S1E3: What Caused the Passenger Pigeon Extinction?
This podcast episode explores the history and dramatic extinction of the Passenger Pigeon, once the most numerous bird on Earth.
In this episode, Gabby and AJ talk to Mark Avery, author of A Message from Martha. As a biologist and conservationist, Mark takes us back in time to reconstruct the biology, habitat, and final era of the Passenger Pigeon. The Passenger Pigeon’s extinction is one of the most dramatic extinction stories of the 20th century, resulting in the loss of the most numerous bird on Earth. This episode explores everything from nesting biology and historical accounts to habitat destruction and the last Passenger Pigeon to die in captivity at the Cincinnati Zoo—Martha.
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Passenger Pigeon Episode Podcast Script
Gabby: AJ, I must say that I absolutely loved your article on the extinction of the passenger pigeon that you put together for the spring issue of Project Upland Magazine. It was so well-researched, and I learned a ton. What inspired you to write it?
AJ: Believe it or not, it’s a taxidermied specimen of a passenger pigeon that exists at the New Hampshire Fish and Game headquarters in Concord. I walk by this bird every time I go there, and I always stop and look at it. When I first noticed it, I happened to be reading the book 1491: New Revelations of the Americas Before Columbus by Charles C. Mann, which mentions the history of the passenger pigeon. Naturally, I got pulled down a rabbit hole—hence my inspiration for the article.
Gabby: That is so cool that you’ve actually seen a passenger pigeon before. Granted, it wasn’t alive, but still. And given that, I’m assuming you’re familiar with A. W. Schorger’s book, The Passenger Pigeon. That author was a professor in the wildlife ecology department I graduated from at UW-Madison. He was an incredible ecologist and actually helped Aldo Leopold start his career.
And as a side note, his first book is called Birds of Dane County, and if anyone has a copy or knows where I can find one, please tell me! I grew up in Dane County, and I would love to add it to my collection.
AJ: Today’s episode is very similar to, I’m sure, the motivation of those ecologists—it’s about a cautionary tale, a story we should never forget. It is the true story of how humans can create such a big impact that they cause the extinction of even the most numerous bird species on Earth.
John Muir: The breast of the male is a fine, rosy red. The lower part of the neck, behind and along the sides, changes from the red of the breast to gold, emerald green, and rich crimson. The general color of the upper parts is grayish blue, the underparts white. The extreme length of the bird is about 17 inches, the finely modeled slender tail about 8 inches, and the extent of the wings 24 inches. The females are scarcely less beautiful.
“Oh, what bonny, bonny birds!” we exclaimed over the first that fell into our hands. “Oh, what colors! Look at their breasts, bonny as roses, and at their necks, aglow with every color, juiced like the wonderful wood ducks.”
—John Muir, The Story of My Boyhood and Youth, 1913.
AJ: I want people to use their imagination for a moment and picture the sky filled with literally millions of birds—so many that the sky darkens in what was called the “blue meteor.” That was their nickname. And there is no equivalent in 2025 that could even help us conceptualize what that was like.
You can think of the biggest bird migrations you’ve ever seen in your life, and they wouldn’t even come close. The scale of this was unforgettable in pretty much any form of writing while this bird still existed on this continent. It was just part of life. And now, it’s gone.
So, to take us back, we’re going to start with a passage from the famous John Audubon.
John Audubon: I observed the pigeons flying from northeast to southwest in greater numbers than I thought I had ever seen before. Feeling an inclination to count the flocks that might pass within the reach of my eye in one hour, I dismounted, seated myself on an eminence, and began to mark with my pencil, making a dot for every flock that passed.
I traveled on and still met more the farther I proceeded. 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 flakes of snow, and the continued buzz of wings had a tendency to lull my senses to repose. Before sunset, I reached Louisville, distant from Hardinsburg, 55 miles.
The pigeons were still passing in undiminished numbers and continued to do so for three days in succession. Let us take a column of one mile in breadth, which is far below the average size, and suppose it passes over us without interruption for three hours, at the rate mentioned above—one mile per minute.
This will give us a parallelogram of 180 miles by one, covering 180 square miles. Allowing two pigeons per square yard, we have 1,115,136,000 pigeons in one flock.
—John James Audubon, Kentucky, 1813.
AJ: So when I wrote the article for the spring issue, I went down one serious rabbit hole on why the passenger pigeon went extinct. I read the books 1491, The Passenger Pigeon, A Feathered River Across the Sky, and A Message from Martha. I found historical accounts written by folks like John Audubon and theories exploring how Indigenous people managed passenger pigeon populations.
