The passenger pigeon, an extinct American bird, was once so common that it flew over the deciduous forests of eastern North America in flocks numbering in the billions.
What were passenger pigeons?
Passenger pigeons were about the size of Europe's stock dove and ate acorns, beech mast and chestnuts. Oaks and beeches produce bumper crops of seeds every few years, and those 'mast' years are synchronised over large areas of forest, so the pigeons were nomadic and each year sought out those forests with greatest mast abundance.
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A pigeon that hatched on the East Coast in New York State, in a good year for acorns there, might next year nest farther inland where there was plentiful beech mast, perhaps in Michigan in the Great Lakes region.
In between it might have wintered well to the south in Georgia or Arkansas, where the autumn seedfall is not buried under deep winter snow.
From hatching until death, a passenger pigeon was always within sight of thousands, possibly millions, of other passenger pigeons that nested in colonies of sometimes hundreds of millions of pairs, fed in huge flocks that scoured the forest floor like living harvesting machines, and migrated as a group - their winter roosts were so dense that the sheer weight of birds broke large branches off mature trees.
It is very difficult for us to imagine this abundance, but many consistent accounts have survived until today.
The artist John lames Audubon painted the species for his magnificent Birds of America. He wrote of passenger pigeons as follows: "The air was literally filled with pigeons; the light of noonday was obscured as by an eclipse; the dung fell in spots. not unlike melting fakes of snow; and the continued buzz of wings had a tendency to lull my senses to repose
Audubon estimated that over a billion birds passed in three hours, and there are many other credible accounts of immense flocks taking several days to pass that could have numbered more than three billion. Unless all of the passenger pigeons had gathered into a single flock for each of these observers, then we can only wonder at what proportion of the whole population they saw. Previous estimates indicated that there might have been three to five billion individuals in total, but I wonder whether it could have been even higher, perhaps touching to billion after years of high productivity.
Recent genetic studies, albeit based on only three individual museum specimens, suggest that passenger pigeons might have undergone large natural population swings - as would be expected from a species whose food supply fluctuated wildly. But this genetic work has produced an estimate of about 300,000 birds for the average population size in historic times (that is, before hunting and habitat loss became intense), which is just one-ten-thousandth of the lowest 19th-century estimates. So even if eyewitness accounts were exaggerated, they differ substantially from the genetic information.
Do you believe the genetics or the ecology? It seems to me unlikely that a species whose largest recorded colony numbered over 100 million birds, at a time when it was highly persecuted and when its forest habitat had been greatly reduced, could have had an average population size of 300,000 in the absence of such persecution and in pristine forests. Something doesn't add up.
Nevertheless despite its population ups and downs the passenger pigeon was still the most abundant bird the world has ever seen, only decades before extinction.
When did passenger pigeons go extinct?
The last passenger pigeons were lost from the wild in 1900, the species lived on in captivity until the very last one, named Martha, died at Cincinnati Zoo at lunchtime on September 1914.
Why did the passenger pigeon go extinct?
There's rarely just one reason why a species becomes extinct. For the passenger pigeon, the main factor was the loss of forests. About half of the USA's native forest was felled by 1872, for firewood, house construction and to clear land for agriculture, thus removing half of the species' habitat. But that's not the whole story - young oaks and beeches produce few acorns and beech mast, so areas of forest with young trees, or areas where old trees were selectively logged, would have furnished less food, too.
The introduction of avian diseases might have played a role as well. The arrival of the house sparrow in North America, introduced from Europe to Brooklyn, New York, in 1851 (and elsewhere subsequently), occurred at the very time of the passenger pigeon's rapid decline in numbers - it's a factor that seems impossible either to prove or to rule out with certainty.
Other animals shared the passenger pigeon's taste for acorns, chestnuts and beech mast. The forests were now scoured not only by the native wild turkeys, bears, jays and a wide range of small rodents, but also by domesticated and feral pigs. Hog-farmers noted that their animals starved if a multitude of pigeons descended on their woods, and there must have been some impact in the other direction - forest pigs would have depleted the food supply of the passenger pigeons. This was no trivial matter - Cincinnati was at the centre of the US pork business and was even nicknamed 'Porkopolis'.
The most eye-catching impact, though that does not make it the most important, was human exploitation. For centuries Native Americans removed passenger pigeons from nesting colonies according to strict rules. Adults were rarely taken and harvest was of the fat young pigeons, or squabs, which were also tastier. Native American bands would relocate to take advantage of the arrival of pigeons in their neighbourhood, but the bird's superabundance swamped any impact of the small human population (in its low millions) - until Europeans invaded the continent.
Between 1850 and 190o, as pigeon numbers plummeted, the US human population soared more than threefold through immigration and a high birth rate. Many of these folk had rifles, and the invention of the telegraph and the spread of the railroad meant that people with guns could travel quickly to wherever the passenger pigeons were nesting to plunder their numbers - and they did!
The culling of passenger pigeons at their colonies provides a case study of unsustainable harvesting. It took place at the time of year that would have the biggest impact on the population, reducing nesting success through disturbance as well as by the direct deaths of adults and squabs. This was uncontrolled plunder, and regulation eventually introduced by state legislatures was not enforced. Trees were felled to get at the nests, sometimes over 100 per tree, and bark was set alight to make the young squabs flee their nests and fall to the ground.
Passenger pigeons were trapped by various ingenious means, and shot in their millions. Trainloads of barrels of pigeons were sent east to New York, Washington and Philadelphia to restaurants and markets.
The passenger pigeon was a valuable resource, but we squandered it. No longer can you order pigeon pie as a treat; no longer can you see the sky darken as the pigeons search for the forests with most acorns and beech mast. We drove the passenger pigeon to extinction with a combination of a growing human population conquering the American continent, better guns, easier travel and improved communications. Perhaps we should reconsider what we mean by progress.
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Images @ Getty