The passenger pigeon or wild pigeon (Ectopistes migratorius) is an extinct species of pigeon that was endemic to North America. Its common name is derived from the French word passager, meaning "passing by", due to the migratory habits of the species. The scientific name also refers to its migratory characteristics. The morphologically similar mourning dove (Zenaida macroura) was long thought to be its closest relative, and the two were at times confused, but genetic analysis has shown that the genus Patagioenas is more closely related to it than the Zenaida doves.
The passenger pigeon was sexually dimorphic in size and coloration. The male was in length, mainly gray on the upperparts, lighter on the underparts, with iridescent bronze feathers on the neck, and black spots on the wings. The female was , and was duller and browner than the male overall. The juvenile was similar to the female, but without iridescence. It mainly inhabited the deciduous forests of eastern North America and was also recorded elsewhere, but bred primarily around the Great Lakes. The pigeon migrated in enormous flocks, constantly searching for food, shelter, and breeding grounds, and was once the most abundant bird in North America, numbering around 3 billion, and possibly up to 5 billion. A very fast flyer, the passenger pigeon could reach a speed of . The bird fed mainly on mast, and also fruits and invertebrates. It practiced communal roosting and communal breeding, and its extreme gregariousness may have been linked with searching for food and predator satiation.
Passenger pigeons were hunted by Native Americans, but hunting intensified after the arrival of Europeans, particularly in the 19th century. Pigeon meat was commercialized as cheap food, resulting in hunting on a massive scale for many decades. There were several other factors contributing to the decline and subsequent extinction of the species, including shrinking of the large breeding populations necessary for preservation of the species and widespread deforestation, which destroyed its habitat. A slow decline between about 1800 and 1870 was followed by a rapid decline between 1870 and 1890. A historical plaque in Indiana states that the last confirmed wild passenger pigeon was shot in Laurel in 1902. The last captive birds were divided in three groups around the turn of the 20th century, some of which were photographed alive. Martha, thought to be the last passenger pigeon, died on September 1, 1914, at the Cincinnati Zoo. The eradication of the species is a notable example of anthropogenic extinction.
Taxonomy
thumb|left|Earliest published illustration of the species (a male), [[Mark Catesby, 1731]]
Swedish naturalist Carl Linnaeus coined the binomial name Columba macroura for both the mourning dove and the passenger pigeon in the 1758 edition of his work Systema Naturae (the starting point of biological nomenclature), wherein he appears to have considered the two identical. This composite description cited accounts of these birds in two pre-Linnean books. One of these was Mark Catesby's description of the passenger pigeon, which was published in his 1731 to 1743 work Natural History of Carolina, Florida and the Bahama Islands, which referred to this bird as Palumbus migratorius, and was accompanied by the earliest published illustration of the species. In the same edition, Linnaeus also named C. canadensis, based on Turtur canadensis, as used by Mathurin Jacques Brisson in 1760. Brisson's description was later shown to have been based on a female passenger pigeon. In 1906 Outram Bangs suggested that because Linnaeus had wholly copied Catesby's text when coining C. macroura, this name should apply to the passenger pigeon, as E. macroura. In 1918 Harry C. Oberholser suggested that C. canadensis should take precedence over C. migratoria (as E. canadensis), as it appeared on an earlier page in Linnaeus' book. In 1952 Francis Hemming proposed that the International Commission on Zoological Nomenclature (ICZN) secure the specific name macroura for the mourning dove, and the name migratorius for the passenger pigeon, since this was the intended use by the authors on whose work Linnaeus had based his description.
Evolution
thumb|Mounted male passenger pigeon, [[Field Museum of Natural History]]
thumb|[[Band-tailed pigeon, a species in the related genus Patagioenas ]]
thumb|The physically similar [[mourning dove is not closely related.]]
The passenger pigeon was a member of the pigeon and dove family, Columbidae. The oldest known fossil of the genus is an isolated humerus (USNM 430960) known from the Lee Creek Mine in North Carolina in sediments belonging to the Yorktown Formation, dating to the Zanclean stage of the Pliocene, between 5.3 and 3.6 million years ago. Its closest living relatives were long thought to be the Zenaida doves, based on morphological grounds, particularly the physically similar mourning dove (now Z. macroura). It was even suggested that the mourning dove belonged to the genus Ectopistes and was listed as E. carolinensis by some authors, including Thomas Mayo Brewer. The passenger pigeon was supposedly descended from Zenaida pigeons that had adapted to the woodlands on the plains of central North America.
