Editing Flightless bird (section)

==Morphological changes and energy conservation==
Two key differences between flying and flightless birds are the smaller wing bones of flightless birds<ref>{{cite journal |doi=10.1111/j.1463-6395.2009.00391.x |title=A shortening of the manus precedes the attenuation of other wing-bone elements in the evolution of flightlessness in birds |journal=Acta Zoologica |volume=91 |pages=115–122 |year=2010 |last1=Nudds |first1=R. L. |last2=Davidson |first2=J. Slove }}</ref> and the absent (or greatly reduced) [[keel (bird anatomy)|keel]] on their breastbone, which anchors muscles needed for wing movement.<ref name=NHM />

Adapting to a cursorial lifestyle causes two inverse morphological changes to occur in the skeleto-muscular system: the pectoral apparatus used to power flight is paedorphically reduced while [[peramorphosis]] leads to enlargement of the pelvic girdle for running.<ref name="Baker, A. J. 2014">{{Cite journal |pmid = 24825849|year = 2014|last1 = Baker|first1 = A. J.|title = Genomic support for a moa-tinamou clade and adaptive morphological convergence in flightless ratites|journal = Molecular Biology and Evolution|volume = 31|issue = 7|pages = 1686–96|last2 = Haddrath|first2 = O.|last3 = McPherson|first3 = J. D.|last4 = Cloutier|first4 = A.|doi = 10.1093/molbev/msu153|doi-access = free}}</ref> Repeated selection for [[Cursorial hunting|cursorial trait]]s across ratites suggests these adaptions comprise a more efficient use of energy in adulthood.<ref name="Phillips, M. J. 2010"/> The name "ratite" comes from the Latin ''ratis'', raft, a vessel with no [[keel]]. Their flat sternum is distinct from the typical sternum of flighted birds because it lacks a keel, like a raft. This structure is the place where flight muscles attach and thus allow for powered flight.<ref name="Smith, J. V. 2012">{{Cite journal |pmid = 22831877|year = 2013|last1 = Smith|first1 = J. V.|title = Ratite nonmonophyly: Independent evidence from 40 novel Loci|journal = Systematic Biology|volume = 62|issue = 1|pages = 35–49|last2 = Braun|first2 = E. L.|last3 = Kimball|first3 = R. T.|doi = 10.1093/sysbio/sys067|doi-access = free}}</ref> However, ratite anatomy presents other primitive characters meant for flight, such as the fusion of wing elements, a cerebellar structure, the presence of a [[pygostyle]] for tail feathers, and an [[alula]] on the wing.<ref name="Cracraft, J. 1974"/> These morphological traits suggest some affinities to volant groups. Palaeognathes were one of the first colonizers of [[Niche (ecology)|novel niches]] and were free to increase in abundance until the population was limited by food and territory. A study looking at energy conservation and the evolution of flightlessness hypothesized intraspecific competition selected for a reduced individual energy expenditure, which is achieved by the loss of flight.<ref name="McNab, B. K. 1994">{{Cite journal |jstor = 2462941|title = Energy Conservation and the Evolution of Flightlessness in Birds|journal = The American Naturalist|volume = 144|issue = 4|pages = 628–642|last1 = McNab|first1 = Brian K.|year = 1994|doi = 10.1086/285697|s2cid = 86511951}}</ref>

