Pigeon Whodunnit

Monday, July 7, 2025

Pigeon Whodunnit

Martha’s ancestor [Source: Public domain, via Wikimedia Commons]

The passenger pigeon is an extinct bird species of North America that was estimated once to number in the billions—maybe three or five billions. It began to enter a dramatic decline by the 1870s, and the last known specimen, a female named “Martha”, died in the Cincinnati Zoo in around 1914.

The expanding European settlement of North America played a direct role in the species’s extinction. The birds’ abundance and easy capture made them a welcome source of meat just as food production industrialized. Passenger pigeons clearly were over-harvested over several decades. They were over-harvested because they were over-abundant. But had they always been present in billions?

The previous consensus view was based on reconstructions from observer accounts. The Wikipedia page has a good discussion of how observations were converted into plausible numbers. In this snippet, the estimates are based on a journal entry by John James Audubon in 1813, and reports from contemporaries:

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.

Facts more tangible and solid than that are as difficult to ascertain as it is to recreate the environments of the past. A lot of unanswerable questions remain, considering that past landscapes and human shaping of them are lost to time, with only interpretations of scattered eyewitness accounts to go by.

More recent genetic research has led to the conclusion that numbers in the billions were extreme outliers for a species whose habitat, which included human predation, was changing rapidly in the centuries after Europeans began arriving in the Americas.

The range and habitat of passenger pigeons in the couple centuries before extinction was centered around the Great Lakes Basin and from the Mississippi watershed eastward: This much is well established. These areas prior to 1492 have come to be considered human-cultivated landscapes. The eastern deciduous forests were subjected to controlled undergrowth burns, and the Native American peoples encouraged certain tree species to grow as a food source—both directly for the nuts, and as mast for the large foraging herbivores such as white-tailed deer, moose, and elk.

By the time the English were making their first attempts to settle the eastern coast of New England, a century of Eurasian infectious disease had already begun to ravage Native American populations. As Charles C. Mann described it in 1491, the English from the 1600s and later in North America happened upon peoples whose numbers had been reduced somewhat, and would rapidly decline as diseases spread faster than the new settlers themselves.

When European explorers began crossing the North American continent, the cultural encounters with Native Americans was like conquering troops encountering the escaped survivors of the Nazi concentration camps as they found them. Their numbers were so drastically reduced that entire cultures and traditions went forever extinct and lost, with only a few individuals here and there to recall the hundreds of generations of civilizational knowledge and learning. Along with this knowledge presumably went the best, least destructive use of passenger pigeons as a reliable food source—not one to be exploited to the species’s destruction.

The North American Native Americans did rely on passenger pigeons as a meat source—this much is well known. And the European newcomers did, too, as improved manufacturing technology made harvesting, preservation, and commodification all the easier. The industrialization improved the speed of exploitation, just as the demand for food grew along with human population numbers.

Was it the disappearance of the Native Americans that caused passenger pigeon populations to soar into the billions? They were no longer available with the skills and knowledge they might have had to manage the cultivated woodland environment. Although what those skills and knowledge were are unknown to us, too. When a pre-literate culture suffers catastrophic collapse in numbers, all of its acquired learning can easily vanish without a trace.

The numbers based on genetic proxies lead to estimates that passenger pigeon were stable in the low-digit millions over tens of thousands of years. As with most species, numbers fluctuated based on the availability of food resources. For passenger pigeons the most common mast is believed to have been tree nuts: white oak and red oak acorns, American chestnuts, and beech tree nut. Some theories say white oak distribution relied to some extent on passenger pigeons. But whether or not passenger pigeons were ever a keystone species—one that keeps an ecosystem functioning—is a subject of ongoing debate.

If we try going back even farther in time to when the first humans appeared in the Western Hemisphere—back during the ice ages—evidence has inspired hypotheses about revolutionary changes in all animal life starting tens of thousands of years ago, along with glacial melting:

As the glaciers receded, radical changes began taking place. Humans invaded the continent. Whether small bands who hunted and gathered at the sea’s edge on the Pacific Coast came earlier, or whether—even more controversially—some ice-edge hunters hopped over the margins of the retreating sea ice from Europe, the general consensus is that most of the new Americans came from Asia over the Bering Land Bridge.

Recent studies by Stuart Fiedel suggest that the people who became Clovis Man may have come down the ice-free corridor that opened along the flank of the Rockies on dogsleds, taking only a few months. And, whether or not you accept the so-called Pleistocene overkill scenario (a meteor strike in northeastern Canada may also have been the culprit), most of the big native mammals—a charismatic megafauna that rivaled or surpassed that of the Serengeti—were gone in less than 1,000 years (incidentally, according to a recent theory, causing their own shift toward deciduous trees). Today’s so-called “American” megafauna—the modern bison, elk, moose, grizzlies, and wolves—are all from the Old World, just like humans. Our only true native large mammals are deer, pronghorn, coyote, and puma.

The rest of this article from the Cornell Lab All About Birds site is here.

Based on that radical change in ecosystems, it seems just as reasonable to assume that the passenger pigeons originally succeeded because Homo sapiens arrived. What was catastrophic for some plants and animals must logically have meant opportunity for other creatures. When the Europeans arrived with diseases that (unintentionally) erased the Native Americans and their cultural learnings, the passenger pigeons enjoyed one last burst of unconstrained population growth. This temporary abundance inspired European settler over-harvesting. And the aggressive practice didn’t cease until the passenger pigeon numbers collapsed and survival became impossible under the birds’ requirements to breed and persist.

What do we learn from all this? The natural world is a perplexing place. And even our best theories about it are forever on shaky foundations, just waiting for one more tranche of apparently peripheral evidence to bring it all crashing down—much like the branches of trees were described to do under the weight of passenger pigeon flocks roosting for a night. That, or else we invent time travel.