MS. DIANE REHMThanks for joining us. I'm Diane Rehm. It's hard for us to imagine now, but several hundred years ago, passenger pigeons made up 25 to 40 percent of birds in North America. The passenger pigeon's abundance seemed to early explorers and settlers inexhaustible. Author and naturalist Joel Greenberg writes about the passenger pigeon's demise and the importance of conservation in his new book, "A Feathered River Across the Sky: The Passenger Pigeon's Flight to Extinction." For this month's environmental outlook, Joel Greenberg joins us from the studios of WBEZ in Chicago.

MS. DIANE REHMThroughout the hour, we'll welcome your calls. 800-433-8850. Send us your email to drshow@wamu.org. Follow us on Facebook or send us a tweet. Good morning, Joel. Thanks for joining us.

MR. JOEL GREENBERGIt's a terrific honor to be on your show, Diane. And I'm very grateful. Thank you.

REHMJoel, it's good to have you. We're all familiar with pigeons, but explain to us what a passenger pigeon is. Surely it's not any one of the ones we see in the parks.

GREENBERGPigeons are an order of birds. Exonomy (sp?) divides organisms into categories. And among the pigeon order, there is one family with 300 species, one of which is -- was the passenger pigeon. The common street pigeon, the rock pigeon, is an Eurasian bird. Partially -- it has been domesticated partially for a long time. One way to make the analogy is carnivores and mammals is an order. And in that order is the feline family. So to say all pigeons are alike, is to say a tiger is the same as a lion, is the same as your kitty lounging on your couch.

REHMSo the...

GREENBERGIt's a very different bird.

REHMSo the passenger pigeon is not a rock pigeon. It's not a messenger pigeon. It's not a carrier pigeon. How different is the passenger pigeon?

GREENBERGIt is in its own genus, one species. But it's most similar to a New World group of pigeons that includes, probably the best known, the band-tailed pigeon, another wild pigeon from the western United States and Canada. So it's a distinctive species. It's unlike any other bird people ever knew, for at least three reasons. So don't -- it looks like a mourning dove on steroids, if you want to get a sense of appearance. But it's not, it turns out. Because of genetic research, it turns out it's not related to the mourning dove particularly, but more related to those other pigeons I mentioned.

REHMI know that you are a research associate at Chicago's Field Museum, but what got you particularly interested in the passenger pigeon? And what happened to it?

GREENBERGWell, I'd been interested in nature for most of my life. And, if you read anything about birds of North America, the passenger pigeon comes up. More recently, I've become very interested in the relationship between people and nature. And I wrote a book in 2002 -- it took 13 years of research -- about the Chicago region. And so I read of those spectacular flights of birds, which are almost unbelievable today, except so many people wrote about it for 300 years, it's really -- you can't doubt that it occurred.

GREENBERGThe impending anniversary of the bird's extinction, this year, seemed to be a good time to do a book on it. And from that book then developed a whole lot of other things that maybe we'll talk about as we proceed. But, so it's a concern -- it's a deep concern for natural history that I have, and how people relate to nature and the lessons that this story, although a hundred years old, still have today, which I think are critically relevant.

REHMWell, talk a little about how the early reactors to this bird -- the European explorers -- what happened when they saw these huge hordes of birds in the sky?

GREENBERGWell, they wrote in amazement what they saw. And they used -- they ran out of common language. They were forced to use phrases that are almost biblical in their spectacularness. One writer from Florida -- several writers said things like, you're not going to believe this, but this is what I saw. One writer in Florida in the 1700's said, "It's so spectacular, I'm not going to tell you, because you're not going to believe it." And some poor Irish captain who saw a huge flight over Philadelphia wrote about what he saw, talked about what he saw.

GREENBERGAnd, when he went to Ireland, they didn't believe him and called him Capitan Pigeon thereafter. So they were amazed because it was unlike anything they had ever seen before, because nothing like this existed anywhere else.

REHMAnd, when you say nothing like this, you're talking about billions, literally billions of pigeons -- these particular pigeons -- flying over the city and taking days to do so.

