Editing Hummingbird (section)

=== 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/>