling hummingbirds to metabolize sugar more efficiently for energy.<ref name=callier/><ref name=osipova/>

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.<ref name="chen"/> Studies of hummingbird metabolism address how a [[bird migration|migrating]] ruby-throated hummingbird can cross {{Convert|800|km|mi|abbr=on}} of the [[Gulf of Mexico]] on a nonstop flight.<ref name="Hargrove"/> 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.<ref name="Hargrove"/><ref name="Skutch, 1973">{{Cite book |last1=Skutch |first1=Alexander F. |url=https://archive.org/details/lifeofhummingbir00skut |title=The Life of the Hummingbird |last2=Singer |first2=Arthur B. |publisher=Crown Publishers |year=1973 |isbn=978-0-517-50572-4 |location=New York |url-access=registration |name-list-style=amp}}</ref> 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 {{convert|50|ft|m}}.<ref name="Hargrove"/>

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 [[respiratory rate|breathing rate]] of 250 breaths per minute at rest.<ref name="Hargrove"/><ref>{{Cite journal |last=Lasiewski |first=Robert C. |year=1964 |title=Body temperatures, heart and breathing rate, and evaporative water loss in hummingbirds |journal=Physiological Zoology |volume=37 |issue=2 |pages=212–223 |doi=10.1086/physzool.37.2.30152332 |s2cid=87037075}}</ref>

===Heat dissipation===

The high metabolic rate of hummingbirds &ndash; especially during rapid forward flight and hovering &ndash; produces increased body heat that requires specialized mechanisms of [[thermoregulation]] for heat dissipation, which becomes an even greater challenge in hot, humid climates.<ref name="powers">{{Cite journal |last1=Powers |first1=Donald R. |last2=Langland |first2=Kathleen M. |last3=Wethington |first3=Susan M. |last4=Powers |first4=Sean D. |last5=Graham |first5=Catherine H. |last6=Tobalske |first6=Bret W. |year=2017 |title=Hovering in the heat: effects of environmental temperature on heat regulation in foraging hummingbirds |journal=Royal Society Open Science |volume=4 |issue=12 |page=171056 |doi=10.1098/rsos.171056 |issn=2054-5703 |pmc=5750011 |pmid=29308244}}</ref> 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 ([[patagium|patagia]]), and feet.<ref name="evang">{{Cite journal |last1=Evangelista |first1=Dennis |last2=Fernández |first2=María José |last3=Berns |first3=Madalyn S. |last4=Hoover |first4=Aaron |last5=Dudley |first5=Robert |year=2010 |title=Hovering energetics and thermal balance in Anna's hummingbirds (''Calypte anna'') |url=https://www.researchgate.net/publication/42638033 |journal=Physiological and Biochemical Zoology |volume=83 |issue=3 |pages=406–413 |doi=10.1086/651460 |issn=1522-2152 |pmid=20350142 |s2cid=26974159}}</ref><ref name="soniak">{{Cite web |first=Matt|last=Soniak |date=2 February 2016 |title=Infrared video shows how hummingbirds shed heat through their eyes and feet |url=https://www.mentalfloss.com/article/74571/infrared-video-shows-how-hummingbirds-shed-heat-through-their-eyes-and-feet |access-date=14 January 2020 |publisher=Mental Floss}}</ref>

While hovering, hummingbirds do not benefit from the heat loss by [[convection|air convection]] during forward flight, except for air movement generated by their rapid wing-beat, possibly aiding convective heat loss from the extended feet.<ref name=powers/><ref name="udvardy">{{Cite journal |first=Miklos D.F.|last=Udvardy |date=1983 |title=The role of the feet in behavioral thermoregulation of hummingbirds |url=https://sora.unm.edu/sites/default/files/journals/condor/v085n03/p0281-p0285.pdf |journal=Condor |volume=85 |issue=3 |pages=281–285 |doi=10.2307/1367060|jstor=1367060 }}</ref> 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.<ref name=powers/> When air temperatures rise above {{Convert|36|C}}, 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]].<ref name=powers/> In cold climates, hummingbirds retract their feet into breast feathers to eliminate skin exposure and minimize heat dissipation.<ref name=udvardy/>