In a 2012 study, the nuclear DNA of the passenger pigeon was analyzed for the first time, and its relationship with the Patagioenas pigeon was confirmed. In contrast to the 2010 study, these authors suggested that their results could indicate that the ancestors of the passenger pigeon and its Old World relatives may have originated in the Neotropical region of the New World.
The cladogram below follows the 2012 DNA study showing the position of the passenger pigeon among its closest relatives: The passenger pigeon had no known subspecies. Hybridization occurred between the passenger pigeon and the Barbary dove (Streptopelia risoria) in the aviary of Charles Otis Whitman (who owned many of the last captive birds around the turn of the 20th century, and kept them with other pigeon species) but the offspring were infertile.
Etymology
thumb|right|Illustration of [[pigeon species from a 1889 zoology book, including the passenger pigeon (lower left)]]
The genus name, Ectopistes, translates as "moving about" or "wandering", while the specific name, migratorius, indicates its migratory habits. The full binomial can thus be translated as "migratory wanderer". The English common name "passenger pigeon" derives from the French word ', which means "to pass by" in a fleeting manner. While the pigeon was extant, the name "passenger pigeon" was used interchangeably with "wild pigeon". The bird also gained some less-frequently used names, including blue pigeon, merne rouck pigeon, wandering long-tailed dove, and wood pigeon. In the 18th century, the passenger pigeon was known as tourte in New France (in modern Canada), but to the French in Europe it was known as tourtre. In modern French, the bird is known as tourte voyageuse or pigeon migrateur, among other names.
In the Native American Algonquian languages, the pigeon was called amimi by the Lenape, ' by the Ojibwe, and ' by the Kaskaskia Illinois. Other names in indigenous American languages include ' in Mohawk, and ', or "lost dove", in Choctaw. The Seneca people called the pigeon ', meaning "big bread", as it was a source of food for their tribes.
Description
left|thumbtime=0:07|thumb|Turntable video of an adult male specimen at [[Naturalis Biodiversity Center]]
The passenger pigeon was sexually dimorphic in size and coloration. It weighed between . It had a bluish-gray head, nape, and hindneck. On the sides of the neck and the upper mantle were iridescent display feathers that have variously been described as being a bright bronze, violet, or golden-green, depending on the angle of the light. The upper back and wings were a pale or slate gray tinged with olive brown, that turned into grayish-brown on the lower wings. The lower back and rump were a dark blue-gray that became grayish-brown on the upper tail-covert feathers. The greater and median wing-covert feathers were pale gray, with a small number of irregular black spots near the end. The primary and secondary feathers of the wing were a blackish-brown with a narrow white edge on the outer side of the secondaries. The two central tail feathers were brownish gray, and the rest were white.
thumb|Skeleton of a male bird, 1914
The passenger pigeon was physically adapted for speed, endurance, and maneuverability in flight, and has been described as having a streamlined version of the typical pigeon shape, such as that of the generalized rock dove (Columba livia). The wings were very long and pointed, and measured from the wing-chord to the primary feathers, and to the secondaries. The tail, which accounted for much of its overall length, was long and wedge-shaped (or graduated), with two central feathers longer than the rest. The body was slender and narrow, and the head and neck were small.
Vocalizations
thumb|[[Musical notes documenting male vocalizations, compiled by Wallace Craig, 1911]]
The noise produced by flocks of passenger pigeons was described as deafening, audible for miles away, and the bird's voice as loud, harsh, and unmusical. It was also described by some as clucks, twittering, and cooing, and as a series of low notes, instead of an actual song. The birds apparently made croaking noises when building nests, and bell-like sounds when mating. During feeding, some individuals would give alarm calls when facing a threat, and the rest of the flock would join the sound while taking off.