But honestly, after writing the article, I still didn’t feel like I had exhausted the subject enough. I wanted to take things a step further. And of course, we reached out to Mark Avery, the author of A Message from Martha.
Gabby: While many museum specimens of passenger pigeons exist today, there isn’t a single person alive on Earth who has seen a living one.
But before we get too much farther, let’s establish what a passenger pigeon is. Or, should I say—was.
Mark: Well, you have to ask, what was the passenger pigeon? The passenger pigeon was a bird that lived in the USA and Canada, but they’ve been extinct now for over a century. In fact, it’s been 110 years.
I’m Mark Avery. I’m a scientist by training. I’ve worked in nature conservation, mostly bird conservation, in the UK. I’m an environmental activist, and I’ve written some books. One of the books I wrote was about the passenger pigeon because I think it’s a really fascinating species with an amazing story.
The passenger pigeon was, for sure—absolutely for sure—the most numerous bird in the world that has ever existed. And yet, it went from the most abundant bird in terms of numbers to extinct within a human generation.
So you have to imagine a time when almost everywhere east of the Mississippi was forest, and the passenger pigeon was a forest bird. It nested in trees, it roosted in trees at night, and it ate the seeds of trees. But its food sources were unpredictable—it was abundant in some places in some years but absent or rare in those same places most of the time.
The passenger pigeon was an amazing bird because it was nomadic. The colonies were vast! They contained tens of millions—hundreds of millions—of pairs of birds. So they needed to be in areas surrounded by forests.
There’s nothing today in North America or Europe that has a lifestyle like that. It was unique.
John Muir: It was a great, memorable day when the first flock of passenger pigeons came to our farm, calling to mind the story we had read about them when we were at school in Scotland. Of all God’s feathered people that sailed the Wisconsin sky, no other bird seemed to us so wonderful.
The beautiful wanderers flew like the winds in flocks of millions from climate to climate in accord with the weather, finding their food—acorns, beech nuts, pine nuts, cranberries, strawberries, huckleberries, juniper berries, hackberries, buckwheat, rice, wheat, oats, corn—in fields and forests thousands of miles apart.
I have seen flocks streaming south in the fall, so large that they flowed from horizon to horizon in an almost continuous stream, all day long, at the rate of 40 or 50 miles an hour.
Like a mighty river in the sky, widening, contracting, descending like falls and cataracts, and rising suddenly here and there in huge, ragged masses, like high-plashing spray.
How wonderful were the distances they flew in a day, in a year, in a lifetime.
—John Muir, The Story of My Boyhood and Youth, 1913.
AJ: Passenger pigeons evolved to live in old-growth forests east of the Mississippi River. They relied on mature oak, beech, chestnut, and other tree species for food, nesting, and survival.
Gabby: That’s fascinating because today, the states east of the Mississippi are no longer dominated by old-growth forests. In fact, they’re nowhere near as thickly forested as they were over a century ago. Plus, the American chestnut tree is nearly extinct itself.
AJ: Where I live in New Hampshire, it’s still 82% forested. But as Avery pointed out in his book, places like Ohio, which were major parts of the passenger pigeon’s range at first European contact, were as much as 95% forested. Avery mentioned that by 1853, Ohio was only 54% forested, by 1883, just 18%, and by 1900, only about 10% of the state had forest left.
Gabby: Clearly, the habitat that passenger pigeons once required no longer exists today. But back then, when it did, millions of these birds traveled hundreds of miles over huge swaths of forest in search of mature, mast-producing trees. Because of their nomadic lifestyle and reliance on vast amounts of acorns, beech nuts, and other nutritious foods, it’s believed that they had a very unique breeding behavior.
Mark: You have to piece this together from lots of little bits of what people said—and some people were a bit unreliable. And since you can’t look them in the eye, because they’ve been dead for over 100 or 150 years, it’s quite difficult to work out which ones you should believe and which ones you shouldn’t.
And they would hatch quite quickly, actually—the young in about two and a half weeks, something like that. And then, I think—this is a bit of speculation—but I think the adults sometimes flew further north and nested again. So it was a big rush to nest successfully, or maybe unsuccessfully, but then move on and have another go.
But everything was rushed all the time because they were trying to exploit a diminishing food resource while it was still there. The pigeons flew north as the snow was melting in the upper part of their range because that meant autumn’s supply of acorns and other food would be uncovered. They could check it out, look at it, and find the best places.
