Passenger Pigeon

From the most common to an extinct bird species

When European explorers reached eastern North America in the 16th century, they were amazed at the huge flocks of birds they found in the ancient forests. Compared to Europe, where large parts of the forests had already been cut down, they were impressed by the immense numbers of birds in the New World. One bird in particular aroused the interest of the settlers: a strikingly beautiful species about 40 centimeters long that bred in the northern forests in summer and migrated south in winter. Due to its seasonal migrations, it was given the name passenger pigeon.

The passenger pigeon was a nomadic bird that migrated across North America in a ceaseless search for food, shelter and nesting sites. They did not travel alone, but in huge swarms that stretched over many kilometers and included thousands, sometimes millions, of individuals. The species was considered the most numerous bird species in the world, with an estimated population of three to five billion birds. Their flocks were so large that they could darken the sky for hours. Zoologist Igor Akimuschkin impressively describes what such a “solar eclipse” caused by pigeons might have looked like:

“These birds appeared in the sky in such dense flocks that the sun was literally no longer visible. It became as dim as during a solar eclipse, from horizon to horizon you could see nothing but flying pigeons, bird droppings fell from the sky like snow, and the incessant rustling of their wings sounded like the roar of the storm wind. Hours passed, but the passenger pigeons were still flying, and the end of the flock was no more visible than its beginning.”

Vom Aussterben bebdroht? p. 36f. 1972. I. Akimuschkin

Accounts from the time, such as that of a Virginia man in 1614, paint a similar picture. He told of swarms he observed in the sky for several hours, so dense that “the firmament was hidden from our eyes.” The famous ornithologist John James Audubon described 1813 one Swarm over Kentucky that took three days to completely pass.

Such reports of the massive swarms that darkened the sky sound almost unbelievable from today’s perspective. But those consistency of numerous independent observations from this period confirms the immense size of the populations. Naturalists such as Alexander Wilson and John James Audubon estimated the size of the passing flocks at over two billion birds. One sighting from 1866 even describes a swarm that covered the sky for more than 14 hours and was estimated at over three billion individuals.

Passenger pigeon – fact sheet

alternative names	Wild pigeon, Migrating dove
scientific names	Ectopistes migratorius, Columba migratoria, Columba canadensis, Ectopistes migratoria, Palumbus migratorius, Columba macroura
native range	North America (USA, Canada)
time of extinction	1914
tauses of extinction	hunting, habitat loss, slow reproduction rate
IUCN status	extinct

Distribution area and migratory behavior of the passenger pigeon

The passenger pigeon, known for its nomadic migratory behavior, was native to large parts of North America east of the Rocky Mountains. Their range extended from the Great Plains in the west to the Atlantic coast in the east and from southern Canada to the northern part of the USA, north of the Mississippi. They preferred the extensive deciduous forests of the East and moved around in huge flocks in constant search of food, nesting places or protection from the weather. These flocks were so large that they often lasted for several days and the birds flew at speeds of over 100 kilometers per hour.

Their migratory behavior was strongly dependent on the respective environmental conditions, especially on the food supply and the weather conditions. The migrations of passenger pigeons were not seasonal, but rather depended on the availability of food, like acorns or nuts. In years when food was plentiful, the flocks stayed in one place longer. In years when care was poor, they moved on. Severe winters and thick snow cover drove the birds south, while mild conditions allowed them to remain in the north.

A special feature of their behavior was the precision with which the passenger pigeons moved as a flock. When birds of prey appeared, they formed compact clouds that quickly changed direction to escape their attackers. The flocks flew in tight formations, which moved through the air in waves or spirals. They were able to further increase their speed through fast and powerful wing beats, in which the wings were held close to the body. This flight technique allowed them to navigate both open landscapes and dense forests at high speeds.

On the ground, however, the pigeons moved in short, jerky steps, always alert and ready to take to the air again in case of danger. The passenger pigeon’s ability to act as a single organism in a flock contributed significantly to its survival. Each bird in the flock precisely followed the leader bird’s movements, allowing them to evade predators in perfect synchrony.

The passenger pigeon as a crop pest

The passenger pigeon mainly fed on nuts, which were available in large quantities in the so-called fattening years, i.e. in years with a particularly rich food supply. In particular, beechnuts, acorns from various American oak species and the nuts of the American chestnut formed the main diet of these birds. In addition, they ate berries and fruits such as blueberries, elderberries, grapes, cherries and mulberries in the summer. Seeds from field maple and elm trees were also on their menu. Although their diet was largely plant-based, they also consumed insects such as caterpillars and earthworms to raise their young. Because passenger pigeons also frequented grain fields, farmers often viewed them as pests because they feared their crops would be destroyed by the birds.