Some flightless varieties of island birds are closely related to flying varieties, implying flight is a significant [[biological cost]].<ref name="McNab, B. K. 1994" /> Flight is the most costly type of locomotion exemplified in the natural world. The energy expenditure required for flight increases proportionally with body size, which is often why flightlessness coincides with body mass.<ref name="Noble, J. C. 1991"/> By reducing large pectoral muscles that require a significant amount of overall metabolic energy, ratites decrease their basal metabolic rate and conserve energy.<ref name="McNab, B. K. 1994" /><ref name="Cubo, J 2000">{{Cite journal | doi=10.1023/A:1011695406277|title = Patterns of correlated character evolution in flightless birds: A phylogenetic approach| journal=Evolutionary Ecology| volume=14| issue=8| pages=693–702|year = 2000|last1 = Cubo|first1 = Jorge| last2=Arthur| first2=Wallace| bibcode=2000EvEco..14..693C | citeseerx=10.1.1.115.1294|s2cid = 951896}}</ref> A study looking at the basal rates of birds found a significant correlation between low basal rate and pectoral muscle mass in kiwis. On the contrary, flightless penguins exhibit an intermediate basal rate. This is likely because penguins have well-developed pectoral muscles for hunting and diving in the water.<ref name="McNab, B. K. 1994"/> For ground-feeding birds, a cursorial lifestyle is more economical and allows for easier access to dietary requirements.<ref name="Phillips, M. J. 2010"/> Flying birds have different wing and feather structures that make flying easier, while flightless birds' wing structures are well adapted to their environment and activities, such as diving in the ocean.<ref>{{Cite journal | doi=10.1073/pnas.1304838110| pmid=23690614| pmc=3677478|title = High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins| journal=Proceedings of the National Academy of Sciences| volume=110| issue=23| pages=9380–9384|year = 2013|last1 = Elliott|first1 = Kyle H.| last2=Ricklefs| first2=Robert E.| last3=Gaston| first3=Anthony J.| last4=Hatch| first4=Scott A.| last5=Speakman| first5=John R.| last6=Davoren| first6=Gail K.| bibcode=2013PNAS..110.9380E| doi-access=free}}</ref>

Species with certain characteristics are more likely to evolve flightlessness. For example, species that already have shorter wings are more likely to lose flight ability.<ref>{{cite journal |last1=McCall |first1=Robert A. |last2=Nee |first2=Sean |last3=Harvey |first3=Paul H. |title=The role of wing length in the evolution of avian flightlessness |journal=Evolutionary Ecology |date=July 1998 |volume=12 |issue=5 |pages=569–580 |doi=10.1023/A:1006508826501 |bibcode=1998EvEco..12..569M |s2cid=37855732 }}</ref> Some species will evolve flatter wings so that they move more efficiently underwater at the cost of their flight.<ref>{{Cite journal |last=Haidr |first=Nadia Soledad |date=June 2023 |title=Ecomorphological variation of the penguin wing |journal=Journal of Morphology |language=en |volume=284 |issue=6 |pages=e21588 |doi=10.1002/jmor.21588 |issn=0362-2525|doi-access=free |pmid=37183492 }}</ref> Additionally, birds that undergo simultaneous wing molt, in which they replace all of the feathers in their wings at once during the year, are more likely to evolve flight loss.<ref>{{Cite journal |last=Terrill |first=Ryan S. |date=2020-12-01 |title=Simultaneous Wing Molt as a Catalyst for the Evolution of Flightlessness in Birds |journal=The American Naturalist |volume=196 |issue=6 |pages=775–784 |doi=10.1086/711416 |pmid=33211563 |s2cid=225249314 }}</ref>

A number of bird species appear to be in the process of losing their powers of flight to various extents. These include the [[Zapata rail]] of [[Cuba]], the [[Okinawa rail]] of [[Japan]], and the [[Laysan duck]] of [[Hawaii]]. All of these birds show adaptations common to flightlessness, and evolved recently from fully flighted ancestors, but have not yet completely given up the ability to fly. They are, however, weak fliers and are incapable of traveling long distances by air.<ref>{{cite book |last=Roots |first=Clive |year=2006 |title=Flightless Birds |url=https://archive.org/details/flightlessbirds00root_463 |url-access=limited |location=Westport, CT |publisher=Greenwood |pages=[https://archive.org/details/flightlessbirds00root_463/page/n156 136]–37|isbn=9780313335457 }}</ref>