GREENBERGCorrect. We don't know with -- really how many there were. The birds were never studied in the wild. Science, itself, you know, hadn't really developed during much of this period. But, when people say the sky was black, you know, over Philadelphia for several hours over the course of the day, or, you know, people would shoot them from their porches in Manhattan-- That's one thing, that's an aspect of the bird, too, which makes it unusual. It affected 19th century culture in many ways. And one reason is, you didn't have to travel anywhere remote to see these hordes.

GREENBERGYou didn't have to go to the western Great Plains if you want -- like you would Bison. If you lived in Montreal or Toronto or Chicago or Columbus or Philadelphia, Boston, there would be -- it wouldn't be necessarily every year, but there would be days of the spring and days in the fall when likely there'd be clouds of birds moving over. So it was a widely observed phenomenon, which is important, because if there's no one there to write about it or see it, you know, we, in future generations, won't know about it.

GREENBERGBut there were writers. There were composers. There were people moved by what they saw. And so that's why there's this amazing record of the birds' existence and destruction.

REHMYou know, we have already had an email -- even before the program began -- from Justina, who says, "This topic is close to my heart. In 2008, I was director of a nature center on a bird refuge. I was asked to prepare a special workshop for a group of visiting sixth through eighth graders on human impacts on animal populations. During my research on specific case studies, I was overwhelmed with sadness about the fate of the passenger pigeon -- the enormity of the loss of the species." She says, "I openly wept, just sat in public with tears streaming down my face.

REHMBut, of course, in those early years, that's not what happened. People, as you said, were shooting at them, trying to get rid of them as quickly as possible."

GREENBERGThere weren't any voices really talking about the possible loss of this resource. They were so abundant, they aggregated in huge numbers so that, you know, even as they declined, somebody could say, Well, there were a million birds in Wisconsin. And maybe there were. But, you know, ten years earlier, there may have been millions of birds. There are some examples though, some really remarkable examples of people who did care. The very first person to really discuss the possibility of them being wiped out was John Audubon. He got it wrong.

GREENBERGHe said that as long as the forests were there, the birds would be there. But Audubon also thought the birds nested multiple times during the year, when in fact they only nest once. He also believed they laid two eggs. They only laid one. But a guy about the same time, who's not nearly as well known, his name was Revoyal (sp?) he was a French visitor, I call him the de Tocqueville of passenger pigeons -- being a foreigner, he saw clearly, and he wrote that if the slaughter continued, the birds would disappear.

GREENBERGOne of the most amazing examples was Junius Brutus Booth, the great actor, and probably more famously, the father of John Wilkes Booth. But Junius Brutus Booth cared deeply about animals and nature. And he was performing in Louisville in 18, I don't know, 10, maybe 1830 or so -- there were flights and people were killing them. So he wanted to have a funeral for the birds as a protest. And there's an account where he got a minister to come. So that's an example.

REHMJoel Greenberg, his new book is "A Feathered River Across The Sky." More, when we come back.

REHMWelcome back. If you've just joined us, Joel Greenberg is with me. He's actually at WBEZ in Chicago where it's even colder than it is here right now. His new book is titled "A Feathered River Across the Sky: The Passenger Pigeon's Flight to Extinction." And this year marks the 100th anniversary since the death of Martha, the last pigeon of its type in this country.

REHMNow we have one wonderful story here, a comment from our website. "My grandfather told us when a passenger pigeon flew over it would nearly darken the sky, took many minutes to clear. He saw these flocks as a child. He said folks would shoot them just to be shooting without even needing to aim at a particular bird. He said there were also flocks of many other birds nearly as numerous." Is that so, Joel?

GREENBERGIt is. North America was incredibly rich. Life pullulated here and so, you know, you had the bizens, you had passenger pigeons, you had shore birds. The Eskimo Curlew was a once abundant bird that was shot into extinction as well. So that's absolutely correct. And, you know, people did kill these birds in a multitude of ways.

REHMAnd one way was actually they became quite trendy in restaurants.