Distribution and habitat
thumb|Mounted specimens representing a [[roosting group in 1890s Johnson County, Iowa, at Denver Museum of Nature and Science]]
The passenger pigeon was found across most of North America east of the Rocky Mountains, from the Great Plains to the Atlantic coast in the east, to the south of Canada in the north, and the north of Mississippi in the southern United States, coinciding with its primary habitat, the eastern deciduous forests. Within this range, it constantly migrated in search of food and shelter. It is unclear if the birds favored particular trees and terrain, but they were possibly not restricted to one type, as long as their numbers could be supported. It originally bred from the southern parts of eastern and central Canada south to eastern Kansas, Oklahoma, Mississippi, and Georgia in the United States, but the primary breeding range was in southern Ontario and the Great Lakes states south through states north of the Appalachian Mountains.
More than 130 passenger pigeon fossils have been found scattered across 25 US states, including in the La Brea Tar Pits of California. These records date as far back as 100,000 years ago in the Pleistocene era, during which the pigeon's range extended to several western states that were not a part of its modern range. The abundance of the species in these regions and during this time is unknown.
Behavior and ecology
thumb|upright|Alert parent bird posing defiantly towards the camera (1896, published 1913)
A skilled flyer, the passenger pigeon is estimated to have averaged 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. 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.
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. 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, 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. 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. while only seven years later, in 1866, one flock in southern Ontario was described as being wide and long, took 14 hours to pass, and held in excess of 3.5 billion birds. Such a number would likely represent a large fraction of the entire population at the time, or perhaps all of it. Some early accounts also suggest that the appearance of flocks in great numbers was an irregular occurrence. 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). This study found evidence that the passenger-pigeon population had been stable for at least the previous 20,000 years. 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." A similar study inferring human population size from genetics (published in 2008, and using human mitochondrial DNA and Bayesian 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.
thumb|[[Study skins in the National Museum of Natural History]]
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. Natural selection can reduce genetic diversity over extended regions of a genome through selective sweeps or background selection. The authors found evidence of a faster rate of adaptive evolution and faster removal of harmful mutations in passenger pigeons compared to band-tailed pigeons, 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 sweeps 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. 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.
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.
The pigeon could eat and digest of acorns per day. At the historic population of three billion passenger pigeons, this amounted to of food a day. 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 red oaks, 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, or after dying. The birds do not seem to have formed as vast breeding colonies at the periphery of their range. 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. As both sexes took care of the nest, the pairs were monogamous for the duration of the nesting. 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. Both parents 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. The entire nesting cycle lasted about 30 days. There is no record of a wild pigeon dying of either disease or parasites.
thumb|left|Live bird showing "highly characteristic attitude"
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.
thumb|Specimen in flying pose, [[Academy of Natural Sciences of Drexel University]]
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 they consumed—passenger pigeons are thought to have influenced both the structure of eastern forests and the composition of the species present there. Their role in creating forest disturbances has been linked to greater vertebrate diversity in forests by creating more niches for animals to fill. 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 canopies to provide more understory light.
The American chestnut trees that provided much of the 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. It has been speculated 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 reservoir hosts of Borrelia burgdorferi.
Relationship with humans
thumb|upright|Illustration of [[Native American men returning from war, illustrated by Peter Rindisbacher in 1825; note passenger pigeon corpses on the floor]]
For fifteen thousand years or more before the arrival of Europeans in the Americas, passenger pigeons and Native Americans coexisted in the forests of what would later become the eastern part of the continental United States. A study published in 2008 found that, throughout most of the Holocene, Native American land-use practices greatly influenced forest composition. The regular use of controlled burns, the girdling of unwanted trees, and the planting and tending of favored trees suppressed the populations of a number of tree species that did not produce nuts, acorns, or fruit, while increasing the populations of numerous tree species that did. In addition, the burning away of forest-floor litter made these foods easier to find, once they had fallen from the trees. Some have argued that such Native American land-use practices increased the populations of various animal species, including the passenger pigeon, by increasing the food available to them, while elsewhere it has been claimed that, by hunting passenger pigeons and competing with them for some kinds of nuts and acorns, Native Americans suppressed their population size. Genetic research may shed some light on this question. A 2017 study of passenger-pigeon DNA found that the passenger-pigeon population size was stable for 20,000 years prior to its 19th-century decline and subsequent extinction, while a 2016 study of ancient Native American DNA found that the Native American population went through a period of rapid expansion, increasing 60-fold, starting about 13–16 thousand years ago. If both of these studies are correct, then a great change in the size of the Native American population had no apparent impact on the size of the passenger-pigeon population. This suggests that the net effect of Native American activities on passenger-pigeon population size was neutral.
thumb|left|upright|Billing pair by [[John James Audubon, from The Birds of America, 1827–1838. This image has been criticized for its scientific inaccuracy.]]
