Editing Passenger pigeon (section)

==Ecology and behavior==
[[File:Bird lore (1913) (14562367319).jpg|thumb|left|upright|Live male in Whitman's aviary, 1896/98]]

The passenger pigeon was [[nomad]]ic, constantly migrating in search of food, shelter, or nesting grounds.<ref name="Fuller 2014 30–47"/> In his 1831 ''Ornithological Biography'', American naturalist and artist [[John James Audubon]] described a migration he observed in 1813 as follows:
{{Blockquote|I dismounted, seated myself on an eminence, and began to mark with my pencil, making a dot for every flock that passed. In a short time finding the task which I had undertaken impracticable, as the birds poured in <!--not a mistake-->in countless multitudes, I rose and, counting the dots then put down, found that 163 had been made in twenty-one minutes. I traveled on, and still met more the farther I proceeded. The air was literally filled with Pigeons; the light of noon-day was obscured as by an eclipse; the dung fell in spots, not unlike melting flakes of snow, and the continued buzz of wings had a tendency to lull my senses to repose&nbsp;... I cannot describe to you the extreme beauty of their aerial evolutions, when a hawk chanced to press upon the rear of the flock. At once, like a torrent, and with a noise like thunder, they rushed into a compact mass, pressing upon each other towards the center. In these almost solid masses, they darted forward in undulating and angular lines, descended and swept close over the earth with inconceivable velocity, mounted perpendicularly so as to resemble a vast column, and, when high, were seen wheeling and twisting within their continued lines, which then resembled the coils of a gigantic serpent&nbsp;... Before sunset I reached Louisville, distant from Hardensburgh fifty-five miles. The Pigeons were still passing in undiminished numbers and continued to do so for three days in succession.<ref name=JAA/>}}

These flocks were frequently described as being so dense that they blackened the sky and as having no sign of subdivisions. The flocks ranged from only {{convert|1.0|m|ft|abbr=on}} above the ground in windy conditions to as high as {{convert|400|m|ft|abbr=on}}. These migrating flocks were typically in narrow columns that twisted and undulated, and they were reported as being in nearly every conceivable shape.<ref name="Blockstein 5"/> A skilled flyer, the passenger pigeon is estimated to have averaged {{convert|100|km/h|mph|abbr=on}} during migration. It flew with quick, repeated flaps that increased the bird's velocity the closer the wings got to the body. It was equally adept and quick flying through a forest as through open space. A flock was also adept at following the lead of the pigeon in front of it, and flocks swerved together to avoid a predator. When landing, the pigeon flapped its wings repeatedly before raising them at the moment of landing. The pigeon was awkward when on the ground, and moved around with jerky, alert steps.<ref name="Blockstein 9">{{cite encyclopedia |last=Blockstein |first=D. E. |editor-last=Poole |editor-first=A. |editor2-last=Gill |editor2-first=F. |title=Passenger Pigeon ''Ectopistes migratorius'' |encyclopedia=[[Birds of North America (book)|Birds of North America]] |page=611 |publisher=The Birds of North America, Inc., [[Cornell Lab of Ornithology]] |location=Philadelphia |year=2002 |url=https://birdsoftheworld.org/bow/species/paspig/cur/introduction |url-access=subscription |access-date=October 8, 2024}}</ref>

[[File:Flock of passenger pigeons.jpg|thumb|Illustration of migrating flocks, [[Frank Bond]], 1920]]

The passenger pigeon was one of the most social of all land birds.<ref name="Blockstein 10"/> Estimated to have numbered three to five billion at the height of its population, it may have been the most numerous bird on Earth; researcher [[Arlie W. Schorger]] believed that it accounted for between 25 and 40 percent of the total land bird population in the United States.<ref name="Schorger 205">{{Harvnb|Schorger|1955|p=205}}</ref> The passenger pigeon's historic population is roughly the equivalent of the number of birds that overwinter in the United States every year in the early 21st century.<ref name="Ellsworth"/> Even within their range, the size of individual flocks could vary greatly. In November 1859, [[Henry David Thoreau]], writing in [[Concord, Massachusetts]], noted that "quite a little flock of [passenger] pigeons bred here last summer,"<ref name="Thoreau">{{cite journal |title=General Notes: Thoreau's Notes on the Passenger Pigeon |journal=The Auk |year=1911 |volume=28 |issue=1 |page=111 |jstor=4071503 |doi=10.2307/4071503 |url=https://books.google.com/books?id=wNoUAAAAYAAJ&pg=PA111}}</ref> while only seven years later, in 1866, one flock in southern [[Ontario]] was described as being {{convert|1.5|km|mi|abbr=on}} wide and {{convert|500|km|mi|abbr=on}} long, took 14 hours to pass, and held in excess of 3.5 billion birds.<ref name="sullivan">{{cite book |title=Hunting for Frogs on Elston, and Other Tales from Field & Street |chapter=The Passenger Pigeon: Once There Were Billions |pages=210–213 |date=April 2004 |last1=Sullivan |first1=J. |last2=Sutton |first2=B. |last3=Cronon |first3=W. |publisher=University of Chicago Press |isbn=978-0-226-77993-5 |url=https://press.uchicago.edu/Misc/Chicago/779939pass.html |access-date=February 29, 2012}}</ref> Such a number would likely represent a large fraction of the entire population at the time, or perhaps all of it.<ref name="Extinct Birds"/> Most estimations of numbers were based on single migrating colonies, and it is unknown how many of these existed at a given time. American writer [[Christopher Cokinos]] has suggested that if the birds flew single file, they would have stretched around the Earth 22 times.<ref name="Fuller 2014 50–69"/>