===Continued presence of wings in flightless birds===
Although [[selection pressure]] for flight was largely absent, the wing structure has not been lost except in the New Zealand moas.<ref name="Baker, A. J. 2014"/> Ostriches are the fastest running birds in the world and emus have been documented running 50&nbsp;km/h.<ref name="Noble, J. C. 1991"/> At these high speeds, wings are necessary for balance and serving as a parachute apparatus to help the bird slow down. Wings are hypothesized to have played a role in [[sexual selection]] in early ancestral ratites and were thus maintained. This can be seen today in both the rheas and ostriches. These ratites utilize their wings extensively for courtship and displays to other males.<ref name="Cracraft, J. 1974"/> Sexual selection also influences the maintenance of large body size, which discourages flight. The large size of ratites leads to greater access to mates and higher [[reproductive success]]. Ratites and tinamous are monogamous and mate only a limited number of times per year.<ref>{{Cite journal | doi=10.1111/j.1095-8312.1985.tb00387.x|title = The mating systems of ratites and tinamous: An evolutionary perspective| journal=Biological Journal of the Linnean Society| volume=25| pages=77–104|year = 1985|last1 = Handford|first1 = Paul| last2=Mares| first2=Michael A.}}</ref> High parental involvement denotes the necessity for choosing a reliable mate. In a climatically stable habitat providing year-round food supply, a male's claimed territory signals to females the abundance of resources readily available to her and her offspring.<ref name="Cubo, J 2000"/> Male size also indicates his protective abilities. Similar to the emperor penguin, male ratites incubate and protect their offspring anywhere between 85 and 92 days while females feed. They can go up to a week without eating and survive only off fat stores. The emu has been documented fasting for as long as 56 days.<ref name="Noble, J. C. 1991"/> If no continued pressures warrant the energy expenditure to maintain the structures of flight, selection will tend towards these other traits.

In [[penguin]]s, wing structure is maintained for use in locomotion underwater.<ref>{{cite book |doi=10.1002/9781119990475.ch6 |chapter=Penguins Past, Present, and Future: Trends in the Evolution of the Sphenisciformes |title=Living Dinosaurs |date=2011 |last1=Ksepka |first1=Daniel T. |last2=Ando |first2=Tatsuro |pages=155–186 |isbn=978-0-470-65666-2 }}</ref> Penguins evolved their wing structure to become more efficient underwater at the cost of their efficiency in the air.<ref>{{cite journal |last1=Elliott |first1=Kyle H. |last2=Ricklefs |first2=Robert E. |last3=Gaston |first3=Anthony J. |last4=Hatch |first4=Scott A. |last5=Speakman |first5=John R. |last6=Davoren |first6=Gail K. |title=High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins |journal=Proceedings of the National Academy of Sciences |date=4 June 2013 |volume=110 |issue=23 |pages=9380–9384 |doi=10.1073/pnas.1304838110 |doi-access=free |pmc=3677478 |pmid=23690614 |bibcode=2013PNAS..110.9380E }}</ref>

The only known species of flightless bird in which wings completely disappeared was the gigantic, herbivorous [[moa]] of [[New Zealand]], hunted to extinction by humans by the 15th century. In moa, the entire [[Shoulder girdle|pectoral girdle]] is reduced to a paired [[scapulocoracoid]], which is the size of a finger.<ref>{{cite journal |last1=Huynen |first1=Leon |last2=Suzuki |first2=Takayuki |last3=Ogura |first3=Toshihiko |last4=Watanabe |first4=Yusuke |last5=Millar |first5=Craig D |last6=Hofreiter |first6=Michael |last7=Smith |first7=Craig |last8=Mirmoeini |first8=Sara |last9=Lambert |first9=David M |title=Reconstruction and in vivo analysis of the extinct tbx5 gene from ancient wingless moa (Aves: Dinornithiformes) |journal=BMC Evolutionary Biology |date=December 2014 |volume=14 |issue=1 |page=75 |doi=10.1186/1471-2148-14-75 |doi-access=free |pmid=24885927 |pmc=4101845 |bibcode=2014BMCEE..14...75H }}</ref>