[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|alt= Pink flamingo with grey legs and long neck pressed against body and head tucked under wings|Many birds, like thisএই [[:en: American Flemingo|American Flamingo]],ৰ tuckদৰে theirবহুতো headচৰায়ে intoশুৱাৰ theirসময়ত backমূৰটো whenসুমুৱাই sleepingলয়]]
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats.<ref>{{Cite journal|last=Gauthier-Clerc |first=Michael |month=May |year=2000 |title=Sleep-Vigilance Trade-off in Gadwall during the Winter Period |journal=The Condor |volume=102 |issue=2 |pages=307–13 |url=http://elibrary.unm.edu/sora/Condor/files/issues/v102n02/p0307-p0313.pdf |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2|format=PDF |last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank|issn=0010-5422}}</ref> [[Swift]]s are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight.<ref>{{Cite journal|journal=The Journal of Experimental Biology|volume=205|pages=905–910|date=1 April 2002|title=Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift ''Apus apus''|first=Johan|last=Bäckman|url=http://jeb.biologists.org/cgi/content/full/205/7/905|pmid=11916987|issue=7|issn=0022-0949|author2=A}}</ref> It has been suggested that there may be certain kinds of sleep which are possible even when in flight.<ref>{{Cite journal|last=Rattenborg|first=Niels C. |year=2006 |title=Do birds sleep in flight? |journal=Die Naturwissenschaften |volume=93 |issue=9 |pages=413–25 |doi=10.1007/s00114-006-0120-3|pmid=16688436|month= September|last1=Rattenborg|first1=NC|issn=0028-1042}}</ref> Some birds have also demonstrated the capacity to fall into [[slow-wave sleep]] one [[Cerebral hemisphere|hemisphere]] of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for [[predator]]s by viewing the outer margins of the flock. This adaptation is also known from [[marine mammal]]s.<ref>{{Cite journal|last=Milius |first=S. |date=6 February 1999|title=Half-asleep birds choose which half dozes |journal=Science News Online |volume=155 |issue= 6|page=86 |url=http://findarticles.com/p/articles/mi_m1200/is_6_155/ai_53965042 |doi=10.2307/4011301 |issn=00368423 |jstor=4011301 }}</ref> Communal roosting is common because it lowers the [[thermoregulation|loss of body heat]] and decreases the risks associated with predators.<ref>{{Cite journal|last=Beauchamp |first=Guy |year=1999 |title=The evolution of communal roosting in birds: origin and secondary losses |journal=Behavioural Ecology |volume=10 |issue=6 |pages=675–87 |url=http://beheco.oxfordjournals.org/cgi/content/full/10/6/675 |doi=10.1093/beheco/10.6.675 }}</ref> Roosting sites are often chosen with regard to thermoregulation and safety.<ref>{{Cite journal|last=Buttemer |first=William A.|year=1985 |title=Energy relations of winter roost-site utilization by American goldfinches (''Carduelis tristis'') |journal=[[Oecologia]] |volume=68 |issue=1 |pages=126–32 |url=http://deepblue.lib.umich.edu/bitstream/2027.42/47760/1/442_2004_Article_BF00379484.pdf |doi=10.1007/BF00379484 |format=PDF}}</ref>
Many sleeping birds bend their heads over their backs and tuck their [[beak|bills]] in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. [[Passerine|Perching birds]] have a tendon locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus ''[[Loriculus]]'' roost hanging upside down.<ref>{{Cite journal|last=Buckley |first=F. G. |date=1 January 1968|title=Upside-down Resting by Young Green-Rumped Parrotlets (''Forpus passerinus'') |journal=The Condor |volume=70 |issue=1 |page=89 |doi=10.2307/1366517 |issn=00105422 |author2=Buckley}}</ref> Some [[hummingbird]]s go into a nightly state of [[torpor]] accompanied with a reduction of their metabolic rates.<ref>{{Cite journal|last=Carpenter |first=F. Lynn |year=1974 |title=Torpor in an Andean Hummingbird: Its Ecological Significance |journal=Science |volume=183 |issue=4124 |pages=545–47 |doi=10.1126/science.183.4124.545 |pmid=17773043 |month= February|issn=0036-8075}}</ref> This [[Adaptation|physiological adaptation]] shows in nearly a hundred other species, including [[owlet-nightjar]]s, [[nightjar]]s, and [[woodswallow]]s. One species, the [[Common Poorwill]], even enters a state of [[hibernation]].<ref>{{Cite journal|last=McKechnie |first=Andrew E. |year=2007 |title=Torpor in an African caprimulgid, the freckled nightjar ''Caprimulgus tristigma'' |journal=Journal of Avian Biology |volume=38 |issue=3 |pages=261–66 |doi=10.1111/j.2007.0908-8857.04116.x |last2=Ashdown |first2=Robert A. M. |last3=Christian |first3=Murray B. |last4=Brigham |first4=R. Mark}}</ref> Birds do not have sweat glands, but they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or by using special behaviours like [[urohidrosis]] to cool themselves.