The passenger pigeon played a religious role for some northern Native American tribes. The Wyandot people (or Huron) believed that every twelve years during the Feast of the Dead, the souls of the dead changed into passenger pigeons, which were then hunted and eaten. Before hunting the juvenile pigeons, the Seneca people made an offering of wampum and brooches to the old passenger pigeons; these were placed in a small kettle or other receptacle by a smoky fire.
French explorer Jacques Cartier was the first European to report on passenger pigeons, during his voyage in 1534. The bird was subsequently observed and noted by historical figures such as Samuel de Champlain and Cotton Mather. Most early accounts dwell on the vast number of pigeons, the resulting darkened skies, and the enormous amount of hunted birds (50,000 birds were reportedly sold at a Boston market in 1771). The early colonists thought that large flights of pigeons would be followed by ill fortune or sickness. When the pigeons wintered outside of their normal range, some believed that they would have "a sickly summer and autumn." In the 18th and 19th centuries, various parts of the pigeon were thought to have medicinal properties. The blood was supposed to be good for eye disorders, the powdered stomach lining was used to treat dysentery, and the dung was used to treat a variety of ailments, including headaches, stomach pains, and lethargy. Though they did not last as long as the feathers of a goose, the feathers of the passenger pigeon were frequently used for bedding. Pigeon feather beds were so popular that for a time in Saint-Jérôme, Quebec, every dowry included a bed and pillows made of pigeon feathers. In 1822, one family in Chautauqua County, New York, killed 4,000 pigeons in a day solely for this purpose.
thumb|upright|Painting of a male, K. Hayashi, c. 1900
The passenger pigeon was featured in the writings of many significant early naturalists, as well as accompanying illustrations. Mark Catesby's 1731 illustration, the first published depiction of this bird, is somewhat crude, according to some later commentators. The original watercolor that the engraving is based on was bought by the British royal family in 1768, along with the rest of Catesby's watercolors. The naturalists Alexander Wilson and John James Audubon both witnessed large pigeon migrations first hand, and published detailed accounts wherein both attempted to deduce the total number of birds involved. The most famous and often reproduced depiction of the passenger pigeon is Audubon's illustration (handcolored aquatint) in his book The Birds of America, published between 1827 and 1838. Audubon's image has been praised for its artistic qualities, but criticized for its supposed scientific inaccuracies. As Wallace Craig and R. W. Shufeldt (among others) pointed out, the birds are shown perched and billing one above the other, whereas they would instead have done this side by side, the male would be the one passing food to the female, and the male's tail would not be spread. Craig and Shufeldt instead cited illustrations by American artist Louis Agassiz Fuertes and Japanese artist K. Hayashi as more accurate depictions of the bird. Illustrations of the passenger pigeon were often drawn after stuffed birds, and Charles R. Knight is the only "serious" artist known to have drawn the species from life. He did so on at least two occasions; in 1903 he drew a bird possibly in one of the three aviaries with surviving birds, and some time before 1914, he drew Martha, the last individual, in the Cincinnati Zoo. In connection with the centennial of Martha's death, the song was cited as evidence of her iconic staturea symbol of the wanton slaughter of these pigeons and the human-caused extinction of the species. and fiction) and illustrated by many notable writers and artists, and is depicted in art to this day, for example in Walton Ford's 2002 painting Falling Bough, and National Medal of Arts winner John A. Ruthven's 2014 mural in Cincinnati, which commemorates the 100th anniversary of Martha's death.