A 2014 genetic study (based on [[coalescent theory]] and on "sequences from most of the [[genome]]" of three individual passenger pigeons) suggested that the passenger pigeon population experienced dramatic fluctuations across the last million years, due to their dependence on availability of [[mast seeding|mast]] (which itself fluctuates). The study suggested the bird was not always abundant, mainly persisting at around 1/10,000 the amount of the several billions estimated in the 1800s, with vastly larger numbers present during outbreak phases.<ref name="Hung 2014">{{cite journal |title=Drastic population fluctuations explain the rapid extinction of the passenger pigeon |journal=[[Proceedings of the National Academy of Sciences]] |volume=111 |issue=29 |pages=10636–10641 |year=2014 |last1=Hung |first1=C. M. |last2=Shaner |first2=P. J. L. |last3=Zink |first3=R. M. |last4=Liu |first4=W. C. |last5=Chu |first5=T. C. |last6=Huang |first6=W. S. |last7=Li |first7=S. H. |display-authors=2 |bibcode=2014PNAS..11110636H |pmid=24979776 |doi=10.1073/pnas.1401526111 |doi-access=free |pmc=4115547}}</ref><ref name="ScienceMag_2014-06-16">{{cite web |title=Humans not solely to blame for passenger pigeon extinction |year=2014 |last1=Williams |first1=S. C. P. |website=Science & AAAS |publisher=[[American Association for the Advancement of Science]] |url=https://www.science.org/content/article/humans-not-solely-blame-passenger-pigeon-extinction |access-date=June 17, 2014}}</ref> Some early accounts also suggest that the appearance of flocks in great numbers was an irregular occurrence.<ref name=Greenway/> These large fluctuations in population may have been the result of a disrupted ecosystem and have consisted of outbreak populations much larger than those common in pre-European times.<ref name="Mann 2011">{{cite book |last=Mann |first=Charles C. |chapter=The Artificial Wilderness |title=[[1491: New Revelations of the Americas Before Columbus]]|edition=2nd |location=New York |publisher=Vintage |year=2011 |pages=365–367|isbn=978-1-4000-4006-3}}</ref> The authors of the 2014 genetic study note that a similar analysis of the human population size arrives at an "[[effective population size]]" of between 9,000 and 17,000 individuals (or approximately 1/550,000th of the peak total human population size of 7 billion cited in the study).<ref name="Hung 2014"/>

For a 2017 genetic study, the authors sequenced the [[genome]]s of two additional passenger pigeons, as well as analyzing the [[mitochondrial DNA]] of 41 individuals.<ref name="Selection"/><ref name="ScienceMag_2017-11-16">{{cite web |title=Four billion passenger pigeons vanished. Their large population may have been what did them in |last1=Pennisi |first1=Elizabeth |author-link=Elizabeth Pennisi |website=Science & AAAS |publisher=[[American Association for the Advancement of Science]] |date=November 16, 2017 |url=https://www.science.org/content/article/four-billion-passenger-pigeons-vanished-their-large-population-may-have-been-what-did |access-date=March 4, 2018}}</ref><ref>{{citation |last=SciShow |title=Why Billions of Passenger Pigeons Died in Under a Century |date=2018-07-02 |url=https://www.youtube.com/watch?v=twr53QVGh0E |access-date=2019-02-15 |url-status=live |archive-url=https://ghostarchive.org/varchive/youtube/20211211/twr53QVGh0E |archive-date=2021-12-11}}{{cbignore}}</ref> This study found evidence that the passenger-pigeon population had been stable for at least the previous 20,000 years.<ref>{{cite news |title=Billions or bust: New genetic clues to the extinction of the passenger pigeon |last=Achenbach |first=Joel |date=November 16, 2017 |newspaper=The Washington Post |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/11/16/billions-or-bust-new-genetic-clues-to-the-extinction-of-the-passenger-pigeon/?noredirect=on}}</ref> The study also found that the size of the passenger pigeon population over that time period was larger than the found in the 2014 genetic study. However, the 2017 study's "conservative" estimate of an "[[effective population size]]" of 13 million birds is still only about 1/300th of the bird's estimated historic population of approximately 3–5 billion before their "19th century decline and eventual extinction."<ref name="Selection">{{cite journal |last1=Murray |first1=G. G. R. |last2=Soares |first2=A. E. R. |last3=Novak |first3=B. J. |last4=Schaefer |first4=N. K. |last5=Cahill |first5=J. A. |last6=Baker |first6=A. J. |last7=Demboski |first7=J. R. |last8=Doll |first8=A. |last9=Da Fonseca |first9=R. R. |last10=Fulton |first10=T. L. |last11=Gilbert |first11=M. T. P. |last12=Heintzman |first12=P. D. |last13=Letts |first13=B. |last14=McIntosh |first14=G. |last15=O’Connell |first15=B. L. |display-authors=2 |date=2017 |title=Natural selection shaped the rise and fall of passenger pigeon genomic diversity |journal=Science |volume=358 |issue=6365 |pages=951–954 |bibcode=2017Sci...358..951M |doi=10.1126/science.aao0960 |pmid=29146814 |s2cid=4779202 |doi-access=free |last16=Peck |first16=M. |last17=Pipes |first17=M.-L. |last18=Rice |first18=E. S. |last19=Santos |first19=K. M. |last20=Sohrweide |first20=A. G. |last21=Vohr |first21=S. H. |last22=Corbett-Detig |first22=R. B. |last23=Green |first23=R. E. |last24=Shapiro |first24=B. |hdl=11250/2480523|hdl-access=free }}</ref> A similar study inferring human population size from genetics (published in 2008, and using human mitochondrial DNA and [[Bayesian inference|Bayesian]] [[coalescent theory|coalescent]] inference methods) showed considerable accuracy in reflecting overall patterns of human population growth as compared to data deduced by other means—though the study arrived at a human [[effective population size]] (as of 1600 AD, for Africa, Eurasia, and the Americas combined) that was roughly 1/1000 of the census population estimate for the same time and area based on anthropological and historical evidence.<ref name="Variation">{{cite journal |last1=Atkinson |first1=Q. D. |last2=Gray |first2=R. D. |last3=Drummond |first3=A. J. |title=mtDNA variation predicts population size in humans and reveals a major southern Asian chapter in human prehistory |journal=Molecular Biology and Evolution |date=2008 |volume=25 |issue=2 |pages=468–474 |pmid=18093996 |doi=10.1093/molbev/msm277 |doi-access=free}}</ref><ref name="Essay">{{cite journal |last1=Biraben |first1=J. N. |title=Essai sur l'évolution du nombre des hommes |trans-title=Essay on the evolution of numbers of mankind |journal=Population |series=French Edition |date=1979 |volume=34 |issue=1 |pages=13–25 |jstor=1531855 |doi=10.2307/1531855}}</ref>