Then they would build their very simple nests—just a few twigs. They laid a single egg and started incubating immediately. When it hatched, the adults—this is a general thing with pigeons—would fly off, pick up food, and bring it back to the nest to feed the young.
The adults would go off, feed, and stuff themselves full of food. Then they produced a secretion, known as pigeon milk, which they fed to their young in the nest. It’s not a very bird-like way of doing it, but because that’s how the passenger pigeon evolved, it must have worked pretty well for tens of thousands of years.
But then, when our species started throwing challenges at them, their biology—well, that’s when it became a problem.
Gabby: Generally speaking, it’s unusual for female birds to lay a single egg. However, this strategy is pretty normal when referring to doves and pigeons.
Mourning doves typically lay 1–3 eggs per clutch and can lay up to six clutches per year. Eurasian collared doves lay 1–2 eggs 3–6 times per year. Rock doves incubate 1–2 eggs 5–6 times a year.
Band-tailed pigeons lay one egg one to two times a year, so apparently, this is a common breeding behavior for birds in the Columbidae family, of which the passenger pigeon was a member.
When you consider that passenger pigeons were estimated to live about six years on average and laid one egg two to four times a year, it’s possible that each bird raised between five and eighteen nestlings throughout its life.
Of course, I’m not trying to estimate nestling survival here, since not every chick would have lived to adulthood, but this nesting strategy would have supported population growth.
AJ: While this breeding strategy was normal for them, it might have made them more vulnerable to extinction once their population started to decline.
Now we’re going to add another unique variable to their nesting strategy—the dilution effect, which, as we’ll see later in the story, could have had catastrophic consequences.
The gist of the dilution effect is that there’s safety in numbers. Any single pigeon is less likely to be predated upon if it’s living in a large group—in this case, a group of millions of birds.
Mark: Pigeons are quite tasty. We eat pigeons. I can buy pigeon breasts—not passenger pigeons, but wood pigeons—in my local supermarket or off a stall. Some farms will sell them to me.
And everything else in the woods eats pigeons too. Many colonial birds that nest together collectively defend their colony and their nests against predators.
But the pigeon—or the dove—isn’t called a sign of peace for nothing. These are pacifist birds. They don’t have sharp beaks, and they’re not aggressive. They’re not even very aggressive toward each other.
Their form of defense against predation—all these things looking at them and thinking, Oh yeah, I quite fancy eating some of that—was to nest in huge colonies.
Predators couldn’t come in from 50 miles away to cash in on this food. It was just the local ones—the ones that lived in the woods nearby, maybe within five or ten miles. But if you’re talking about a colony of 100 million passenger pigeons or more, then the impact of those predators was diluted.
That’s why it’s called the dilution effect. Simply by nesting in vast numbers, even if every predator nearby wanted to eat them—and they were eating some—they couldn’t eat enough to make much of a difference because the pigeons were so numerous. That’s what we’d expect, really. So I think that’s probably true.
That does mean they had to nest in very large numbers, though. And as the passenger pigeon declined, that became less and less effective—until, eventually, it stopped working altogether.
John Lawson: This day, one of our company, with a Sapona Indian who attended Stewart, went back for the horses. In the meantime, we went to shoot pigeons, which were so numerous in these parts that you might see many millions in a flock. They sometimes split off the limbs of stout oaks and other trees upon which they roosted at night.
You may find several Indian towns, of not above 17 houses, that have more than 100 gallons of pigeon oil, or fat. They use it with pulse, or bread, as we do butter, and it makes the ground as white as a sheet with their dung. The Indians take a light and go among them at night, bringing away some thousands, killing them with long poles as they roost in the trees.
At this time of the year, the flocks, as they pass by, in great measure obstruct the light of the day.
—John Lawson, 1709.
AJ: As a passenger pigeon, you don’t want to be unique. You don’t want to stand out. You want to be one in the crowd—and hopefully not one on the edge of the crowd. The birds further away from the edge would have felt a greater sense of safety and stood a lesser chance of predation.
Gabby: Exactly. Nesting sites, in particular, drew a lot of predators, including humans like market hunters and in earlier Indigenous histories. Unlike market hunters, Indigenous people had very strict ethics when it came to engaging with these massive nesting sites. There is even evidence of people being exiled for breaking these concepts.
AJ: The book 1491 mentioned a theory that Indigenous people were actually keeping the passenger pigeon population in check. The book argues that European diseases caused the deaths of a significant portion of the Indigenous population. The removal of this human predator then became the reason why the passenger pigeon population exploded.