Hunting
left|thumb|Depiction of a shooting in northern Louisiana, Smith Bennett, 1875
The passenger pigeon was an important source of food for the people of North America. Native Americans ate pigeons, and tribes near nesting colonies would sometimes move to live closer to them and eat the juveniles, killing them at night with long poles. Many Native Americans were careful not to disturb the adult pigeons, and instead ate only the juveniles as they were afraid that the adults might desert their nesting grounds; in some tribes, disturbing the adult pigeons was considered a crime. Away from the nests, large nets were used to capture adult pigeons, sometimes up to 800 at a time. Low-flying pigeons could be killed by throwing sticks or stones. At one site in Oklahoma, the pigeons leaving their roost every morning flew low enough that the Cherokee could throw clubs into their midst, which caused the lead pigeons to try to turn aside and in the process created a blockade that resulted in a large mass of flying, easily hit pigeons.
thumb|upright|1881 spread showing methods of trapping pigeons for shooting contests
What may be the earliest account of Europeans hunting passenger pigeons dates to January 1565, when the French explorer René Laudonnière wrote of killing close to 10,000 of them around Fort Caroline in a matter of weeks:
This amounted to about one passenger pigeon per day for each person in the fort.
After European colonization, the passenger pigeon was hunted with more intensive methods than the more sustainable methods practiced by the natives. The flavor of the flesh of passenger pigeons varied depending on how they were prepared. In general, juveniles were thought to taste the best, followed by birds fattened in captivity and birds caught in September and October. It was common practice to fatten trapped pigeons before eating them or storing their bodies for winter. The birds were frequently shot either in flight during migration or immediately after, when they commonly perched in dead, exposed trees. The pigeons proved difficult to shoot head-on, so hunters typically waited for the flocks to pass overhead before shooting them. Trenches were sometimes dug and filled with grain so that a hunter could shoot the pigeons along this trench. Hunters largely outnumbered trappers, and hunting passenger pigeons was a popular sport for young boys. In 1871, a single seller of ammunition provided three tons of powder and 16 tons (32,000 lb) of shot during a nesting. In the latter half of the 19th century, thousands of passenger pigeons were captured for use in the sports shooting industry. The pigeons were used as living targets in shooting tournaments, such as "trap-shooting", the controlled release of birds from special traps. Competitions could also consist of people standing regularly spaced while trying to shoot down as many birds as possible in a passing flock. The pigeon was considered so numerous that 30,000 birds had to be killed to claim the prize in one competition. Tunnel nets were also used to great effect, and one particularly large net was capable of catching 3,500 pigeons at a time. These nets were used by many farmers on their own property as well as by professional trappers. Food would be placed on the ground near the nets to attract the pigeons. Decoy or "stool pigeons" (sometimes blinded by having their eyelids sewn together) were tied to a stool. When a flock of pigeons passed by, a cord would be pulled that made the stool pigeon flutter to the ground, making it seem as if it had found food, and the flock would be lured into the trap. Salt was also frequently used as bait, and many trappers set up near salt springs. At least one trapper used alcohol-soaked grain as bait to intoxicate the birds and make them easier to kill. Another method of capture was to hunt at a nesting colony, particularly during the period of a few days after the adult pigeons abandoned their nestlings, but before the nestlings could fly. Some hunters used sticks to poke the nestlings out of the nest, while others shot the bottom of a nest with a blunt arrow to dislodge the pigeon. Others cut down a nesting tree in such a way that when it fell, it would also hit a second nesting tree and dislodge the pigeons within. In one case, of large trees were speedily cut down to get birds, and such methods were common. Sulfur was sometimes burned beneath the nesting tree to suffocate the birds, which fell out of the tree in a weakened state.
thumb|upright|Trapper Albert Cooper with blind [[decoy pigeons for luring wild birds, c. 1870]]
By the mid-19th century, railroads had opened new opportunities for pigeon hunters. While it was once extremely difficult to ship masses of pigeons to eastern cities, railway access permitted pigeon hunting to become commercialized. An extensive telegraph system was introduced in the 1860s, which improved communication across the United States, making it easier to spread information about the whereabouts of pigeon flocks. A single hunter is reported to have sent three million birds to eastern cities during his career. In 1874, at least 600 people were employed as pigeon trappers, a number which grew to 1,200 by 1881. Pigeons were caught in such numbers that by 1876, shipments of dead pigeons were unable to recoup the costs of the barrels and ice needed to ship them. The price of a barrel full of pigeons dropped to below fifty cents, due to overstocked markets. Passenger pigeons were instead kept alive so their meat would be fresh when killed, and sold once their market value had increased. Thousands of birds were kept in large pens, though the bad conditions led many to die from lack of food and water, and by fretting (gnawing) themselves; many rotted away before they could be sold.