The 2017 passenger-pigeon genetic study also found that, in spite of its large population size, the genetic diversity was very low in the species. The authors suggested that this was a side-effect of [[natural selection]], which theory and previous empirical studies suggested could have a particularly great impact on species with very large and cohesive populations.<ref>{{cite journal |last1=Sackton |first1=Timothy B. |last2=Hartl |first2=Daniel L. |last3=Corbett-Detig |first3=Russell B. |title=Natural selection constrains neutral diversity across a wide range of species |date=2015-04-10 |journal=PLOS Biology |language=en |volume=13 |issue=4 |page=e1002112 |pmid=25859758 |doi=10.1371/journal.pbio.1002112 |doi-access=free |pmc=4393120}}</ref><ref>{{cite journal |last1=Przeworski |first1=Molly |last2=Andolfatto |first2=Peter |last3=Venkat |first3=Aarti |last4=Ségurel |first4=Laure |last5=Meyer |first5=Wynn K. |last6=Matute |first6=Daniel R. |last7=Bullaughey |first7=Kevin |last8=Leffler |first8=Ellen M. |title=Revisiting an old riddle: what determines genetic diversity levels within species? |date=2012-09-11 |journal=PLOS Biology |language=en |volume=10 |issue=9 |page=e1001388 |pmid=22984349 |doi=10.1371/journal.pbio.1001388 |doi-access=free |pmc=3439417}}</ref> Natural selection can reduce genetic diversity over extended regions of a genome through '[[selective sweep]]s' or '[[background selection]]'. The authors found evidence of a faster rate of [[adaptation|adaptive evolution]] and faster removal of harmful mutations in passenger pigeons compared to [[band-tailed pigeon]]s, which are some of passenger pigeons' closest living relatives. They also found evidence of lower genetic diversity in regions of the passenger pigeon genome that have lower rates of [[genetic recombination]]. This is expected if natural selection, via [[selective sweep]]s or [[background selection]], reduced their genetic diversity, but not if population instability did. The study concluded that earlier suggestion that population instability contributed to the extinction of the species was invalid.<ref name="Selection"/> Evolutionary biologist A. Townsend Peterson said of the two passenger-pigeon genetic studies (published in 2014 and 2017) that, though the idea of extreme fluctuations in the passenger-pigeon population was "deeply entrenched," he was persuaded by the 2017 study's argument, due to its "in-depth analysis" and "massive data resources."<ref name="ScienceMag_2017-11-16"/>

[[File:Ectopistes migratoriusAAP042CA.jpg|thumb|left|Juvenile (left), male (center), female (right), [[Louis Agassiz Fuertes]], 1910]]

A [[communally roosting]] species, the passenger pigeon chose roosting sites that could provide shelter and enough food to sustain their large numbers for an indefinite period. The time spent at one roosting site may have depended on the extent of human persecution, weather conditions, or other, unknown factors. Roosts ranged in size and extent, from a few acres to {{convert|260|km2|sqmi|abbr=on}} or greater. Some roosting areas would be reused for subsequent years, others would only be used once.<ref name="Fuller 2014 30–47"/> The passenger pigeon roosted in such numbers that even thick tree branches would break under the strain. The birds frequently piled on top of each other's backs to roost. They rested in a slumped position that hid their feet. They slept with their bills concealed by the feathers in the middle of the breast while holding their tail at a 45-degree angle.<ref name="Blockstein 9"/> Dung could accumulate under a roosting site to a depth of over {{convert|0.3|m|ft|1|abbr=on}}.<ref name="Blockstein 8"/>

[[File:Bird-lore (1913) (14568824740).jpg|thumb|upright|Alert parent bird posing defiantly towards the camera (1896, published 1913)]]

If the pigeon became alert, it would often stretch out its head and neck in line with its body and tail, then nod its head in a circular pattern. When aggravated by another pigeon, it raised its wings threateningly, but passenger pigeons almost never actually fought. The pigeon bathed in shallow water, and afterwards lay on each side in turn and raised the opposite wing to dry it.<ref name="Blockstein 9"/>

The passenger pigeon drank at least once a day, typically at dawn, by fully inserting its bill into lakes, small ponds, and streams. Pigeons were seen perching on top of each other to access water, and if necessary, the species could alight on open water to drink.<ref name="Blockstein 8">{{Harvnb|Blockstein|2002|p=8}}</ref> One of the primary causes of natural mortality was the weather, and every spring many individuals froze to death after migrating north too early. In captivity, a passenger pigeon was capable of living at least 15 years; [[Martha (pigeon)|Martha]], the last known living passenger pigeon, was at least 17 and possibly as old as 29 when she died. It is undocumented how long wild pigeons lived.<ref name="Blockstein 15"/>