While this theory is hotly debated, the book presents fossil evidence to support it. We also have early written records indicating that passenger pigeon fat was critical to many Indigenous people’s survival, much as we more commonly understand the importance of bear fat as a winter food source. It was a major staple of their diet.
Gabby: So the truth is, our early written evidence of the passenger pigeon population—which comes entirely from European encounters—could actually be describing a population that had experienced exponential growth.
This really makes me think about the type of impact passenger pigeons had on the landscape—creating forest disturbances, spreading nutrients through their droppings across vast areas, and the cascading effects that can happen in two directions: overpopulation and underpopulation.
Mark: I mean, the fertilizing effect would be spread over a pretty large area—unless they all decided to hold it in until they got back to the colony. And gosh, they must’ve been smelly places anyway.
But that would have been spread over quite a large area. Absolutely, all those birds—even in a forest—were bound to have an effect. I think the massive amount of deposition and the sheer weight of birds would have wrecked some areas of forest.
But that’s a bit like fire and other natural disturbances. It destroys patches—sometimes quite large patches—but then it just grows back. It takes time. Trees take a long time to grow, but it would have been part of the natural process.
I think one of the things we kind of know is that there wasn’t a massive change in the species composition of American forests before and after the passenger pigeon. So it’s not as though, when the passenger pigeon declined and went extinct in the wild, two or three species of native trees suddenly became more common at the expense of others.
But there has been another suggested impact. A billion passenger pigeons eating all those protein-rich seeds—if they weren’t eaten by passenger pigeons, they were probably going to be eaten by other things.
One species that may have benefited is deer. Their numbers have certainly gone up. That might be due to other factors as well, but they’ve increased significantly in the last 50 years. That doesn’t take us right back to the time of the passenger pigeon, but as deer populations have gone up, so have tick populations. And diseases spread by ticks have increased in people in the eastern U.S.
So some have suggested that it’s conceivable—and I think it is conceivable—that removing that massive seed consumption by passenger pigeons could have affected the biomass of deer, and probably rodents as well. And all of these animals carry ticks, which spread diseases that can affect you and me if we go for a walk in the woods.
Gabby: So much of the passenger pigeon story takes place in the past. However, this species’ legacy continues today, especially at the Cincinnati Zoo in Ohio.
There, zoo-goers can observe a statue of Martha, the last passenger pigeon—the bird for which Mark Avery’s book is named—along with an empty cage and a plaque dedicated to her and her species’ existence.
Mark: If you go to the Cincinnati Zoo—I’ve been there twice to look at the cage where Martha was kept and where she died. I have sat there on a busy day in the zoo, and the only thing I looked at was this cage where a passenger pigeon died and that bronze statue.
Hardly anybody else looks at the cage. Everybody goes off to see the gorillas.
If I were eight years old, I’d want to see the gorillas, not an empty cage.
The last passenger pigeon on Earth. Just imagine that. The last individual of a species on Earth died in a cage around lunchtime, local time, on September 1, 1914. And when that bird, who was called Martha, died, there were no more passenger pigeons in the world.
But nobody really stops to look at the bronze statue, and nobody reads the plaque on the statue, which says, in effect, This is where the last passenger pigeon, called Martha, died.
We ought to do a bit better with nature conservation in the future. And that, I think, is the message from Martha.
AJ: One of the most intriguing factors about the final chapter of the passenger pigeon’s history is its seemingly abrupt disappearance. While there is no doubt that human exploitation through market hunting played a role, Joel Greenberg wrote in A Feathered River Across the Sky, I have struggled to accept as sufficient the purported factors that reduced a billion or more birds to zero in four decades.
So how did we go from all these birds to suddenly nothing? This led people to come up with some wild ideas.
Gabby: One of the most imaginative theories regarding the extinction of the last passenger pigeon populations appears in William Mershon’s 1907 book, The Passenger Pigeon. In the chapter titled A Novel Theory of Extinction, Mershon suggests that the last flocks flew out over the ocean or the Great Lakes in search of food and nesting grounds, ultimately perishing there.
This theory is not accepted today.
AJ: I love old conspiracy theories in the birding world. They have an extra level of insanity that I find pretty entertaining. Although I am by no means an expert on this topic, it seems that these books collectively tell a long story that begins in 1492.
We have a massive shift in human populations, we have market hunting, and I think the actual death kiss was deforestation.
Gabby: I’m going to have to agree that the destruction of old-growth forests is really the smoking gun here. And I’m not saying that all forests need to be old growth. Instead, our forests need to be a complex matrix of young and old-growth stands with varying age classes that meet an array of wildlife needs.