GREENBERGThey -- as food, you know, they were food of the poor. They cost pennies apiece in the markets. They -- some people didn't like them much but they also were served in Delmonico's for example, America's perhaps first great gourmet restaurant. It was served at banquets for President Johnson when he visited New York for the first time, and others as well.

REHMWhat happened in 1840 when the slaughter really became far more widespread?

GREENBERGWhat really turned the tide -- I suppose what really resulted in the increase of killing was the introduction of two technologies. There was railroads. The tentacles of the railroads were now spreading, so they could reach to remote areas in Michigan or Missouri, even Oklahoma later. And so it was possible to kill these birds in huge numbers and then send them to the burgeoning cities of the Midwest and east.

GREENBERGSecondly, telegraphs expanded as well. And the telegraph operators were specifically instructed to let people know when there were flocks of pigeons -- passenger pigeon flocks roosts in their vicinity. So while prior to that a roost say in some remote hamlet in the Missouri Ozarks could host these birds for say months at a time. Now people would know about it. And as a consequence of this market developing, there were people, anywhere from 500 to maybe 2,000, who did nothing but hunt these birds throughout the year.

REHMSo you talk about a major massacre like the one that happened at Kettle Creek, Penn. What happened there?

GREENBERGWell, there were -- wherever particularly the big nestings and the birds tended, in almost all cases, nested where there were masting oaks and beech. And so Pennsylvania, Wisconsin, Michigan, Minnesota, there were other places, where the birds congregated. So in Kettle Creek, like so many other places, you would have people literally coming from all over the country to kill the birds. You had -- they turned these towns into boon towns complete with, you know, packers and clerks.

GREENBERGAnd at one nesting in Michigan there was even talk of trollops. I mean, I just have this idea of hookers following the pigeon flocks to service all these men. But they were -- you know, the locals became involved. Indians often -- in New York the reservation for Seneca closed down and it was a holiday. So people streamed to where these nestings were. So what you described in that one place was repeated, you know, for decades.

REHMAnd then there was Henry Bergh, one of the lone voices who tried to sound the alarm. What happened?

GREENBERGWell, Bergh, a wealthy New Yorker, really had an aversion to cruelty. He started the Prevention -- he worked on preventing the cruelty to animals and later to children. And so one of the things that claimed passenger pigeons were live shooting meets. I mean, nowadays pigeons are made out of clay. Then they were made out of blood and feathers. And some of these gatherings, wealthy people would bet on who would win. One match in Long Island, very close to where Bergh lived, claimed 40,000 birds in one contest. They would catch these live birds in these remote areas, bring them in.

GREENBERGYou can image the mortality of how many birds would die in the process of bringing them, you know, from Missouri to New York. And he was repelled by that. And so he was warned that because of the wealthy prominence of some of the participants that he was risking some of his other goals, some of his other projects. But he stood fast and New York was the first state to ban these live shooting meets.

GREENBERGThere still are one or two live bird meets I believe still in the country, but basically it was his efforts that stopped -- the other thing that stopped it of course was the depletion of the birds.

REHMSure.

GREENBERGThey just ran out of passenger pigeons.

REHMWhat about Teddy Roosevelt? How did he get into the picture?

GREENBERGWell, Teddy Roosevelt, of course probably our greatest conservation president, knew the birds. He reported seeing birds in the early 20th century in a retreat in Virginia and wrote people, friends of his about it. I'm really pleased that the American Museum of Natural History as part of this anniversary has an exhibit based on a bird that Teddy Roosevelt shot. So, you know, there's this great legacy. It's a sad story and, you know, people kill these birds but, you know, they're -- we can use them now to tell the story and to get people to think about current issues.

REHMSo what you're saying is that even though Teddy Roosevelt got into the preservation movement, he still shot at least one of these birds.

GREENBERGIt's interesting that a lot of the people who -- a guy named Ruthven Deane for one -- I mean, people who cared at the very end when the birds were disappearing. When they were younger and the birds were still common, they participated in it. The first book ever written about the passenger pigeon was written by W. B. Mershon of Saginaw. He actually was mayor there. And he was, in his own words, a game hog. He would shoot lots of pigeons and other birds. And as he saw -- this, you know, was an important part of his life and as he realized that the birds were disappearing, you know, he devoted resources and time to trying to save them.