The bird is believed to have played a significant ecological role in the composition of [[pre-Columbian era|pre-Columbian]] forests of eastern North America. For instance, while the passenger pigeon was extant, forests were dominated by [[Quercus alba|white oaks]]. This species germinated in the fall, therefore making its seeds almost useless as a food source during the spring breeding season, while [[Quercus rubra|red oaks]] produced acorns during the spring, which the pigeons devoured. The absence of the passenger pigeon's seed consumption may have contributed to the modern dominance of red oaks. Due to the immense amount of dung present at roosting sites, few plants grew for years after the pigeons left. Also, the accumulation of flammable debris (such as limbs broken from trees and foliage killed by excrement) at these sites may have increased both the frequency and intensity of [[wildfire|forest fires]], which would have favored [[fire ecology#Fire tolerance|fire-tolerant species]], such as [[Quercus macrocarpa|bur oaks]], [[Quercus velutina|black oaks]], and [[Quercus alba|white oaks]] over less fire-tolerant species, such as red oaks, thus helping to explain the change in the composition of eastern forests since the passenger pigeon's extinction (from white oaks, bur oaks, and black oaks predominating in presettlement forests, to the "dramatic expansion" of red oaks today).<ref name="Ellsworth">{{cite journal |last=Ellsworth |first=J. W. |author2=McComb, B. C. |title=Potential Effects of Passenger Pigeon Flocks on the Structure and Composition of Presettlement Forests of Eastern North America |journal=[[Conservation Biology (journal) |Conservation Biology]] |volume=17 |issue=6 |pages=1548–1558 |year=2003 |bibcode=2003ConBi..17.1548E |doi=10.1111/j.1523-1739.2003.00230.x |s2cid=55427679}}</ref>

A study released in 2018 concluded that the "vast numbers" of passenger pigeons present for "tens of thousands of years" would have influenced the evolution of the tree species whose seeds they ate. Those masting trees that produced seeds during the spring nesting season (such as red oaks) evolved so that some portion of their seeds would be too large for passenger pigeons to swallow (thus allowing some of their seeds to escape predation and grow new trees). White oak, in contrast, with its seeds sized consistently in the edible range, evolved an irregular masting pattern that took place in the fall, when fewer passenger pigeons would have been present. The study further concluded that this allowed white oaks to be the dominant tree species in regions where passenger pigeons were commonly present in the spring.<ref name="evolve">{{cite journal |last1=Novak |first1=B. J. |last2=Estes |first2=J. A. |last3=Shaw |first3=H. E. |last4=Novak |first4=E. V. |last5=Shapiro |first5=B. |title=Experimental investigation of the dietary ecology of the extinct passenger pigeon, ''Ectopistes migratorius'' |journal=Frontiers in Ecology and Evolution |date=2018 |volume=6 |page=20 |doi=10.3389/fevo.2018.00020 |doi-access=free|bibcode=2018FrEEv...6...20N }}</ref>

With the large numbers in passenger pigeon flocks, the excrement they produced was enough to destroy surface-level vegetation at long-term roosting sites, while adding high quantities of nutrients to the [[ecosystem]]. Because of this—along with the breaking of tree limbs under their collective weight and the great amount of [[mast seeding|mast]] they consumed—passenger pigeons are thought to have influenced both the structure of eastern forests and the composition of the species present there.<ref name="Ellsworth"/> Due to these influences, some ecologists have considered the passenger pigeon a [[keystone species]],<ref name="Hung 2014"/> with the disappearance of their vast flocks leaving a major gap in the ecosystem.<ref name="McCauley 2016">{{cite journal |title=A mammoth undertaking: harnessing insight from functional ecology to shape de-extinction priority setting |journal=[[Functional Ecology (journal) |Functional Ecology]] |volume=31 |issue=5 |pages=1008–1009 |year=2016 |last1=McCauley |first1=D. J. |last2=Hardesty-Moore |first2=M. |last3=Halpern |first3=B. S. |last4=Young |first4=H. S. |last5=Seddon |first5=P. |display-authors=2 |doi=10.1111/1365-2435.12728 |s2cid=19791653 |url=https://escholarship.org/content/qt5cd8p3cr/qt5cd8p3cr.pdf?t=ozrwe2}}</ref> Their role in creating [[forest dynamics|forest disturbances]] has been linked to greater vertebrate diversity in forests by creating more niches for animals to fill.<ref>{{cite journal |last1=Hutchinson |first1=Todd F. |last2=Yaussy |first2=Daniel A. |last3=Long |first3=Robert P. |last4=Rebbeck |first4=Joanne |last5=Sutherland |first5=Elaine Kennedy |title=Long-term (13-year) effects of repeated prescribed fires on stand structure and tree regeneration in mixed-oak forests |date=December 2012 |journal=Forest Ecology and Management |volume=286 |pages=87–100 |bibcode=2012ForEM.286...87H |issn=0378-1127 |doi=10.1016/j.foreco.2012.08.036}}</ref> To help fill that ecological gap, it has been proposed that modern land managers attempt to replicate some of their effects on the ecosystem by creating openings in forest [[canopy (biology)|canopies]] to provide more [[understory]] light.<ref name="Buchanan 2012">{{cite journal |last=Buchanan |first=M. L. |author2=Hart, J. L. |title=Canopy disturbance history of old-growth ''Quercus alba'' sites in the eastern United States: Examination of long-term trends and broad-scale patterns |journal=[[Forest Ecology and Management]] |volume=267 |issue=267 |pages=28–39 |year=2012 |bibcode=2012ForEM.267...28B |doi=10.1016/j.foreco.2011.11.034}}</ref>

The [[American chestnut]] trees that provided much of the [[mast seeding|mast]] on which the passenger pigeon fed was itself almost driven to extinction by an imported Asian fungus ([[chestnut blight]]) around 1905. As many as thirty billion trees are thought to have died as a result in the following decades, but this did not affect the passenger pigeon, which was already extinct in the wild at the time.<ref name="Fuller 2014 30–47"/>

