Passenger Pigeon

The passenger pigeon was nomadic, constantly migrating in search of food, shelter, or nesting grounds. In his 1831 Ornithological Biography, American naturalist and artist John James Audubon described a migration he observed in 1813 as follows: 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 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... 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... 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. 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 1.0 m (3.3 ft) above the ground in windy conditions to as high as 400 m (1,300 ft). These migrating flocks were typically in narrow columns that twisted and undulated, and they were reported as being in nearly every conceivable shape. A skilled flyer, the passenger pigeon is estimated to have averaged 100 km/h (62 mph) 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 as adept and quick at 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. The passenger pigeon was one of the most social of all land birds. 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. 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. 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," while only seven years later, in 1866, one flock in southern Ontario was described as being 1.5 km (0.93 mi) wide and 500 km (310 mi) 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. 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. 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 (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. Some early accounts also suggest that the appearance of flocks in great numbers was an irregular occurrence. 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. 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). For a 2017 genetic study, the authors sequenced the genomes of two additional passenger pigeons, as well as analyzing the mitochondrial DNA of 41 individuals. 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 had been larger than the 2014 genetic study had found. 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. 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 particular 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. 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.” 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 260 km (100 sq mi) or greater. Some roosting areas would be reused for subsequent years, others would only be used once. 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. Dung could accumulate under a roosting site to a depth of over 0.3 m (1.0 ft). 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. 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. 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 a wild pigeon lived. The bird is believed to have played a significant ecological role in the composition of pre-Columbian forests of eastern North America. For instance, while the passenger pigeon was extant, forests were dominated by white oaks. This species germinated in the fall, therefore making its seeds almost useless as a food source during the spring breeding season, while red oaks produced acorns during the spring, which were devoured by the pigeons. 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 forest fires, which would have favored fire-tolerant species, such as bur oaks, black oaks, and 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). 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 that they ate the seeds of — specifically, that 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), while white oaks, 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. 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. Due to these influences, some ecologists have considered the passenger pigeon a keystone species, with the disappearance of their vast flocks leaving a major gap in the ecosystem. Their role in creating forest disturbances has been linked to greater vertebrate diversity in forests by creating more niches for animals to fill, as well as contributing to a healthy forest fire cycle in the forests, as it has been found that forest fires have increased in prevalence since the extinction of the passenger pigeon, which seems to go against the idea that the tree limbs and branches they would bring down served as fuel for the fires. 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. 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. 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.

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. Though the western forests were ecologically similar to those in the east, these were occupied by band-tailed pigeons, which may have kept out the passenger pigeons through competitive exclusion. The passenger pigeon wintered from Arkansas, Tennessee, and North Carolina south to Texas, the Gulf Coast, and northern Florida, though flocks occasionally wintered as far north as southern Pennsylvania and Connecticut. It preferred to winter in large swamps, particularly those with alder trees; if swamps were not available, forested areas, particularly with pine trees, were favored roosting sites. There were also sightings of passenger pigeons outside of its normal range, including in several Western states, Bermuda, Cuba, and Mexico, particularly during severe winters. It has been suggested that some of these extralimital records may have been due to the paucity of observers rather than the actual extent of passenger pigeons; North America was then unsettled country, and the bird may have appeared anywhere on the continent except for the far west. There were also records of stragglers in Scotland, Ireland, and France, although these birds may have been escaped captives, or the records incorrect. 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.

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 westwards 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. In 1856 Bénédict Henry Révoil may have been one of the first writers to voice concern about the fate of the passenger pigeon, after witnessing a hunt in 1847: Everything leads to the belief that the pigeons, which cannot endure isolation and are forced to flee or to change their way of living according to the rate at which North America is populated by the European inflow, will simply end by disappearing from this continent, and, if the world does not end this before a century, I will wager... that the amateur of ornithology will find no more wild pigeons, except those in the Museums of Natural History. By the 1870s, the decrease in birds was noticeable, especially after the last large-scale nestings and subsequent slaughters of millions of birds in 1874 and 1878. By this time, large nestings only took place in the north, around the Great Lakes. The last large nesting was in Petoskey, Michigan, in 1878 (following one in Pennsylvania a few days earlier), where 50,000 birds were killed each day for nearly five months. The surviving adults attempted a second nesting at new sites, but were killed by professional hunters before they had a chance to raise any young. Scattered nestings are reported into the 1880s, but the birds were now wary, and commonly abandoned their nests if persecuted. By the time of these last nestings, laws had already been enacted to protect the passenger pigeon, but these proved ineffective, as they were unclearly framed and hard to enforce. H. B. Roney, who had witnessed the Petoskey slaughter, led campaigns to protect the pigeon, but was met with resistance, and accusations that he was exaggerating the severity of the situation. Few offenders were prosecuted, mainly some poor trappers, but the large enterprises were not affected. In 1857, a bill was brought forth to the Ohio State Legislature seeking protection for the passenger pigeon, yet a Select Committee of the Senate filed a report stating that the bird did not need protection, being "wonderfully prolific", and dismissing the suggestion that the species could be destroyed. 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 3 km (1.9 mi) 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. Small flocks are known to have existed at this point, since large numbers of birds were still being sold at markets. Thereafter, only small groups or individual birds were reported, many of which were shot on sight.