Passenger pigeons fed in huge flocks that moved across the fields and forests, eating immense quantities of nuts and grain. The birds pecked nuts from the ground or picked them directly from the trees. Constantly on the move, the pigeons in the back rows flew over their counterparts in the front rows to reach fresh food. Their anatomy gave them impressive gathering and digesting efficiency: A passenger pigeon’s crop could reach the size of an orange and stored large amounts of food.The birds also had a powerful gizzard that allowed them to crush even hard nuts.

The mass appearance of passenger pigeons sometimes caused immense damage. The birds roamed fields and ate large quantities of grain. In some regions, passenger pigeons were considered a pest because they could destroy almost entire crops within a few hours. The forests also suffered from the swarms: the weight of the pigeons broke thick branches on which they sat en masse, and the droppings that fell from the trees onto the forest floor destroyed the vegetation. The consumption of nuts, especially acorns, also depleted the natural resources of forests. The American ornithologist Alexander Wilson estimated the size of a flock he observed in 1810 at over 2.2 billion birds and calculated a daily feed amount of around 600 million liters of acorns.

A newspaper report in the Marshall County Republican from 1857 illustrates the population’s concern at the time about the damage caused by pigeons. It called for the birds to be shot and used as food:

Despite the damage caused, passenger pigeons played an important role in the ecosystem, helping to create new habitats for other species by disturbing forests and clearing ground areas. After their extinction, they left an ecological gap in the forests of eastern North America that permanently changed the natural balance.

Pursuit and hunting of passenger pigeons

For centuries, the passenger pigeon was one of the most heavily hunted bird species in North America and served as an important source of food. The indigenous peoples already hunted pigeons on a large scale, but were careful not to kill any adult birds that were busy raising the young so as not to endanger the continued existence of the brood. Hunting was often done at night by knocking the birds out of their nests with long sticks. The use of nets was also common and led to the capture of hundreds of pigeons at once. Often it was even enough to throw sticks or stones at low-flying birds to kill them. The fat from the pigeons was so valuable that the indigenous peoples even used it as a butter substitute.

With the arrival of European settlers, hunting changed fundamentally. While the indigenous population practiced sustainable hunting methods, colonization led to radical, commercialized hunting. American ornithologist John James Audubon reported on a large-scale hunt in 1813, when the people of the Ohio River shot continuously for days at huge passing flocks:

“The people were all armed, and the banks of the Ohio were full of men and boys, who shot continually at the passenger pigeons that flew lower as they crossed the river. Countless birds were thus killed. For a week or more the population lived on nothing but pigeon meat and talked about nothing but pigeons.”

The Auk, the Dodo, and the Oryx. Vanishing and Vanished Creatures. p. 154f. 1965 R. Silverberg

As early as 1565, French explorer René Laudonnière reported killing 10,000 pigeons within a few weeks in Fort Caroline. Hunting increased dramatically in the 19th century. Passenger pigeons were considered easy prey and were caught and killed in large numbers. Their meat served as a cheap source of food for the growing population of the United States, and with the commercialization of hunting, passenger pigeon hunting became a full-time profession after 1860. Traders delivered tons of pigeon meat to the cities, where it was sold not only as food but also as pig feed.

A message from Indiana State Sentinel from 1857 illustrates the extent of the hunt:

In addition to the meat, other parts of the passenger pigeon were also marketed. In The Doomsday Book of Animals (1981), David Day describes how even the stomachs, intestines, and the droppings of birds were sold as remedies for various ailments such as gallstones, stomach pain, dysentery, colic, sore eyes, fever and epilepsy. The down and feathers of passenger pigeons were also used in pillows and quilts, and there was also a market for live pigeons.

In addition to commercial hunting, the passenger pigeon was also used as a target in shooting competitions. During the so-called “trap shooting,” the pigeons were released from traps and the participants competed to shoot as many birds as possible. Such competitions were so popular that the winners often shot thousands of birds, although the prize was sometimes only paid out when over 30,000 pigeons were killed.

Diverse and efficient hunting methods

The hunt for passenger pigeons was as diverse as it was efficient. In addition to the use of guns, huge nets, sulfur fumes, clubs and long poles were used. A Massachusetts family is said to have killed 1,200 birds in a single night by knocking the pigeons from their roosts with sticks. Tunnel nets, which could catch up to 3,500 birds at once, were particularly devastating. Another method was to use decoys—so-called “stool pigeons”. These pigeons had their eyes sewn shut so they couldn’t escape, and were tied to poles to attract passing flocks. Salt was also used as bait, as pigeons specifically searched for salt sources. Some trappers even used alcohol-soaked grain to stun the birds and make them easier to catch.