After the disappearance of the passenger pigeon, the population of another acorn feeding species, the [[white-footed mouse]], grew exponentially because of the increased availability of the seeds of the oak, beech, and chestnut trees.<ref>{{cite journal |jstor=1312946 |title=Of Mice and Mast |journal=BioScience |volume=46 |issue=5 |pages=323 |year=1996 |last1=Ostfeld |first1=Richard S. |last2=Jones |first2=Clive G. |last3=Wolff |first3=Jerry O. |doi=10.2307/1312946 |doi-access=free |s2cid=89496723}}</ref> It has been speculated<ref>{{cite journal |title=Lyme Disease and the Passenger Pigeon? |journal=Science |volume=279 |issue=5358 |pages=1831c–1831 |year=1998 |last1=Blockstein |first1=D. E. |bibcode=1998Sci...279.1831B |pmid=9537894 |doi=10.1126/science.279.5358.1831c |s2cid=45065236}}</ref> that the extinction of passenger pigeons may have increased the prevalence of tick-borne [[lyme disease]] in modern times as white-footed mice are the [[natural reservoir|reservoir hosts]] of ''[[Borrelia burgdorferi]]''.<ref>{{cite book |title=The Annihilation of Nature: Human Extinction of Birds and Mammals |last2=Ehrlich |first2=A. H. |last3=Ehrlich |first3=P. R. |publisher=Johns Hopkins University Press |year=2015 |isbn=978-1-4214-1718-9 |location=Baltimore, Maryland |pages=33–35 |via=Open Edition |last1=Ceballos |first1=G.}}</ref>

===Diet===
[[File:Passenger pigeons (Ectopistes migratorius) in autumn 1.jpg|left|thumb|Taxidermied specimens mounted as if foraging for [[Quercus palustris|pin oak]] acorns, at the [[American Museum of Natural History]].]]
[[Beech]]es and [[oak]]s produced the [[mast seeding|mast]] needed to support nesting and roosting flocks.<ref name="Blockstein 7"/> The passenger pigeon changed its diet depending on the season. In the fall, winter, and spring, it mainly ate beechnuts, [[acorn]]s, and chestnuts. During the summer, berries and softer fruits, such as [[blueberry|blueberries]], [[grape]]s, [[Prunus|cherries]], [[Morus rubra|mulberries]], [[Phytolacca americana|pokeberries]], and [[Cornus canadensis|bunchberry]], became the main objects of its consumption. It also ate [[Oligochaeta|worms]], [[caterpillar]]s, [[snail]]s, and other invertebrates, particularly while breeding.<ref name="Fuller 2014 30–47"/><ref name="Blockstein 6">{{Harvnb|Blockstein|2002|p=6}}</ref> It took advantage of cultivated grains, particularly [[buckwheat]], when it found them. It was especially fond of [[salt]], which it ingested either from brackish springs or salty soil.<ref name="Blockstein 7">{{Harvnb|Blockstein|2002|p=7}}</ref>

Mast occurs in large quantities in different places at different times, and rarely in consecutive years, which is one of the reasons why the large flocks were constantly on the move. As mast is produced during autumn, there would have to be a large amount of it left by the summer, when the young were reared. It is unknown how they located this fluctuating food source, but their eyesight and flight powers helped them survey large areas for places that could provide food enough for a temporary stay.<ref name="Extinct Birds"/><ref name="Fuller 2014 30–47"/>

[[File:Passenger Pigeon organs.jpg|thumb|upright|Internal organs of Martha, the last individual: cr. denotes the crop, gz. the [[gizzard]], 1915]]

The passenger pigeon foraged in flocks of tens or hundreds of thousands of individuals that overturned leaves, dirt, and snow with their bills in search of food. One observer described the motion of such a flock in search of mast as having a rolling appearance, as birds in the back of the flock flew overhead to the front of the flock, dropping leaves and grass in flight.<ref name="Fuller 2014 30–47"/><ref name="Blockstein 6"/> The flocks had wide leading edges to better scan the landscape for food sources.<ref name="Blockstein 7"/>

When nuts on a tree loosened from their caps, a pigeon would land on a branch and, while flapping vigorously to stay balanced, grab the nut, pull it loose from its cap, and swallow it whole. Collectively, a foraging flock was capable of removing nearly all fruits and nuts from their path. Birds in the back of the flock flew to the front in order to pick over unsearched ground; however, birds never ventured far from the flock and hurried back if they became isolated. It is believed that the pigeons used social cues to identify abundant sources of food, and a flock of pigeons that saw others feeding on the ground often joined them.<ref name="Blockstein 6"/> During the day, the birds left the roosting forest to forage on more open land.<ref name="Blockstein 5"/> They regularly flew {{convert|100|to|130|km|mi|abbr=on}} away from their roost daily in search of food, and some pigeons reportedly traveled as far as {{convert|160|km|mi|abbr=on}}, leaving the roosting area early and returning at night.<ref name="Fuller 2014 30–47"/><ref name="Ellsworth"/>