Decline and conservation attempts
thumb|left|upright|Male and female by [[Louis Agassiz Fuertes, frontispiece of William Butts Mershon's 1907 The Passenger Pigeon]]
The notion that the species could be driven to extinction was alien to the early colonists, because the number of birds did not appear to diminish, and also because the concept of extinction was yet to be defined. The bird seems to have been slowly pushed westward after the arrival of Europeans, becoming scarce or absent in the east, though there were still millions of birds in the 1850s. The population must have been decreasing in numbers for many years, though this went unnoticed due to the apparent vast number of birds, which clouded their decline. Public protests against trap-shooting erupted in the 1870s, as the birds were badly treated before and after such contests. Conservationists were ineffective in stopping the slaughter. A bill was passed in the Michigan legislature making it illegal to net pigeons within of a nesting area. In 1897, a bill was introduced in the Michigan legislature asking for a 10-year closed season on passenger pigeons. Similar legal measures were passed and then disregarded in Pennsylvania. The gestures proved futile and, by the mid-1890s, the passenger pigeon had almost completely disappeared, and was probably extinct as a breeding bird in the wild.
For many years, the last confirmed wild passenger pigeon was thought to have been shot near Sargents, Pike County, Ohio on March 24, 1900, when a boy named Press Clay Southworth killed a female bird with a BB gun. The boy did not recognize the bird as a passenger pigeon, but his parents identified it, and sent it to a taxidermist. The specimen, nicknamed "Buttons" due to the buttons used instead of glass eyes, was donated to the Ohio Historical Society by the family in 1915. The reliability of accounts after the Ohio, Illinois, and Indiana birds are in question. Ornithologist Alexander Wetmore claimed that he saw a pair flying near Independence, Kansas, in April 1905. On May 18, 1907, U.S. President Theodore Roosevelt claimed to have seen a "flock of about a dozen two or three times on the wing" while on retreat at his cabin in Pine Knot, Virginia, and that they lit on a dead tree "in such a characteristically pigeon-like attitude"; this sighting was corroborated by a local gentleman whom he had "rambled around with in the woods a good deal" and whom he found to be "a singularly close observer." In 1910, the American Ornithologists' Union offered a reward of $3,000 for discovering a nest— .
Most captive passenger pigeons were kept for exploitative purposes, but some were housed in zoos and aviaries. Audubon alone claimed to have brought 350 birds to England in 1830, distributing them among various noblemen, and the species is also known to have been kept at London Zoo. Being common birds, these attracted little interest, until the species became rare in the 1890s. By the turn of the 20th century, the last known captive passenger pigeons were divided in three groups; one in Milwaukee, one in Chicago, and one in Cincinnati. There are claims of a few further individuals having been kept in various places, but these accounts are not considered reliable today. The Milwaukee group was kept by David Whittaker, who began his collection in 1888, and possessed fifteen birds some years later, all descended from a single pair.
right|upright=1.5|thumb|"The Folly of 1857 and the Lesson of 1912", frontispiece to [[William Temple Hornaday|William T. Hornaday's Our vanishing wild life (1913), showing Martha in life, the endling of the species.]]