GREENBERGSo people who lived in that transition period, many of them, you know, changed their attitudes. And I think that's, you know, a heartening element of the story as well.

REHMAll right, Joel. So now tell us about Martha.

GREENBERGIt's rare that we know the exact date with some degree of precision when a bird becomes extinct. So Martha was probably born in Chicago to Charles Otis Whitman, a professor at University of Chicago. He maintained a small flock for study. In 1902 she was sent to the Cincinnati Zoo. The Cincinnati Zoo is the second oldest zoo in the country. And she spent the rest of her life in the zoo. She shared her cage with other birds, eventually with George. The thought is that both birds were named after the Washingtons.

GREENBERGGeorge died in 1911 and so for three years she lived alone. She lived as the last of the species, which just decades earlier filled the sky. She became decrepit, weak. People apparently used to throw sand at her, zoo visitors, to get her to move. And they actually cordoned off her little area. Probably around 1:00 in the afternoon -- we don't know with absolute certainty, on September 1, 1914 they found her -- her keeper found her dead in her cage. She was put in a block of ice, what had long been an agreement with the Smithsonian Institute, the National Museum not far presumably from where you are.

REHMExactly.

GREENBERGAnd she was -- this was hot. This was in, you know, early September -- put in a huge block of ice, sent to the Smithsonian. The ice had mostly melted. She was then necropsied and stuffed. And she had been on display for a long time. She's been off now for a while but will be on -- Smithsonian is also creating an exhibit around Martha. So if Martha had died several -- you know, maybe five, six, seven years earlier, we would be less certain that she was the last one. But there aren't any credible reports of wild birds for years before her death.

GREENBERGAnd so to have a name, you know, and it's really pretty -- you mentioned your listener who was moved to tears. And to think about loneliness or to think about, you know, being the last of your kind when just -- now Martha lived her whole life in a cage. And so it's ironic the only flying she ever did was after death. She flew to Los Angeles once in first class and then flew to Cincinnati. The only traveling she ever did in life was on a train when Whitman took her to Woods Hall (sp?) which he founded the research center there. So Martha is an icon and, you know, adds power to the story.

REHMAnd Luke in Caledonian, Mich. asks, "Is there any way through DNA from any existing specimens that the passenger pigeon could be brought back?" Before you answer let me just say you're listening to "The Diane Rehm Show." Any response to that?

GREENBERGYes. The Long Now (sp?) Foundation out of California has been very interested in promoting de-extinction. And I was privileged to attend perhaps their first meeting in Boston. So you have some really smart people who -- geneticists who are working on trying to bring passenger pigeons back. There are about 1500 to 2,000 dead passenger pigeons in collections throughout the world. And they have access to a number of these.

GREENBERGSo it's not cloning. It's a process where you take the DNA for passenger pigeons, introduce it into a close-living relative like the band-tailed. And so I -- you know, only time will tell how much they succeed. But there is an effort to do that.

REHMWhy was it called the passenger pigeon?

GREENBERGIt's based on French which I have trouble pronouncing with the right accent. The idea being these were birds -- they were highly vagile birds moving constantly. They could be anyplace at any time. And so that's really how they got that name.

REHMWhereas the carrier pigeon obviously did, in fact, carry messages from one place to another.

GREENBERGCorrect. One is a semi-domestic bird, the other was a wild bird.

REHMAll right. I'm going to open the phones, 800-433-8850. First to Timothy in Baltimore, Md. You're on the air.

TIMOTHYHi. Thank you for taking my call and thanks for this discussion. I think it's really important to talk about this because one of the lasting legacies of the passenger pigeon I think is the lessons we've learned, environmental impact that humans have and how we can affect diversity. But my question is -- and I'm not entirely sure if -- or how to ask it exactly -- but has there been a lasting impact other then in diversity? Have we noticed any changes in food chain, anything like that? Or do we miss the passenger pigeon more for nostalgic reasons? I'll hang up and listen for your answer. Thank you very much for taking my call.