=== Kidney function ===

The dynamic range of metabolic rates in hummingbirds<ref>{{Cite journal |last1=Suarez |first1=R.K. |last2=Gass |first2=C.L. |year=2002 |title=Hummingbirds foraging and the relation between bioenergetics and behavior |journal=Comparative Biochemistry and Physiology |series=Part A |volume=133 |issue=2 |pages=335–343 |doi=10.1016/S1095-6433(02)00165-4 |pmid=12208304}}</ref> requires a parallel dynamic range in [[kidney]] function.<ref name="Bakken et al">{{Cite journal |last1=Bakken |first1=B.H. |last2=McWhorter |first2=T.J. |last3=Tsahar |first3=E. |last4=Martinez del Rio |first4=C. |year=2004 |title=Hummingbirds arrest their kidneys at night: diel variation in glomerular filtration rate in Selasphorus platycercus |journal=The Journal of Experimental Biology |volume=207 |issue=25 |pages=4383–391 |doi=10.1242/jeb.01238 |pmid=15557024 |doi-access=free|bibcode=2004JExpB.207.4383B |hdl=2440/55466 |hdl-access=free }}</ref> 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 [[renal function|glomerular filtration rates]] (GFR) in amounts proportional to water consumption, thereby avoiding [[water intoxication|overhydration]].<ref name="Bakken et al"/><ref name="ajp">{{Cite journal |last1=Bakken |first1=B.H. |last2=Sabat |first2=P. |year=2006 |title=Gastrointestinal and renal responses to water intake in the green-backed firecrown (Sephanoides sephanoides), a South American hummingbird |journal=AJP: Regulatory, Integrative and Comparative Physiology |volume=291 |issue=3 |pages=R830–836 |doi=10.1152/ajpregu.00137.2006 |pmid=16614056 |hdl-access=free |s2cid=2391784 |hdl=10533/177203|url=http://americanae.aecid.es/americanae/es/registros/registro.do?tipoRegistro=MTD&idBib=3228740 }}</ref> During brief periods of water deprivation, however, such as in nighttime torpor, GFR drops to zero, preserving body water.<ref name="Bakken et al"/><ref name="ajp"/>

Hummingbird kidneys also have a unique ability to control the levels of [[electrolyte]]s 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 (nutrient)|mineral]] quality.<ref>{{Cite journal |last1=Lotz |first1=Chris N. |last2=Martínez Del Rio |first2=Carlos |year=2004 |title=The ability of rufous hummingbirds ''Selasphorus rufus'' to dilute and concentrate urine |journal=Journal of Avian Biology |volume=35 |pages=54–62 |doi=10.1111/j.0908-8857.2004.03083.x}}</ref> Morphological studies on Anna's hummingbird kidneys showed adaptations of high [[capillary]] density in close proximity to [[nephron]]s, allowing for precise regulation of water and electrolytes.<ref name="ajp"/><ref>{{Cite journal |author1=Beuchat, C.A. |author2=Preest, M.R. |author3=Braun, E.J. |year=1999 |title=Glomerular and medullary architecture in the kidney of Anna's Hummingbird |journal=Journal of Morphology |volume=240 |issue=2 |pages=95–100 |doi=10.1002/(sici)1097-4687(199905)240:2<95::aid-jmor1>3.0.co;2-u |pmid=29847878 |s2cid=44156688}}</ref>