Felling trees was also a common method of getting to the breeding pigeons. Entire forest areas were cut down to capture nests and young birds. The trees were often felled so that they fell onto other nesting trees, destroying several nests at once. A particularly drastic method was to set trees on fire to drive away the pigeons or to throw the young birds out of their nests. Sulfur fumes were also used under the nesting trees to suffocate the pigeons, making them easier to catch.

With the introduction of telegraphs to quickly disseminate information about nesting sites and railways to transport the captured pigeons commercial hunting increased dramatically in the 19th century. In the 1850s, huge quantities of pigeons were shipped to the eastern United States, where they were sold in cities for as little as five cents per bird. In 1851, approximately 1.8 million pigeons were delivered to municipal markets in Plattsburgh, New York alone. Hunting became a lucrative business, and professional pigeon catchers, so-called “pigeoners” followed the flocks all year round to catch and sell the birds.

When the market was finally flooded with pigeon meat and prices fell dramatically, the birds were often kept alive until demand increased again. However, the poor keeping conditions often meant that many pigeons starved or died in the cages before they could be sold.

Etta S. Wilson, who witnessed the persecution of passenger pigeons as a young girl, remembers the cruel extent of the hunt in the 1930s:

“The ghastly business continued day and night. Bird glue lay everywhere and covered the ground in a thick layer. Pots in which sulfur was burned spewed out their deadly fumes in various places, where the birds suffocated. Earth spirits in human form (…) went around with sticks and clubs and knocked down bird’s nests, while others felled trees and broke off the overloaded branches to catch the young birds. (…) Pigs came to the Nesting place to fatten on the fallen birds (…). Of the countless thousands of crushed, dead, fallen birds, only comparatively few could be picked up, and yet wagonloads were driven out of the nesting place in almost continuous succession, while the ground was still covered with living, dying, dead and decaying birds.

Der Gesang des Dodo. P. 411. 2001. D. Quammen

Despite the obvious decline in stocks the hunt continued until the 1890s. When pigeon meat became almost worthless due to oversupply, hardly anyone took measures to protect the animals.

There were fewer and fewer …

At the beginning of the 19th century, the passenger pigeon was the most common bird in North America and one of the most numerous bird species in the world. But a dramatic decline began in the second half of the century, eventually leading to the extinction of the species. To this day, this rapid collapse remains partly mysterious. Although the species had already been decimated along the Atlantic coast by intensive hunting in the 17th and 18th centuries, populations in the Midwest and around the Great Lakes were considered stable until the mid-19th century.

From the 1870s onwards, however, the population decline became obvious. The last large nesting colonies became the target of massive hunting. In 1871, almost the entire remaining population of about 135 million birds was breeding in Wisconsin—only a tenth of the once estimated population. Nevertheless, the hunt continued unabated: millions of pigeons were killed and sold at markets. At the last large breeding colony in Petoskey, Michigan, up to 50,000 birds were shot every day for almost five months in 1878.

David Day reports that in 1896 there were only about 250,000 passenger pigeons left. One of the last large nesting flocks gathered in the Green River forest near Bowling Green, Ohio, in April of this year. Thanks to the telegraph and railway, hunters quickly found out about the situation and flocked from all parts of the country. The result was devastating: 200,000 pigeons were killed, another 40,000 mutilated or discarded, and countless chicks too young to be collected died or were abandoned to predators. All the prey was loaded into freight wagons to be transported to the markets in the East. But the trains derailed, the carcasses rotted in the hot sun, and eventually the 200,000 dead birds were dumped into a ravine.

As the population continued to decline, the few surviving pigeons tried to find new breeding grounds. But hunters pursued them mercilessly, killing the adult birds and preventing the animals from raising their young. In the 1880s there were only a few breeding colonies with tens of thousands of birds left, but breeding success was lacking. The population was dwindling rapidly, and the last large flock was seen in 1888. From the 1890s onwards, groups of more than a hundred birds were a rarity, and after 1895 sightings of just ten pigeons were already considered exceptional.

Protective measures for the passenger pigeon

Several states enacted laws regulating the trade in passenger pigeons, but these were often inconsistent, unclearly formulated, were hardly enforced or simply came too late. The large hunting companies continued their activities unhindered. In 1857, a committee in Ohio rejected the proposal for a protective law, stating:

“The passenger pigeon needs no protection. The animal, which is incredibly fertile, has at its disposal the endless forests of the north as its breeding ground and travels hundreds of kilometers in search of food, is here today and there tomorrow; no normal destruction can decimate it or inflict noticeable losses on the myriads produced annually.”