The passenger pigeon's very elastic mouth and throat and a joint in the lower bill enabled it to swallow acorns whole. It could store large quantities of food in its [[crop (anatomy)#Birds|crop]], which could expand to about the size of an orange, causing the neck to bulge and allowing a bird quickly to grab any food it discovered. The crop was described as being capable of holding at least 17 acorns or 28 beechnuts, 11 grains of corn, 100 maple seeds, plus other material; it was estimated that a passenger pigeon needed to eat about {{convert|61|cm3|in3|abbr=on}} of food a day to survive. If shot, a pigeon with a crop full of nuts would fall to the ground with a sound described as like the rattle of a bag of marbles. After feeding, the pigeons perched on branches and digested the food stored in their crop overnight<ref name="Fuller 2014 30–47"/><ref name="Blockstein 6"/><ref name="Ellsworth"/> with the aid of a muscular [[gizzard]], which often [[gastrolith|contained gravel]].<ref>{{cite journal|url=https://birdsoftheworld.org/bow/species/paspig/cur/foodhabits|title=Passenger Pigeon (''Ectopistes migratorius''): Diet and Foraging|website=All About Birds|publisher=Cornell Lab of Ornithology|last=Blockstein|first=D.E.|year=2020|editor-last1=Poole|editor-first1=A.F.|editor-last2=Gill|editor-first2=F.B.|location=Ithaca, New York, USA|version=1.0|doi=10.2173/bow.paspig.01|access-date=11 October 2024}}</ref>

The pigeon could eat and digest {{convert|100|g|oz|abbr=on}} of acorns per day.<ref>{{cite journal |title=Human-wildlife competition and the passenger pigeon: Population growth from system destabilization |journal=[[Human Ecology (journal) |Human Ecology]] |volume=13 |issue=4 |pages=389–410 |year=1985 |last1=Neumann |first1=T. W. |bibcode=1985HumEc..13..389N |doi=10.1007/BF01531152 |s2cid=153426755}}</ref> At the historic population of three billion passenger pigeons, this amounted to {{convert|210,000,000|L|gal|abbr=on}} of food a day.<ref name="Ellsworth"/> The pigeon could regurgitate food from its crop when more desirable food became available.<ref name="Fuller 2001">{{cite book |last=Fuller |first=E. |title=Extinct Birds |edition=Revised |publisher=Comstock Publishing Associates |year=2001 |location=Ithaca, New York |isbn=978-0-8014-3954-4 |pages=96–97}}</ref> A 2018 study found that the dietary range of the passenger pigeon was restricted to certain sizes of seed, due to the size of its gape. This would have prevented it from eating some of the seeds of trees such as [[Quercus rubra|red oaks]], [[Quercus velutina|black oaks]], and the [[American chestnut]]. Specifically, the study found that between 13% and 69% of red oak seeds were too large for passenger pigeons to have swallowed, that only a "small proportion" of the seeds of black oaks and American chestnuts were too large for the birds to consume, and that all white oak seeds were sized within an edible range. They also found that seeds would be completely destroyed during digestion, which therefore hindered dispersal of seeds this way. Instead, passenger pigeons may have spread seeds by [[regurgitation (digestion)|regurgitation]], or after dying.<ref name="evolve"/>

===Reproduction===
[[File:Birdonnest.jpg|thumb|upright|Nesting captive bird, wary of the photographer]]

Other than finding roosting sites, the migrations of the passenger pigeon were connected with finding places appropriate for this [[communally breeding]] bird to nest and raise its young. It is not certain how many times a year the birds bred; once seems most likely, but some accounts suggest more. The nesting period lasted around four to six weeks. The flock arrived at a nesting ground around March in southern latitudes, and some time later in more northern areas.<ref name="Fuller 2014 30–47"/><ref name="Schorger 205"/> The pigeon had no site fidelity, often choosing to nest in a different location each year.<ref name="Blockstein 15"/> The formation of a nesting colony did not necessarily take place until several months after the pigeons arrived on their breeding grounds, typically during late March, April, or May.<ref name="Blockstein 11"/>

The colonies, which were known as "cities", were immense, ranging from {{convert|49|ha|acre|abbr=on}} to thousands of hectares in size, and were often long and narrow in shape (L-shaped), with a few areas untouched for unknown reasons. Due to the topography, they were rarely continuous. Since no accurate data was recorded, it is not possible to give more than estimates on the size and population of these nesting areas, but most accounts mention colonies containing millions of birds. The largest nesting area ever recorded was in central Wisconsin in 1871; it was reported as covering {{convert|2,200|km2|sqmi|abbr=on}}, with the number of birds nesting there estimated to be around 136,000,000. As well as these "cities", there were regular reports of much smaller flocks or even individual pairs setting up a nesting site.<ref name="Fuller 2014 30–47"/><ref name="Blockstein 11">{{Harvnb|Blockstein|2002|p=11}}</ref> The birds do not seem to have formed as vast breeding colonies at the periphery of their range.<ref name=Greenway/>

Courtship took place at the nesting colony.<ref name="Blockstein 10">{{Harvnb|Blockstein|2002|p=10}}</ref> Unlike other pigeons, courtship took place on a branch or perch. The male, with a flourish of the wings, made a "keck" call while near a female. The male then gripped tightly to the branch and vigorously flapped his wings up and down. When the male was close to the female, he then pressed against her on the perch with his head held high and pointing at her.<ref name="Blockstein 9"/> If receptive, the female pressed back against the male.<ref name="Blockstein 10"/> When ready to mate, the pair [[social grooming|preened]] each other. This was followed by the birds [[beak#Billing|billing]], in which the female inserted its [[beak|bill]] into and clasped the male's bill, shook for a second, and separated quickly while standing next to each other. The male then scrambled onto the female's back and copulated, which was then followed by soft clucking and occasionally more preening.<ref name="Blockstein 10"/> John James Audubon described the courtship of the passenger pigeon as follows:

[[File:Nestandegg.jpg|thumb|left|Nest and egg in Whitman's aviary]]
{{Blockquote|Thither the countless myriads resort, and prepare to fulfill one of the great laws of nature. At this period the note of the Pigeon is a soft coo-coo-coo-coo much shorter than that of the domestic species. The common notes resemble the monosyllables kee-kee-kee-kee, the first being the loudest, the others gradually diminishing In power. The male assumes a pompous demeanor, and follows the female, whether on the ground or on the branches, with spread tail and drooping wings, which it rubs against the part over which it is moving. The body is elevated, the throat swells, the eyes sparkle. He continues his notes, and now and then rises on the wing, and flies a few yards to approach the fugitive and timorous female. Like the domestic Pigeon and other species, they caress each other by billing, in which action, the bill of the one is introduced transversely into that of the other, and both parties alternately disgorge the contents of their crop by repeated efforts.<ref name=JAA/>}}