===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 {{Convert|1500|m|ft}} to {{Convert|5200|m|ft}} where the [[Blood gas tension|partial pressure of oxygen]] in the air is reduced, a condition of [[hypoxia (medical)|hypoxic challenge]] for the high metabolic demands of hummingbirds.<ref name="projecto">{{Cite journal |last1=Projecto-Garcia |first1=Joana |last2=Natarajan |first2=Chandrasekhar |last3=Moriyama |first3=Hideaki |last4=Weber |first4=Roy E. |last5=Fago |first5=Angela |last6=Cheviron |first6=Zachary A. |last7=Dudley |first7=Robert |last8=McGuire |first8=Jimmy A. |last9=Witt |first9=Christopher C. |last10=Storz |first10=Jay F. |display-authors=3 |date=2013-12-02 |title=Repeated elevational transitions in hemoglobin function during the evolution of Andean hummingbirds |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=110 |issue=51 |pages=20669–20674 |bibcode=2013PNAS..11020669P |doi=10.1073/pnas.1315456110 |issn=0027-8424 |pmc=3870697 |pmid=24297909 |doi-access=free}}</ref><ref name="guardian">{{Cite news |date=13 December 2013 |title=How do hummingbirds thrive in the Andes? |work=The Guardian |url=https://www.theguardian.com/science/grrlscientist/2013/dec/13/grrlscientist-hummingbirds-andes-hemoglobin-evolution |access-date=15 August 2022}}</ref><ref name="Lim">{{Cite journal |last1=Lim |first1=Marisa C.W. |last2=Witt |first2=Christopher C. |last3=Graham |first3=Catherine H. |last4=Dávalos |first4=Liliana M. |date=2019-05-22 |title=Parallel molecular evolution in pathways, genes, and sites in high-elevation hummingbirds revealed by comparative transcriptomics |journal=Genome Biology and Evolution |volume=11 |issue=6 |pages=1573–1585 |doi=10.1093/gbe/evz101 |issn=1759-6653 |pmc=6553505 |pmid=31114863}}</ref> In Andean hummingbirds living at high elevations, researchers found that the oxygen-carrying protein in blood {{Ndash}} [[hemoglobin]] {{Ndash}} had increased oxygen-[[Ligand (biochemistry)|binding affinity]], and that this adaptive effect likely resulted from evolutionary [[mutation]]s within the hemoglobin molecule via specific amino acid changes due to natural selection.<ref name=projecto/><ref name=guardian/><ref name="gayman">{{Cite web |first=Deann|last=Gayman |date=2 December 2013 |title=New study reveals how hummingbirds evolved to fly at high altitude |url=https://news.unl.edu/newsrooms/today/article/new-study-reveals-how-hummingbirds-evolved-to-fly-at-high-altitude |access-date=15 August 2022 |publisher=Department of Communication and Marketing, University of Nebraska-Lincoln}}</ref>

===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.<ref name="greig">{{cite journal | last1=Greig | first1=Emma I. | last2=Wood | first2=Eric M. | last3=Bonter | first3=David N. | title=Winter range expansion of a hummingbird is associated with urbanization and supplementary feeding | journal= Proceedings of the Royal Society B: Biological Sciences| volume=284 | issue=1852 | date=5 April 2017 | issn=0962-8452 | pmid=28381617 | pmc=5394677 | doi=10.1098/rspb.2017.0256 | page=20170256}}</ref><ref name="battey">{{cite journal | last=Battey | first=C. J. | title=Ecological release of the Anna's hummingbird during a northern range expansion | journal=The American Naturalist| volume=194 | issue=3 | year=2019 | issn=0003-0147 | pmid=31553208 | doi=10.1086/704249 | pages=306–315| s2cid=164398193 | doi-access=free | bibcode=2019ANat..194..306B }}</ref> 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.<ref name=greig/><ref name=battey/>