Der Gesang des Dodo. P. 412. 2001. D. Quammen

In the 1870s there was increasing public protest against the brutal hunting methods, but these protests had little impact. In 1897, a law was passed in Michigan that banned the trapping of pigeons within two miles of nesting sites and called for a ten-year closed season for passenger pigeons. Similar measures were taken in Pennsylvania but were largely ignored.

H. B. Roney, who witnessed the Petoskey massacre in 1878, led campaigns to protect pigeons. But his efforts were met with resistance and he was accused of exaggerating the severity of the situation.

How did the world’s most common bird species become extinct?

American author Robert Silverberg aptly summarized the disappearance of the passenger pigeon in 1965:

“In 1800, the population of the United States was 5,927,000, while the number of passenger pigeons was in the billions. By 1900, there were 76,094,000 Americans and virtually no passenger pigeons left. Within that century, a species that represented 25 to 40 percent of the total bird population of the United States was wiped out.”

The Auk, the Dodo, and the Oryx. Vanishing and Vanished Creatures. P. 158. 1965 R. Silverberg

How could it be that what was once the most common bird species in the world became extinct in less than 100 years? Many were stunned by the sudden disappearance of a species that was thought unlikely to become extinct due to its enormous numbers. This astonishment led to a variety of adventurous theories. Some believed that a priest had cursed the doves for harming his congregation. Others suspected that they drowned en masse in the Gulf of Mexico or the Atlantic Ocean; Pigeons have reportedly even been washed up on Russia’s shores. Still others speculated that the birds fled to Chile, Peru or Bolivia to escape hunting.

Ornithologist Arlie William Schorger refuted many of these imaginative theories in The Passenger Pigeon: Its Natural History and Extinction (1955). Instead, he cited more plausible reasons for the extinction, including forest fires, epidemics such as Newcastle disease, the deforestation of deciduous forests that served as a source of food, and climatic changes. Mainly, however, he—like many other scientists—blamed excessive hunting. Because of their large numbers, hunting passenger pigeons was particularly easy and lucrative.

The history of the passenger pigeon and its flock behavior are reminiscent of the fate of the Rocky Mountain locust, which also once lived in large numbers in North America. It too was once so numerous that its extinction seemed impossible, and yet it disappeared in the early 20th century.

Habitat loss and hunting

Although John James Audubon observed the mass slaughter of passenger pigeons in Kentucky in 1813, he—like many contemporaries—did not believe that the species could become extinct through hunting alone:

“Persons unfamiliar with these birds might naturally assume that such terrible slaughter would soon wipe out the species. But I have come to the conclusion, by long observation, that only the gradual reduction of our forests can bring about their decline.”

The Auk, the Dodo, and the Oryx. Vanishing and Vanished Creatures. P. 157. 1965 R. Silverberg

Argentine ornithologist Enrique H. Bucher argued in his 1992 study The Causes of Extinction of the Passenger Pigeon that it was not primarily hunting, but rather the loss of habitat that caused the extinction of the passenger pigeon. He based this thesis on the fact that the passenger pigeon is fed by mourning doves (Zenaida) that originally lived in the open grasslands of North America and later adapted to forest habitats. This secondary adaptation may have made the species particularly vulnerable to changes in these forests.

The loss of forest areas destabilized the large breeding colonies of passenger pigeons, which affected their social system. The birds relied on living in dense flocks, and as the population declined, these structures broke down, making reproduction difficult. The loss of their habitat due to the clearing of forests for agricultural land and cities further increased these problems.

Many scientists believe that a combination of hunting and habitat loss led to the passenger pigeon’s disappearance. Between 1800 and 1820, the U.S. population doubled, reaching 20 million by 1845. By 1860, there were already 30 million people living in the United States, and by 1870 the number had risen to 49 million. To make room for this growing population, cities were expanded, railroads crisscrossed the country, and forests were cleared. Between 1850 and 1910, approximately 728,000 square kilometers of forest were converted into farmland. Although there were still large areas of forest in eastern North America, these may not have been sufficient to sustain the huge populations of passenger pigeons in the long term.

In his book Extinct Birds (2017), Julian Hume points out that the passenger pigeon was closely associated with the deciduous forest region in the eastern United States and Canada and, despite its wide distribution, never settled in other types of forest. Their breeding areas were also limited by the presence of beech and oak trees. Hume suspects that the forests of western North America, although similar to eastern vegetation, were already occupied by the related band-tailed pigeon (Patagioenas fasciata) were settled, which prevented the passenger pigeon from spreading into these areas.