After observing captive birds, Wallace Craig found that this species did less charging and strutting than other pigeons (as it was awkward on the ground), and thought it probable that no food was transferred during their brief billing (unlike in other pigeons), and he therefore considered Audubon's description partially based on analogy with other pigeons as well as imagination.<ref name="Craig 1911"/><ref name="Blockstein 9"/>

[[File:Pigeon migrateur MHNT.jpg|thumb|Preserved egg, [[Muséum de Toulouse]]]]

Nests were built immediately after pair formation and took two to four days to construct; this process was highly synchronized within a colony.<ref name="Blockstein 11"/> The female chose the nesting site by sitting on it and flicking her wings. The male then carefully selected nesting materials, typically twigs, and handed them to the female over her back. The male then went in search of more nesting material while the female constructed the nest beneath herself. Nests were built between {{convert|2.0|and|20.1|m|ft|abbr=on}} above the ground, though typically above {{convert|4.0|m|ft|abbr=on}}, and were made of 70 to 110 twigs woven together to create a loose, shallow bowl through which the egg could easily be seen. This bowl was then typically lined with finer twigs. The nests were about {{convert|150|mm|in|abbr=on}} wide, {{convert|61|mm|in|abbr=on}} high, and {{convert|19|mm|in|abbr=on}} deep. Though the nest has been described as crude and flimsy compared to those of many other birds, remains of nests could be found at sites where nesting had taken place several years prior. Nearly every tree capable of supporting nests had them, often more than 50 per tree; one hemlock was recorded as holding 317 nests. The nests were placed on strong branches close to the tree trunks. Some accounts state that ground under the nesting area looked as if it had been swept clean, due to all the twigs being collected at the same time, yet this area would also have been covered in dung.<ref name="Fuller 2014 30–47"/><ref name="Fuller 2001"/><ref name="Blockstein 12">{{Harvnb|Blockstein|2002|p=12}}</ref> As both sexes took care of the nest, the pairs were [[monogamy in animals|monogamous]] for the duration of the nesting.<ref name="Blockstein 9"/>

[[File:Young passenger pigeon.jpg|thumb|left|Live nestling or squab]]

Generally, the eggs were laid during the first two weeks of April across the pigeon's range.<ref name="Blockstein 11"/> Each female laid its egg immediately or almost immediately after the nest was completed; sometimes the pigeon was forced to lay it on the ground if the nest was not complete.<ref name="Blockstein 13">{{Harvnb|Blockstein|2002|p=13}}</ref> The normal clutch size appears to have been a single egg, but there is some uncertainty about this, as two have also been reported from the same nests.<ref name="Fuller 2014 30–47"/> Occasionally, a second female laid its egg in another female's nest, resulting in two eggs being present.<ref name="Blockstein 14"/> The egg was white and oval shaped and averaged {{convert|40|by|34|mm|in|abbr=on}} in size.<ref name="Blockstein 12"/> If the egg was lost, it was possible for the pigeon to lay a replacement egg within a week.<ref name="Blockstein 13"/> A whole colony was known to re-nest after a snowstorm forced them to abandon their original colony.<ref name="Blockstein 15">{{Harvnb|Blockstein|2002|p=15}}</ref> Both parents [[egg incubation|incubated]] The egg for 12 to 14 days, with the male incubating it from midmorning to midafternoon and the female incubating it the rest of the time.<ref name="Fuller 2014 30–47"/><ref name="Blockstein 13"/>

Upon hatching, the nestling (or squab) was blind and sparsely covered with yellow, hairlike [[down feather|down]].<ref name="Blockstein 13"/> The nestling developed quickly and within 14 days weighed as much as its parents. During this brooding period both parents took care of the nestling, with the male attending in the middle of the day and the female at other times. The nestlings were fed [[crop milk]] (a substance similar to [[curd]], produced in the crops of the parent birds) exclusively for the first days after hatching. Adult food was gradually introduced after three to six days. After 13 to 15 days, the parents fed the nestling for a last time and then abandoned it, leaving the nesting area ''en masse''. The nestling begged in the nest for a day or two, before climbing from the nest and fluttering to the ground, whereafter it moved around, avoided obstacles, and begged for food from nearby adults. It was another three or four days before it fledged.<ref name="Fuller 2014 30–47"/><ref name="Blockstein 14">{{Harvnb|Blockstein|2002|p=14}}</ref> The entire nesting cycle lasted about 30 days.<ref name="Fuller 2001"/> It is unknown whether colonies re-nested after a successful nesting.<ref name="Blockstein 15"/> The passenger pigeon sexually matured during its first year and bred the following spring.<ref name="Blockstein 14"/>

[[Alfred Russel Wallace]], in his historic 1858 paper [[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection]], used the passenger pigeon as an example of an immensely successful species despite laying fewer eggs than most other birds:

{{Blockquote|It would therefore appear that, as far as the continuance of the species and the keeping up the average number of individuals are concerned, large broods are superfluous. On the average all above one become food for hawks and kites, wild cats, and weasels, or perish of cold and hunger as winter comes on. This is strikingly proved by the case of particular species; for we find that their abundance in individuals bears no relation whatever to their fertility in producing offspring. Perhaps the most remarkable instance of an immense bird population is that of the passenger pigeon of the United States, which lays only one, or at most two eggs, and is said to rear generally but one young one. Why is this bird so extraordinarily abundant, while others producing two or three times as many young are much less plentiful? The explanation is not difficult. The food most congenial to this species, and on which it thrives best, is abundantly distributed over a very extensive region, offering such differences of soil and climate, that in one part or another of the area the supply never fails. The bird is capable of a very rapid and long-continued flight, so that it can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food begins to fail in one place is able to discover a fresh feeding-ground. This example strikingly shows us that the procuring a constant supply of wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given species, since neither the limited fecundity, nor the unrestrained attacks of birds of prey and of man are here sufficient to check it. In no other birds are these peculiar circumstances so strikingly combined. Either their food is more liable to failure, or they have not sufficient power of wing to search for it over an extensive area, or during some season of the year it becomes very scarce, and less wholesome substitutes have to be found; and thus, though more fertile in offspring, they can never increase beyond the supply of food in the least favourable seasons.<ref>{{citation |last=Darwin |given1=Charles |author-link1=Charles Darwin |surname2=Wallace |given2=Alfred Russel |author-link2=Alfred Russel Wallace |year=1858 |title=On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection |journal=Zoological Journal of the Linnean Society |volume=3 |issue=9  |pages=46–62 |doi=10.1111/j.1096-3642.1858.tb02500.x |doi-access=free |url=https://darwin-online.org.uk/content/frameset?itemID=F350&viewtype=text&pageseq=1 |access-date=14 January 2007}}</ref>}}

===Predators and parasites===
[[File:Passenger pigeon.jpg|thumb|upright|Immature bird; the young were vulnerable to predators after leaving the nest]]

Nesting colonies attracted large numbers of predators, including [[American mink]]s (''Neogale vison''), [[long-tailed weasel]]s (''Neogale frenata''), [[American marten]]s (''Martes americana''), and [[raccoon]]s (''Procyon lotor'') that preyed on eggs and nestlings, [[birds of prey]], such as [[owl]]s, [[hawk]]s, and [[eagle]]s that preyed on nestlings and adults, and [[gray wolf|wolves]] (''Canis lupus''), [[fox]]es (''Urocyon cinereoargenteus'' and ''Vulpes vulpes''), [[bobcat]]s (''Lynx rufus''), [[American black bear]]s (''Ursus americanus''), and [[cougar]]s (''Puma concolor'') that preyed on injured adults and fallen nestlings. Hawks of the genus ''[[Accipiter]]'' and falcons pursued and preyed upon pigeons in flight, which in turn executed complex aerial maneuvers to avoid them; [[Cooper's hawk]] (''Accipiter cooperii'') was known as the "great pigeon hawk" due to its successes, and these hawks allegedly followed migrating passenger pigeons.<ref name="Blockstein 10"/> While many predators were drawn to the flocks, individual pigeons were largely protected due to the sheer size of the flock, and overall little damage could be inflicted on the flock by predation.<ref name="Blockstein 10"/> Despite the number of predators, nesting colonies were so large that they were estimated to have a 90% success rate if not disturbed.<ref name="Blockstein 15"/> After being abandoned and leaving the nest, the very fat juveniles were vulnerable to predators until they were able to fly. The sheer number of juveniles on the ground meant that only a small percentage of them were killed; [[predator satiation]] may therefore be one of the reasons for the extremely social habits and communal breeding of the species.<ref name="Fuller 2014 30–47"/><ref name="Fuller 2014 72–88"/>

Two parasites have been recorded on passenger pigeons. One species of [[Philopteridae|phtilopterid]] [[louse]], ''[[Columbicola extinctus]]'', was originally thought to have lived on just passenger pigeons and to have become [[coextinct]] with them. This was proven inaccurate in 1999 when ''C.&nbsp;extinctus'' was rediscovered living on band-tailed pigeons.<ref name="Clayton1999">{{cite journal |last=Clayton |first=D. H. |author2=Price, R. D. |title=Taxonomy of New World Columbicola (Phthiraptera: Philopteridae) from the Columbiformes (Aves), with descriptions of five new species |journal=Annals of the Entomological Society of America |volume=92 |issue=5 |pages=675{{ndash}}685 |year=1999 |doi=10.1093/aesa/92.5.675 |url=https://darwin.biology.utah.edu/PDFs/42.pdf |access-date=October 8, 2024 |url-status=live |archive-date=April 25, 2012 |archive-url=https://web.archive.org/web/20120425124314/http://darwin.biology.utah.edu/PubsHTML/PDF-Files/42.pdf}}</ref><ref name="Price2000"/> This, and the fact that the related louse ''C.&nbsp;angustus'' is mainly found on cuckoo-doves, further supports the relation between these pigeons, as the phylogeny of lice broadly mirrors that of their hosts.<ref name=Johnson/> Another louse, ''[[Campanulotes|Campanulotes defectus]]'', was thought to have been unique to the passenger pigeon, but is now believed to have been a case of a contaminated specimen, as the species is considered to be the still-extant ''[[Campanulotes|Campanulotes flavus]]'' of Australia.<ref name="Price2000">{{cite journal |last=Price |first=R. D. |author2=Clayton, D. H. |author3=Adams, R. J. |title=Pigeon Lice Down Under: Taxonomy of Australian ''Campanulotes'' (Phthiraptera: Philopteridae), with a description of ''C. durdeni'' n.sp. |journal=[[Journal of Parasitology]] |volume=86 |issue=5 |pages=948–950 |year=2000 |jstor=3284803 |pmid=11128516 |doi=10.2307/3284803 |url=https://darwin.biology.utah.edu/PDFs/47.pdf |access-date=October 8, 2024 |url-status=live |archive-url=https://web.archive.org/web/20100610153220/http://darwin.biology.utah.edu/PubsHTML/PDF-Files/47.pdf |archive-date=June 10, 2010}}</ref> There is no record of a wild pigeon dying of either disease or parasites.<ref name="Blockstein 15"/>