The Cincinnati Zoo, one of the oldest zoos in the United States, kept passenger pigeons from its beginning in 1875. The zoo kept more than twenty individuals in a cage. Recognizing the decline of the wild populations, Whitman and the Cincinnati Zoo consistently strove to breed the surviving birds, including attempts at making a rock dove foster passenger pigeon eggs. In 1902, Whitman gave a female passenger pigeon to the zoo; this was possibly the individual later known as Martha, which would become the last living member of the species. Other sources argue that Martha was hatched at the Cincinnati Zoo, lived there for 25 years, and was the descendant of three pairs of passenger pigeons purchased by the zoo in 1877. It is thought this individual was named Martha because her last cage mate was named George, thereby honoring George Washington and his wife Martha, though it has also been claimed she was named after the mother of a zookeeper's friends.
upright|thumb|left|Martha at the [[Smithsonian Museum, 2015]]
In 1909, Martha and her two male companions at the Cincinnati Zoo became the only known surviving passenger pigeons. One of these males died around April that year, followed by George, the remaining male, on July 10, 1910. Martha died of old age on September 1, 1914, and was found lifeless on the floor of her cage. It was claimed that she died at 1 p.m., but other sources suggest she died some hours later. At the time, it was suggested that Martha might have died from an apoplectic stroke, as she had suffered one a few weeks before dying. Her body was frozen into a block of ice and sent to the Smithsonian Institution in Washington, where it was skinned, dissected, photographed, and mounted. As she was molting when she died, she proved difficult to stuff, and previously shed feathers were added to the skin. Martha was on display for many years, but after a period in the museum vaults, she was put back on display at the Smithsonian's National Museum of Natural History in 2015.
Extinction causes
[[File:Campers luck, on the Alleghenies, by R. A. Bonine.png|thumb|right|Hunted animals in the
Allegheny Mountains including passenger pigeons at the lower right, late 1800s]]
The main reasons for the extinction of the passenger pigeon were the massive scale of hunting, the rapid loss of habitat, and the extremely social lifestyle of the bird, which made it highly vulnerable to the former factors. Deforestation was driven by the need to free land for agriculture and expanding towns, but also due to the demand for lumber and fuel. About 728,000 km<sup>2</sup> (180 million acres) were cleared for farming between 1850 and 1910. Though there are still large woodland areas in eastern North America, which support a variety of wildlife, it was not enough to support the vast number of passenger pigeons needed to sustain the population. In contrast, very small populations of nearly extinct birds, such as the kākāpō (Strigops habroptilus) and the takahē (Porphyrio hochstetteri), have been enough to keep those species extant to the present. The combined effects of intense hunting and deforestation has been referred to as a "Blitzkrieg" against the passenger pigeon, and it has been labeled one of the greatest and most senseless human-induced extinctions in history. an example of the Allee effect.
left|thumb|Pigeons being shot to save crops in Iowa, 1867
The 2014 genetic study that found natural fluctuations in population numbers prior to human arrival also concluded that the species routinely recovered from lows in the population, and suggested that one of these lows may have coincided with the intensified hunting by humans in the 1800s, a combination which would have led to the rapid extinction of the species. A similar scenario may also explain the rapid extinction of the Rocky Mountain locust (Melanoplus spretus) during the same period. Speaking on May 11, 1947, Leopold remarked:
Potential resurrection of the species
thumb|Passenger pigeons (above) exhibited alongside other recently extinct birds, [[Natural History Museum, London]]
Today, at least 1,532 passenger pigeon skins (along with 16 skeletons) still exist, spread across many institutions all over the world. A hindrance to cloning the passenger pigeon is the fact that the DNA of museum specimens has been contaminated and fragmented, due to exposure to heat and oxygen. American geneticist George M. Church has proposed that the passenger pigeon genome can be reconstructed by piecing together DNA fragments from different specimens. The next step would be to splice these genes into the stem cells of rock pigeons (or band-tailed pigeons), which would then be transformed into egg and sperm cells, and placed into the eggs of rock pigeons, resulting in rock pigeons bearing passenger pigeon sperm and eggs. The offspring of these would have passenger pigeon traits, and would be further bred to favor unique features of the extinct species. The American non-profit organization Revive & Restore is currently pursuing the idea.
The general idea of re-creating extinct species has been criticized, since the large funds needed could be spent on conserving currently threatened species and habitats, and because conservation efforts might be viewed as less urgent. In the case of the passenger pigeon, since it was very social, it is unlikely that enough birds could be created for revival to be successful, and it is unclear whether there is enough appropriate habitat left for its reintroduction. Furthermore, the parent pigeons that would raise the cloned passenger pigeons would belong to a different species, with a different way of rearing young.