All or nothing

Biologist Tim Halliday stated in his 1980 study The Extinction of the Passenger Pigeon hunting is questioned as the sole reason for the extinction of the species:

“The puzzling thing about the demise of the passenger pigeon is that the decline of the species continued in the final years of its existence at a rate for which the mere fact that it was preyed upon by humans is insufficient to explain it.”

The Extinction of the Passenger Pigeon. Ectopistes migratorius and its Relevance to Contemporary Conservation. 1980. T. R. Halliday

Halliday argued that the rapid decline in the population of the passenger pigeon in recent years had happened too quickly to be explained by hunting alone. He also ruled out habitat loss as the sole cause, as there were still sufficiently large forest areas with oak and beech trees at the time of the dramatic decline. Instead, Halliday suggested that social factors played a crucial role: Colony size and reproductive success were closely related, and although the species was still relatively common, there were not enough offspring to compensate for the losses.

Halliday was of the opinion that the passenger pigeon as a species either had to exist in massive flocks or not exist at all. As hunting and persecution caused populations to fall below a critical threshold necessary for the species’ survival, its social structure collapsed. This led to inefficient foraging, disrupted mating behavior and a decline in breeding success. So the passenger pigeon became extinct because it could not survive in small groups and its rarity was at odds with its social ecology.

This threshold value described by Schorger, which indicates the minimum number of individuals required for the survival of a species, is also known as the Allee effect. In Passenger Pigeon/Allee Effect (2024), ecologist Kevin T. Shoemaker explains that the Allee effect played a crucial role in the passenger pigeon, which was a highly gregarious and social species. With the drastic decline in populations due to hunting and deforestation, their social structures collapsed. The pigeons were no longer able to reproduce effectively or protect themselves from predators. This negative cycle accelerated the eventual extinction of the passenger pigeon.

Natural fluctuations in the passenger pigeon population

A study conducted by National Taiwan Normal University in 2014 showed that the passenger pigeon population was already subject to strong fluctuations before the arrival of Europeans in North America. These natural fluctuations ultimately made the species more vulnerable to human interference such as intensive hunting and habitat loss. By analyzing ancient DNA (aDNA), scientists found that the effective population of passenger pigeons over the last million years was significantly smaller than the 19th century estimate of three to five billion birds. The population experienced repeated dramatic increases and decreases, probably caused by climatic changes and fluctuations in the food supply.

A key finding of the study is that the passenger pigeon became particularly vulnerable to human disturbance during periods of low population numbers. When intensive hunting and deforestation in the 19th century coincided with a natural population low for the species, it was no longer able to recover. The social behaviors of passenger pigeons, which relied on large flocks and colonies to breed successfully and protect themselves from predators, collapsed once the population fell below a critical size.

In Bedrohte und ausgerottete Tiere (1965), Swiss zoologist Vinzenz Ziswiler put forward the theory that the social breeding behavior of passenger pigeons played a decisive role:

“In socially breeding animals such as the passenger pigeon, which often breeds in several hundred specimens on a tree, the presence of conspecifics has a stimulating effect on breeding. It is conceivable that individual pairs were no longer in the mood to breed. This is supported by the fact that it was not possible to obtain offspring from individual pairs in captivity.”

Bedrohte und ausgerottete Tiere. 1965. V. Ziswiler

The hypothesis suggests that passenger pigeons rely on the dense presence of other members of their species for successful reproduction, which made their survival even more difficult in times of declining populations.

Diseases as a possible cause of the extinction of the passenger pigeon

William W. Thompson describes in his book The Passenger Pigeon (1922) a disease that is said to have decimated the last populations of passenger pigeons. This disease, which he calls a “cancerous tumor,” manifested itself as yellowish-white coating in the birds’ mouths that made it impossible for them to eat. Both wild and captive pigeons were reported to have been affected in the 1880s.

The “canker sores” described by Thompson could indicate a disease called trichomoniasis, which is caused by the parasite Trichomonas gallinaeis. This disease primarily affects pigeons and birds of prey and leads to painful ulcers in the mouth and throat, which make swallowing and eating difficult and are often fatal. While there is no concrete evidence that passenger pigeons were affected by the disease, trichomoniasis may have dealt the final blow to an already severely reduced population.

Another possible, but less convincing, hypothesis is Newcastle disease, a highly contagious viral disease that often causes severe losses in birds. Since passenger pigeons lived in huge, dense flocks, such a disease could easily have spread throughout the colonies and had devastating consequences. However, the role of Newcastle disease is considered small and speculative compared to the main causes such as excessive hunting and habitat loss. There is no clear evidence that Newcastle disease was a major contributor to the extinction of the passenger pigeon.

