Passenger pigeons’ biology may share blame in extinction
MINNEAPOLIS -- A hundred years ago, Martha died and an environmental cautionary tale was born.
On Sept. 1, 1914, Martha, a passenger pigeon, fell off her perch at the Cincinnati Zoo and dropped dead. She was 29.
She also was the last known example of her species, the sole survivor of what once was the most numerous bird in North America and possibly the world.
Estimates of the numbers of passenger pigeons in the mid-1800s ranged up to 5 billion. At that time, as many as two out of every five birds in America was a passenger pigeon, according to another estimate.
Accounts by John Audubon and others of the staggering numbers of passenger pigeons that were common in the 19th century sound nearly mythical, describing them as sort of a cross between a natural disaster and a natural wonder, a manifestation of nature’s abundance and fertility.
They formed “superflocks” a mile-wide and hundreds of miles long that eclipsed the sun and took days to pass. Their millions of wings made a noise mistaken for a tornado or thunder when they passed overhead at 60 miles an hour. They could devastate a farm crop. Their nesting colonies could cover more than 100 square miles. And when they roosted, their multitudes caused trees to topple and dung to pile up a foot deep.
But they were still no match for hungry humans with modern weapons.
In the course of a few decades, habitat destruction and unfettered hunting wiped out the birds.
The abrupt extinction of the passenger pigeon, “the greatest human-caused extinction in recorded history,” became a metaphor for the destructive rapacity of man.
“No one thought you could cause something as common as the passenger pigeon to go extinct. It was inconceivable,” said Robert Zink, University of Minnesota professor and curator of birds at the school’s Bell Museum.
However, recent research co-authored by Zink suggests that maybe the bird’s own biology shares a bit of the blame.
The new study — “Drastic population fluctuations explain the rapid extinction of the passenger pigeon” published this summer in the Proceedings of the National Academy of Sciences — indicated that while passenger pigeons were booming in the early 19th century, that wasn’t always the case.
Analysis of DNA scraped from the toe pads of museum specimens more than a century old, including three birds housed at the Bell Museum, have showed that the creatures were a so-called “outbreak” species, similar to plague locusts or lemmings.
In other words, they underwent large swings in population size, which could have made them vulnerable to determined human depredation.
The study suggests that a natural downturn in the bird’s numbers occurred in the late 19th century. That, unfortunately, was when commercial hunting of the birds really got going.
It would have been a one-two combination that spelled curtains for passenger pigeons, the study’s authors believe.
“We hypothesize that a downward trend in its population size occurred simultaneously with human exploitation in the late 1800s and that the combination of the two triggered its rapid extinction,” the study said.
Zink said the acorn-gobbling bird was able to recover from population lows before caused by North American glaciers and bad acorn years.
But it couldn’t overcome the clearing of forests for farming and logging and hunting on an industrial scale.
Aided by a telegraph system to relay alerts of where the birds were flocking and a railroad system to quickly get there, professional pigeon killers filled up railway boxcars with what became a 19th century staple of the American diet.
“There’s no question we’re the reason the passenger pigeon is extinct,” Zink said.
“The principal attraction is the birds were cheap,” said Joel Greenberg, author of the recently published book, “A Feathered River Across the Sky: The Passenger Pigeon’s Flight to Extinction.”
“They were the cheapest terrestrial protein,” he said.
They were also easy to find and kill.
Disappearance and denial
Passenger pigeons aren’t particularly clever or elusive. But according to Zink, they didn’t have to be. Their huge numbers meant that natural predators couldn’t kill enough of them to make a difference in their survival as a species. At least until humans with 19th century technology came along.
A single shotgun blast into a dense flock could bring down a dozen birds. A kid with a pole could knock them out of a tree or even out of the sky. They were also netted, poisoned, burned out of their roosts and gassed with burning sulfur.
Minnesota’s Pigeon River and Pigeon Falls are named after passenger pigeons. But according to Zink, so are the stool pigeon and the clay pigeon.
The birds were slaughtered in such numbers that surplus carcasses were used to feed hogs and fill potholes, according to Greenberg.
Even as their numbers dwindled, the killing continued. Some people were in a state of denial about the disappearance of the bird, believing stories they had flown to South America and changed their appearance.
“That sounds crazy to us now, but at the time, the crazier alternative was that the passenger pigeon would go extinct,” said Elisabeth Condon, as assistant scientist with the University of Minnesota’s Natural Resources Research Institute.
Hunting efforts actually intensified as the flocks shrank, Condon said.
“People were really eager to shoot the last passenger pigeon,” she said.
According to Zink, it wasn’t necessary to shoot the last bird. We just had to fragment their habitat and bring their numbers down to a low enough level where their own biology made it hard for them to recover. Without large colonies and flocks, the passenger pigeon had a hard time breeding, scouting for acorns and resisting predators.
The last wild nest and egg of a passenger pigeon was found in Minneapolis in 1895, according to Greenberg. The last credible report of a wild bird bagged by a hunter occurred in 1902. The second to the last bird living bird, named George, died in captivity in 1910 after failing to reproduce with Martha.
Martha, who had spent her life in captivity, endured the last four years of her life as a lonely, tragic celebrity. Greenberg said her death may be unique in bird extinctions in that we know the day when a species came to an end.
When Martha died, her body was packed in a 300-pound block of ice and shipped by train to the Smithsonian Institution.
“She never flew in her life,” Greenberg said. But in death, she flew first-class twice on an airplane, once to be displayed in San Diego in 1966 and once on a visit back to the Cincinnati Zoo in 1974 for a dedication of a building named in her honor.
The death of Martha and billions of her kin are credited with helping to spur the modern conservation movement in this country. But their disappearance may foreshadow depletions of another seemingly unlimited resource, ocean fisheries.
“The story of the passenger pigeon looms large, unlike any other bird,” Greenberg said.
And the story may not end if something called “The Great Passenger Pigeon Comeback” turns out to be a success.
That’s the “flagship project” of a San Francisco-based organization called Revive & Restore dedicated to “de-extinction,” or using cutting-edge DNA science to bring extinct animals back to life.
The project would use passenger pigeon DNA harvested from museum specimens and DNA of the closely related band-tail pigeon to create a bird that would look and behave like a passenger pigeon.
“What is created is going to be up for debate by a lot of people,” said Ben Novak, lead researcher on the project. “It’s a lot like passenger pigeon 2.0.”
Novak said if everything goes right, the Adam and Eve of this species reset might be created in about eight years. He said by using surrogate band-tail pigeon parents, several hundred or even a thousand new passenger pigeons could be created in another five years for “soft release” in an enclosed wild environment.
“I can see wanting to bring it back because it’s pretty clear we caused its demise,” Zink said of the project. “I think the motivation is apologetic. We’re sorry for the passenger pigeon, we’re sorry we did this to you.”
But Zink said the project would be no more than a “scientific curiosity.”
He said the thousands of square miles of mature oak forests that passenger pigeons need to support large colonies don’t exist anymore.
Greenberg also noted passenger pigeons resemble mourning doves, the most hunted game bird in America.
If the re-created passenger pigeons “escaped and flew to a place where they’re hunting mourning doves, they’d probably shoot those too,” he said.
“That would be a shame. How many times can we cause something to go extinct?” Greenberg said.
The Pioneer Press is a media partner with Forum News Service.