How was the world’s most abundant bird driven to extinction in less than a century? That’s the core question British naturalist Mark Avery asks in his new book, A Message from Martha: The Extinction of the Passenger Pigeon and Its Relevance Today. As the 100th anniversary of the death of Martha, the last living passenger pigeon, approaches this September, we got a chance to chat with Avery about how the pigeons’ biology played a part in their extinction, how the conservation movement has developed since Martha’s demise, and what species loss says about our own humanity:
(Note: This interview has been lightly edited for clarity and length.)
WSF: Passenger pigeons numbered in the billions across their habitat in the eastern United States; they gathered and nested in such large groups that their flocks blocked out the sun. And yet their population nose-dived with almost inconceivable speed, and we happen to know the exact moment when the entire species went extinct. What were the primary forces behind their decline?
MA: Cutting down half of the USA’s forests by 1870 was the main factor. Passenger pigeons nested in forests, roosted in forests, and fed on the fruits of the forest: acorns, beech mast [nuts of the beech tree], and chestnuts. But we followed that up with an industrialized slaughter of the birds—mostly for food, but also for sport—and this was aided by the invention of the telegraph, the spread of the railroads, better firearms, and the lack of regulation of hunting. It was a slaughter rather than a harvest. Trainloads of millions of passenger pigeons were sent from Wisconsin and Michigan back east to the restaurants of New York and Philadelphia.
But I think it was the birds’ biology that made them vulnerable in the end. Passenger pigeons relied on ‘swamping’ natural predators when their numbers were high. When their numbers fell, they just couldn’t cope with the toll taken by hawks, raccoons, squirrels, falcons, bears, and other predators.
But that was in the wild. The last passenger pigeon, named Martha—who died in the Cincinnati Zoo on September 1, 1914—was captive-bred. It’s surprising that we didn’t do better in keeping a captive population alive. Then again, compared with the flocks of billions of birds darkening the skies, maybe a few birds in a cage would just have looked rather sad and pathetic.
WSF: At what point does a species lose the ability to regenerate their population? Do we know when that “tipping point” was reached with the passenger pigeon?
MA: The biology of the bird was important. Other species have been saved from extinction when there have only been a handful of individuals left alive. When the passenger pigeon was reduced to millions of birds rather than billions, however, I think the impact of natural predators overwhelmed them.
Their huge abundance made this tasty pigeon able to cope with great losses—there were never enough local predators really to dent the success of a colony with more than 100 million nests, but when the colonies were of millions, then local predators’ impacts were great enough to finish the species off. That’s what I think, but I’d love to have a time machine to be able to travel back and do the research to know if I’m right.
WSF: Did their sheer numbers lead farmers and hunters to believe, incorrectly, the pigeon population couldn’t be diminished by human factors?
MA: Yes. Europeans settling in North America arrived from a continent depleted of wildlife over centuries and arrived in one of great richness. No one thought that the passenger pigeon, or the beaver, or bison, or Eskimo curlew could ever be brought to the point of extinction. Also, consider that the passenger pigeon was nomadic—the great flocks of billions of birds sought out the areas that were rich in tree nuts—and these varied from year to year. Some years the pigeons would nest in New York and others in Wisconsin, so they were always a ‘here today and gone tomorrow’ bird, and it was less obvious that their numbers were falling. The passenger pigeon population went from at least three billion to zero in a matter of 60 years—an incredibly rapid decline.
WSF: In one part of the book, you relay a story about a young boy hunting near his family’s farm. He couldn’t identify the passenger pigeon he had shot and showed it to his mother. She instantly recalled seeing enormous flocks of them as a girl, but none since. How did this generation gap play into conservation movements?
MA: People missed the pigeons that they had seen in huge numbers in their youths. And at the same time bison, pronghorn and other species were being slaughtered too. It can be no coincidence that the National Parks movement, the Sierra Club, the Audubon Society, and the American Ornithologists’ Union all came into existence around this time. And leaders of the conservation movement, such as Aldo Leopold and John Muir, wrote and spoke about passenger pigeons.
WSF: What is the “message” that Martha offers in the book’s title?
MA: We aren’t better off because we drove the passenger pigeon to extinction. We are morally diminished by our role in its loss from the planet—the commonest bird the world has ever seen. If we had done better and saved it, we would be as rich and as happy as we are today, but wouldn’t have it on our conscience. And that applies to all the other species we have driven extinct or brought to the brink. A century ago, we could just about get away with saying we didn’t know what we were doing. Now we do know. We know enough. The question is: ‘Do we care enough?’ That is what Martha would ask us today.
Check out this excerpt from A Message from Martha, in which Avery describes ornithologist Alexander Wilson’s attempt to calculate the number of birds in one passenger pigeon population in the early 1800s:
Wilson’s estimate of the numbers in the single flock he saw in Kentucky one summer in the early nineteenth century (more than two billion) depended on an estimated flight speed of 60mph, a passage of birds of four hours, and a density of birds of three per square yard. He then does the math correctly: 4 (hours) x 60 (mph) x 1760 (yards/mile) x 1760 (yards in width) x 3 (birds per square yard) = 2,230,272,000. Wilson described this estimate of numbers as an ‘inconceivable multitude’ but also that the estimate was probably ‘far below the actual amount.’ From what Wilson wrote, a duration of four hours and a flock width of a mile were both likely to be underestimates rather than overestimates, and so we could regard this flock as likely to have numbered between two and three billion birds. However perfect this estimate of this one flock might be, it is unlikely that Wilson watched all the passenger pigeons alive in the USA and Canada on that day in Kentucky. Was this flock half, or a quarter, or even a tenth of the total population of the species at that time?
Excerpted from A Message from Martha: The Extinction of the Passenger Pigeon and Its Relevance Today by Mark Avery. Copyright © 2014 by Mark Avery. With permission of the publisher, Bloomsbury USA. All rights reserved.