The International Union for Conservation of Nature (IUCN) emphasizes that the it is difficult to determine the exact cause of the passenger pigeon’s extinction. The most important factors, however, include the large-scale deforestation of the deciduous trees that served as the birds’ main source of food, as well as the expansion of the railway and telegraph network. The latter allowed hunters to locate nesting colonies more efficiently and quickly transport the captured birds to markets. In addition, excessive hunting, the breakdown of social structures within the flocks and Newcastle disease are cited as other factors that contributed to the rapid disappearance of the species.

Extinction as a chain reaction

The extinction of the passenger pigeon in 1914 triggered a chain reaction that also brought the end of the passenger pigeon mite—a parasite that was completely dependent on its host. Since this mite lived exclusively on the passenger pigeon, it could not survive after its host disappeared. This phenomenon highlights an often underestimated dynamic: when a keystone species goes extinct, many dependent species that live in close interaction with that species also disappear.

Specialized parasites that only live on a specific host are particularly affected by such chain reactions. In such cases the loss of a single species is enough to endanger the survival of several others. The passenger pigeon mite is one of the few well-documented examples of this type of addiction. Little is known about its biology, except that it was exclusively specialized for the passenger pigeon, which made its survival impossible after the host extinction.

When did the passenger pigeon become extinct?

While the passenger pigeon still nested in flocks of millions in the early 1880s, by 1888 it was already a rarity to see more than 175 birds at once. Within a few decades their population declined dramatically, from an estimated three billion to zero.

For a long time it was assumed that the last wild passenger pigeon, a female, was shot in March 1900 in the US state of Ohio. Writer Joel Greenberg, however, points this out in his book A Feathered River Across the Sky (2014) referred to a later case: He came across a report of a male shot and subsequently destroyed near Laurel, Indiana, in April 1902. However, the last officially confirmed wild bird was a male shot and stuffed near Oakford, Illinois, in March 1901. This specimen is now located at Millikin University in Decatur, Illinois.

In his book Die ausgestorbenen Vögel der Welt (1986), Dieter Luther suggests that the passenger pigeon probably became extinct in the wild between 1907 and 1909, as there were several unconfirmed sightings during this period.

American ornithologist Alexander Wetmore also reported in Game Birds of Prairie, Forest and Tundra (1936) that in 1905, when he was almost 20 years old, he spotted a pair of flying passenger pigeons in Kansas. However, most sightings after 1901 are believed to be confused with mourning doves (Zenaida macroura) evaluated.

The Cincinnati Zoo, one of the oldest zoos in the United States, has kept passenger pigeons since it opened in 1875. British ornithologist Julian P. Hume writes in Extinct Birds (2017) that originally a flock of 26 birds—13 females and 13 males—came to the zoo. During their time there, the birds raised 23 young. In 1908 there were only three pigeons left that were too old to breed, two males and one female. The two male birds died in 1909 and 1910.

In 1909 and 1910 a reward of $1,500 was offered for the detection of a breeding pair, unfortunately without success. The last passenger pigeon—her name was Martha—died at the age of 29 on September 1, 1914.

Martha’s body was frozen in a block of ice and taken to the Smithsonian Institute in Washington to be examined. Although billions of passenger pigeons have previously been killed, hardly anyone paid attention to the birds’ anatomy, behavior or ecology. Martha was skinned, dissected, photographed and ultimately stuffed.

Passenger pigeon: Behavior and ecology

The passenger pigeon played a crucial role in the North American forest ecosystem. This nomadic bird species fed mainly on mast, such as acorns and chestnuts, thereby influencing the distribution and growth of certain tree species.

In a study conducted in 2003, the biologists Joshua W. Ellsworth and Brenda C. McComb examined the impact of passenger pigeons on the forests of eastern North America prior to European settlement. They found that the flocks of pigeons caused significant disturbance in the forests by breaking branches and depositing nutrients from their droppings. These may have increased the frequency and intensity of forest fires. Additionally, the researchers suspect that eating acorns may have promoted white oak dominance. After the extinction of the passenger pigeon in the early 20th century, this probably contributed to the spread of the northern red oak. Because of their profound impact on forests the passenger pigeon is considered a keystone species whose disappearance left a significant ecological gap.