All the conversations we have about birds—no matter what road we take—it always leads back to habitat.
In this case, I really think about a few things. First, these birds ran out of options for their migration. They couldn’t clean out food sources from mature trees because there weren’t any left. In other words, they experienced food shortages.
This, combined with the introduction of new European avian diseases that they had no previous immunity to, was a death sentence. When you combine wildlife diseases with the destruction of quality habitat, survival becomes really difficult.
For the passenger pigeon, without habitat and large numbers, their dilution effect survival strategy made them extremely vulnerable.
AJ: Exactly, and take it a step further. Because now, with reduced populations, the dilution strategy no longer worked.
Greenberg notes that by the 1870s, passenger pigeons had begun abandoning their nesting sites. In his words, With the ever-increasing intensity of exploitation, the birds became less tolerant of disturbances and quicker to give up their breeding aspirations.
Gabby: As forests vanished, the availability of suitable nesting and wintering locations dwindled. This scarcity increased the pigeons’ vulnerability to predation. With fewer places to go, their movements became even more predictable—even for humans engaged in market hunting.
So if they stopped successfully breeding, especially with only one egg per clutch, we can really account for the dramatic, almost overnight decline.
Habitat loss caused a cascading effect that we couldn’t stop.
AJ: We know what it’s like to live without the passenger pigeon. So let’s dive into something we’ve explored before—the idea of what ifs.
What if these birds were still here today? And the brutal question—could they survive? Or even, should they?
Mark: I think it would be pretty difficult. And I know that some people are looking at a Jurassic Park-type solution—getting DNA from museum specimens and trying to bring passenger pigeons back.
I’m not thrilled by that prospect. There are better things you could spend the money on. Of course, it’s not my money, so…
But if you could bring passenger pigeons back, let’s imagine we could get hundreds of millions of them again.
Would you want to be at Chicago O’Hare Airport and be told that your flight isn’t leaving today because an enormous flock of passenger pigeons is flying over the flight path of the landing and takeoff routes?
You’d be thrilled, wouldn’t you? I don’t think so.
And even if they didn’t disrupt airports—which they undoubtedly would sometimes—we do know that they became agricultural pests to some extent. There’s an awful lot of wheat grown in America, and they wouldn’t be very welcome in today’s world.
I’d rather imagine how they used to be and wish it was still a bit more like that. But we can’t bring back the habitat.
Gabby: The fact that modern North America literally could not support this bird today, even if we could bring them back, is quite the reality check. This is ecology boiled down to its harshest degree.
And this isn’t the only game bird this has happened to—for example, the heath hen. Additionally, there are birds like the Gunnison sage grouse and lesser prairie chicken that are flying down this same path to extinction today.
AJ: Despite our human desire for a simple explanation of how this species went extinct, I don’t think we’ll ever know for sure.
We didn’t even address the theories of cascading effects their extinction had on other species—from plants and bugs to predators and herbivores.
I hope this all serves as a reminder that humans have an impact—and when unchecked, the ability to cause the extinction of even the most numerous bird species on Earth.
Ecology does not often have immediate benefits, but it should always be the bedrock of how we proceed.
We will never see darkened skies. We will never hear the deafening sound. We will never have the privilege of their existence today.
But we did our best to travel back in time—and to remember this forgotten icon.
Gabby: One thing we don’t need a time machine to see is the Passenger Pigeon Monument at Wyalusing State Park in Wisconsin.
Aldo Leopold wrote about it in his 1953 essay A Monument for a Lost Bird, and this excerpt wraps up our episode quite nicely:
“Men still live who, in their youth, remember pigeons. Trees still live who, in their youth, were shaken by a living wind. But a decade hence, only the oldest oaks will remember, and at long last, only the hills will know.”
Mark: When I’ve traveled around the USA, I’m not surprised by this—and this isn’t a criticism—but I find hardly anybody who’s ever heard of a passenger pigeon.
Now, I know they’ve been gone for over a century, so nobody misses the passenger pigeon. It’s only, I think, people like you—because you’ve asked to talk to me about this—and people like me, because I’m a biologist and a conservationist. I find these stories really interesting.
A few bird watchers really, really try to remember, but there’s nothing for us to remember.
None of us has seen a passenger pigeon alive, and certainly, none of us has seen a flock of passenger pigeons a mile wide flying overhead all day long.
Our species has forgotten them. We don’t think of them.