During cold temperatures, Anna's hummingbirds gradually gain weight during the day as they convert sugar to fat.<ref name="Beuchat">{{cite journal |author1=Beuchat, C.A. |author2=Chaplin, S.B. |author3=Morton, M.L. |journal=Physiological Zoology|pages=280–295|volume=52|issue=3 |year=1979|title=Ambient temperature and the daily energetics of two species of hummingbirds, ''Calypte anna'' and ''Selasphorus rufus''|doi=10.1086/physzool.52.3.30155751 |s2cid=87185364 }}</ref><ref name= Powers>{{cite journal |last=Powers|first=D. R. |url=http://www.dpowerslab.com/wp-content/uploads/2011/09/PZ1991.pdf |jstor=30158211|title=Diurnal variation in mass, metabolic rate, and respiratory quotient in Anna's and Costa's hummingbirds|journal=Physiological Zoology|volume=64|issue= 3 |year=1991|pages=850–870|doi=10.1086/physzool.64.3.30158211|s2cid=55730100}}</ref> 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]].<ref name="shankar">{{Cite journal |last1=Shankar |first1=Anusha |last2=Schroeder |first2=Rebecca J. |last3=Wethington |first3=Susan M. |last4=Graham |first4=Catherine H. |last5=Powers |first5=Donald R. |date=May 2020 |title=Hummingbird torpor in context: duration, more than temperature, is the key to nighttime energy savings |url=https://onlinelibrary.wiley.com/doi/10.1111/jav.02305 |journal=Journal of Avian Biology |language=en |volume=51 |issue=5 |pages=jav.02305 |doi=10.1111/jav.02305 |issn=0908-8857 |s2cid=216458501}}</ref>

While their range was originally limited to the [[chaparral]] of California and [[Baja California Peninsula|Baja California]], it expanded northward to [[Oregon]], [[Washington (state)|Washington]], and [[British Columbia]], and east to [[Arizona]] over the 1960s to 1970s.<ref name=battey/> This rapid expansion is attributed to the widespread planting of [[flora|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 [[biological dispersal|dispersal]].<ref name="test">{{cite journal|url=https://birdsoftheworld.org/bow/species/annhum/1.0/introduction|vauthors=Clark CJ, Russell SM|date=2012|title=Anna's hummingbird (''Calypte anna'')|journal=The Birds of North America Online |publisher=The Birds of North America, Cornell University Laboratory of Ornithology|doi=10.2173/bna.226|url-access=subscription}}</ref> In the Pacific Northwest, the fastest growing populations occur in regions with breeding-season cold temperatures similar to those of its native range.<ref name=battey/> 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.<ref name=greig/><ref name=battey/> 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.<ref name=greig/>

Anna's hummingbird is the official city bird of [[Vancouver, British Columbia]], Canada,<ref>{{cite web|url=https://vancouver.ca/parks-recreation-culture/official-city-bird.aspx|title=Official City Bird: Anna's Hummingbird|publisher=City of Vancouver|date=2019|access-date=6 November 2019}}</ref> 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.<ref name="green">{{cite web |last1=Green|first1=Gregory A. |title=Anna's Hummingbird: Our winter hummingbird |url=https://www.birdwatchingdaily.com/news/species-profiles/annas-hummingbird-our-winter-hummingbird/# |publisher=BirdWatching |access-date=6 November 2019 |date=2 October 2018}}</ref>

=== 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.<ref name="Bakken et al"/>

During nighttime torpor, [[body temperature]] in a Caribbean hummingbird was shown to fall from 40 to 18&nbsp;°C,<ref>{{Cite journal |last1=Hainsworth |first1=F.R. |last2=Wolf |first2=L.L. |year=1970 |title=Regulation of oxygen consumption and body temperature during torpor in a hummingbird, Eulampis jugularis |journal=Science |volume=168 |issue=3929 |pages=368–369 |bibcode=1970Sci...168..368R |doi=10.1126/science.168.3929.368 |pmid=5435893 |s2cid=30793291}}</ref> with heart and [[breathing rate]]s slowing dramatically (heart rate of roughly 50 to 180 bpm from its daytime rate of higher than 1000 bpm).<ref>{{Cite journal |last=Hiebert |first=S.M. |year=1992 |title=Time-dependent thresholds for torpor initiation in the rufous hummingbird (''Selasphorus rufus'') |journal=Journal of Comparative Physiology B |volume=162 |issue=3 |pages=249–255 |doi=10.1007/bf00357531 |pmid=1613163 |s2cid=24688360}}</ref> Recordings from a ''[[Metallura phoebe]]'' hummingbird in noctural torpor at around {{Convert|3800|m|ft}} in the Andes mountains showed that body temperature fell to 3.3&nbsp;°C (38&nbsp;°F), the lowest known level for a bird or non-hibernating mammal.<ref name="wolf">{{Cite journal |last1=Wolf |first1=Blair O. |last2=McKechnie |first2=Andrew E. |last3=Schmitt |first3=C. Jonathan |last4=Czenze |first4=Zenon J. |last5=Johnson |first5=Andrew B. |last6=Witt |first6=Christopher C. |year=2020 |title=Extreme and variable torpor among high-elevation Andean hummingbird species |journal= Biology Letters|volume=16 |issue=9 |page=20200428 |doi=10.1098/rsbl.2020.0428 |issn=1744-9561 |pmc=7532710 |pmid=32898456}}</ref><ref>{{Cite news |last=Greenwood |first=Veronique |date=2020-09-08 |title=These hummingbirds take extreme naps. Some may even hibernate. |language=en-US |work=The New York Times |url=https://www.nytimes.com/2020/09/08/science/hummingbirds-torpor-hibernation.html |access-date=2020-09-09 |issn=0362-4331}}</ref> 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&nbsp;°C per minute and rewarming at 1–1.5&nbsp;°C per minute.<ref name=wolf/> 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.<ref name=wolf/>