The extinction of the passenger pigeon also led to further ecological changes. For example, the population of the white-footed mouse (Peromyscus leucopus), which also used acorns as a food source, increased significantly after the pigeons disappeared. Ecologist David E. Blockstein speculated in his 1998 article Lyme Disease and the Passenger Pigeon?, that the disappearance of the passenger pigeon may have contributed to the increased spread of tick-borne Lyme disease. Since white-footed mice act as pathogen reservoirs for Borrelia burgdorferi, the causative agent of Lyme disease, the increase in their population after the extinction of passenger pigeons could also have promoted the spread of the disease. With more food resources available to the mice, their numbers increased, possibly promoting the spread of Lyme disease in the region.

The vocalizations of the passenger pigeon: From bell-like to croaking

The noise of a flock of passenger pigeons has often been described as deafening, audible for miles and characterized by loud, harsh, unmelodic calls. In addition to these loud voices, there were also gurgling, chirping, and cooing sounds that consisted of deep sounds rather than actual singing. The birds made croaking noises while building their nests, but they made bell-like sounds when mating. In dangerous situations, individual pigeons called out the alarm, whereupon the entire flock picked up the warning calls and flew off together.

In 1911, American behavioral scientist Wallace Craig published a detailed study of the gestures and calls of passenger pigeons, based on observations in human care. He identified several calls, including a simple, harsh “keck” used for contact, as well as a more frequent and variable “kee-kee-kee” used as a warning call. The birds used a soft “keeho” to communicate with their partner. Craig suspected that the loud and piercing calls of the passenger pigeon were due to their life in dense colonies, where only the loudest sounds were heard.

She only laid one egg per brood

The breeding behavior of passenger pigeons was closely linked to social life in huge colonies. These colonies, often referred to as “cities,” were home to millions of birds and spanned hundreds to thousands of acres. The nests were built close together in the trees, often more than 50 nests per tree. A colony existed for several weeks and nest building began in the spring, between March and May depending on the region.

The nests were built from simple twigs that the male collected and gave to the female, who completed the nest. Construction was synchronized and typically took two to four days. The nests were located at heights of two to 20 meters and were often so shallow that the egg was visible from below.

The passenger pigeon usually only lays a single oval, white egg per brood. Both parents took part in brood care: The male bird took over breeding during the day and the female at night. The incubation period was twelve to 14 days, and after hatching the chicks grew quickly. Within about two weeks they reached the same weight as their parents. During this time, both parents fed them crop milk, a nutritious substance produced in their crop glands. After 13 to 15 days the parents left the nest and the chick was left to fend for itself.

It seems paradoxical that the passenger pigeon, despite its huge flocks, only lays one egg per brood. While other species maintain their populations through high reproductive rates, the passenger pigeon was so numerous because of the lack of natural enemies—apart from birds of prey such as hawks and eagles. She lived in an almost inexhaustible habitat with plenty of food. But when people began to hunt pigeons en masse, the species could no longer compensate for the losses with its low reproductive rate. Even under optimal conditions, it was not enough to counteract the dramatic decline.

Hardly anyone paid attention to the anatomy of the passenger pigeon

The internal anatomy of the passenger pigeon has rarely been described in detail. In 1914 the American ornithologist Robert W. Shufeldt examined the skeleton of a male bird and did not notice any noticeable differences from other pigeon species: “There is nothing special about the skeleton of this species—in fact it is a typical columbiform skeleton, the kind you would find anywhere.”

Finally in A Magnificent Flying Machine: The Anatomy of the Passenger Pigeon (2015), British ornithologist Julian P. Hume took another look at the anatomy of the passenger pigeon and discovered some striking differences. Particularly striking were the large pectoral muscles, which indicated the passenger pigeon’s enormous flying power. The Musculus supracoracoideus, which connects the shoulder blade, the wishbone and the breastbone, was relatively large at 33.4 millimeters and had more robust joint ends than in other pigeon species. The sternum was also particularly large and strong compared to other pigeons. The wing bones (humerus, radius, ulna and carpometacarpus) were shorter but more robust. The leg bones, on the other hand, largely corresponded to those of other pigeon species.

Taxonomy: From mourning dove to passenger pigeon

The taxonomic history of the passenger pigeon dates back to the 18th century and is remarkably complex. In his Systema Naturae, published in 1758, Swedish naturalist Carl Linnaeus coined the binomial name Columba macroura, which he used for both the passenger pigeon and the mourning dove, since he considered both species to be identical.

Linnaeus’ description was based on two older works: Mark Catesby’s Natural History of Carolina, Florida and the Bahama Islands (1731–1743), in which the passenger pigeon was referred to as Palumbus migratorius, and George Edwards’ 1743 description of the mourning dove. It is believed that Linnaeus never saw a specimen of these birds himself and his description was based entirely on these earlier reports and illustrations. In the 1766 edition of Systema Naturae Linnaeus changed the names: The passenger pigeon became Columba migratoria and the mourning dove Columba carolinensis.

In 1827, the British naturalist William John Swainson transferred the passenger pigeon from the genus Columba into the newly created, monotypic genus Ectopistes, mainly due to their long wings and wedge-shaped tail. This led to the official name of the passenger pigeon being changed to Ectopistes migratorius. Discussions about the correct scientific name of the species continued into the 20th century, until finally, in 1955, until the International Commission on Zoological Nomenclature (ICZN) determined that migratorius was the valid species name for the passenger pigeon. Since then, the passenger pigeon has officially borne the name Ectopistes migratorius.

Evolution of the passenger pigeon

The passenger pigeon belongs to the pigeon family (Columbidae). Due to morphological and physical similarities, ornithologists such as Enrique H. Bucher assumed that the passenger pigeon was closely related to mourning doves (Zenaida) is related. Some scientists even classified the mourning dove as belonging to the genus Ectopistes.

Despite these similarities passenger pigeons were clearly different from mourning doves, for example through their larger body size, the lack of a facial stripe, their pronounced sexual dimorphism, iridescent neck feathers and smaller clutches. In a study published in 2002, American geneticist Beth Shapiro analyzed ancient DNA (aDNA) from museum specimens of the passenger pigeon for the first time. Their analysis revealed that the passenger pigeon is more closely related to the cuckoo-doves (Macropygia), while mourning doves are more closely related to quail-doves (Geotrygon) and ground-foraging doves of the genus Leptotila.

Another significant genetic study, The Flight of the Passenger Pigeon, published by ornithologist Kevin P. Johnson in 2010, confirmed that the passenger pigeon is most closely related to pigeons of the genus Patagioenas, including the band-tailed pigeon, which is common in western North America. This genus is closely related to Southeast Asian pigeon genera such as Turacoena, cuckoo-doves and Reinwardtoena. The authors of the study suggest that the ancestors of passenger pigeons may have colonized the New World from Southeast Asia via the Pacific or via Beringia, the former land bridge between Asia and North America.

A study published in 2012, examined nuclear DNA from passenger pigeons for the first time, confirmed the close genetic relationship with the New World pigeons of the genus Patagioenas. However, unlike the 2010 results, these findings suggested that the ancestors of the passenger pigeon and its Old World relatives came not from Southeast Asia, but from the Neotropical region of the New World—an area that includes Central and South America and the Caribbean.

The passenger pigeon had no known subspecies. It has been reported that when kept in cages, they get along with the Barbary dove (Streptopelia risoria) crossed, but the offspring were sterile.

Genetic engineering: The return of the passenger pigeon?

Supported by the non-profit organization Revive & Restore biologists at the University of California, Santa Cruz launched the Passenger Pigeon Project in 2012 to bring the extinct passenger pigeon back to life. The goal of this project is not only to bring back this once widespread bird species, but also to restore its lost ecological role in the forests of eastern North America.The project aims to serve as a model for the revival of extinct species (de-extinction) and show how lost ecological functions can be restored through the return of a keystone species.

Historically, the huge flocks of passenger pigeons played a crucial role in the ecosystem. Due to their dense population and the large amounts of excrement they dropped, the pigeons caused natural disturbances in the forest. These disturbances stimulated regeneration, promoted biodiversity and contributed to the long-term health of forests. Since the extinction of the passenger pigeon, these natural disturbance factors have been missing, which has led to a reduction in regenerative cycles and had a negative impact on the biodiversity of the region. Reintroducing a stable population could therefore restore ecological balance in forests and help promote biodiversity.

The Passenger Pigeon Project is based on a multi-stage genetic process. First, the genome of the scaly-necked pigeon, a close relative of the passenger pigeon, will be completely sequenced and compared with that of the extinct passenger pigeon. Through targeted genetic modifications, the characteristics of passenger pigeons will be reconstructed in living scaly-necked pigeons. These genetically modified pigeons are then bred in breeding programs and later released into the wild to create a new generation of passenger pigeons that can resume their original role in nature. To find out how reintroducing the passenger pigeon would affect America’s forests, researchers are studying old trees in Wisconsin to determine whether conditions would still be suitable for the birds.

The project has already made important progress. The band-tailed pigeon genome has been completely decoded and researchers have begun to identify and edit the relevant genes. However, significant challenges remain, including creating a sufficiently large population and providing suitable habitat for the return of passenger pigeons. Critics, including the WWF, express concerns that the passenger pigeon’s former habitat has changed so much that a successful reintroduction is questionable. It is also being discussed whether the considerable financial resources spent on this ambitious project could not be better invested in protecting endangered species and habitats.