During torpor, to prevent [[dehydration]], the [[glomerular filtration rate|kidney function]] declines, preserving needed compounds, such as glucose, water, and nutrients.<ref name="Bakken et al"/> The circulating [[hormone]], [[corticosterone]], is one signal that arouses a hummingbird from torpor.<ref>{{Cite journal |last1=Hiebert |first1=S.M. |last2=Salvante |first2=K.G. |last3=Ramenofsky |first3=M. |last4=Wingfield |first4=J.C. |year=2000 |title=Corticosterone and nocturnal torpor in the rufous hummingbird (''Selasphorus rufus'') |journal=General and Comparative Endocrinology |volume=120 |issue=2 |pages=220–234 |doi=10.1006/gcen.2000.7555 |pmid=11078633}}</ref>

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.<ref>{{Cite journal |last1=Powers |first1=D.R. |last2=Brown |first2=A.R. |last3=Van Hook |first3=J.A. |year=2003 |title=Influence of normal daytime fat deposition on laboratory measurements of torpor use in territorial versus nonterritorial hummingbirds |url=https://digitalcommons.georgefox.edu/cgi/viewcontent.cgi?article=1025&context=bio_fac |journal=Physiological and Biochemical Zoology |volume=76 |issue=3 |pages=389–397 |doi=10.1086/374286 |pmid=12905125 |s2cid=6475160|url-access=subscription }}</ref> 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.<ref name=shankar/> 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.<ref name=shankar/>

=== 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.<ref name="rpbo">{{Cite web |date=2010 |title=The hummingbird project of British Columbia |url=http://rpbo.org/hummingbirds.php |access-date=25 June 2016 |publisher=Rocky Point Bird Observatory, Vancouver Island, British Columbia |archive-date=2 February 2017 |archive-url=https://web.archive.org/web/20170202002338/http://rpbo.org/hummingbirds.php |url-status=dead }}</ref> Among the better-known North American species, the typical lifespan is probably 3 to 5 years.<ref name="rpbo"/> For comparison, the smaller [[shrew]]s, among the smallest of all mammals, seldom live longer than 2 years.<ref name="Churchfield">{{Cite book |last=Churchfield |first=Sara |title=The natural history of shrews |publisher=Cornell University Press |year=1990 |isbn=978-0-8014-2595-0 |pages=35–37}}</ref> 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.<ref>{{Cite web |title=Longevity Records Of North American Birds |url=https://www.pwrc.usgs.gov/BBL/longevity/Longevity_main.cfm |access-date=26 January 2021 |publisher=United States Geological Survey}}</ref> 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]].<ref name="BBL">{{cite web |url=http://www.pwrc.usgs.gov/BBL/homepage/long3930.cfm |title=Longevity Records AOU Numbers 3930–4920 |publisher=Patuxent Wildlife Research Center, Bird Banding Laboratory |date=2009-08-31 |accessdate=2009-09-27}}</ref>