REHMThanks for calling.

GREENBERGThat's a very good question. May -- difficult to answer because the birds were never studied while alive. People -- it's impossible for me to conceive that the loss of so much life in such a short period of time did not have profound impacts. Scientists looking back have conjectured, for example, that their competition with mice, which are the reservoir for Lyme disease might've kept Lyme disease limited. David Blockstein came up with that, you know, thoughtful idea.

GREENBERGThere is an insect called the American burying beetle which -- whose population collapsed. And they need carcasses of a certain size of which the passenger pigeon was right sized. So some have suggested that the decline of this beetle, albeit decades later, but might've been affected. This bird undoubtedly, you know, affected flora. So Professor Reed Noss, who's written about southern grasslands, thinks that the ecosystem known as cane breaks, which had several unique species, things like Bachman's warbler and Swainson's warbler, that they were maintained in part because the passenger pigeon promoted fire.

GREENBERGSame thing with their promoting the spread of oak. Again, they would land in the trees, they would break the branches, they would open the canopy drying out the bottom, promoting fire. And fire has profound effects on an ecosystem. So there are a number of ways that this bird almost certainly, you know, affected the wooded ecosystems. We just don't know exactly what those were.

REHMHave we seen other creatures who've gone extinct in that short a period of time?

GREENBERGProbably. I mean, but nothing with the abundance of the passenger pigeon. I mean, island species, the Stevens inlet wren. I think the White House keeper's cat brought in five and they died as a consequence. But...

REHM...nothing...

GREENBERG...nothing quite like this.

REHMJoel Greenberg. His new book is titled "A Feathered River Across the Sky: The Passenger Pigeon's Flight to Extinction." Short break, we'll be right back.

REHMA number of people have written in asking about possible other explanations for the extinction of the passenger pigeon. Joel Greenberg is with me and he's written a book about the passenger pigeon's extinction. It's titled, "A Feathered River Across the Sky." Here's an email from David, Joel, he says, "I was led to believe we did not kill all the birds. I was told they nested in the Northern islands near Greenland. They had to share the nesting grounds with puffins and other birds. Once their numbers dropped below a critical mass they were unable to compete and hold onto their nest sites. A couple of years of this and wah-lah, no more birds." What do you say to that, Joel?

GREENBERGWell, the first part, there are no trees on those islands in Greenland.

REHMSo they would have been unlikely to go there?

GREENBERGAbsolutely. So the first part, there's no evidence of that. Habitat loss was certainly a factor. There were probably plenty of food for them, that's from people who did write about it in the early 20th century. But there were fewer places where they could nest and thus, it was easier to access those places. So whereas originally they would nest as far as the East Coast, the nestings were restricted to the Great Lakes, by and large. There were some exceptions in Pennsylvania. So there were fewer places more accessible. There's no doubt that the loss of timber would have prevented them probably from finding adequate breeding spots.

GREENBERGThe idea that they needed large numbers to successfully nest is very possible. The birds tended, when they got together, to breed in synchrony so all the birds were ready to breed. And there's other factors, too, to suggest that they needed large numbers. But the point isn't that we killed every last one. The point is we killed them to the point where it reached a downward spiral, where possible other factors did take place.

REHMRight.

GREENBERGBut we're the ones that started that descent.

REHMAll right. To Tunnel Hill, Ill. Hi, Chris.

CHRISHi, Diane. Hi, Joel. Thanks for taking my call.

REHMSure.

GREENBERGHello.

CHRISI had read an article talking about how the extinction of the passenger pigeon kind of coincided with the rapid extinction of the American chestnut and how you had a very quick loss of up to 40 percent of the Eastern woodlands. That would be a pretty big mast source for that many birds. I wonder if there's any kind of truth to that theory. And also the lesson involved in kind of the non-linear cause and effect that goes on when you're talking about conservation efforts.

REHMAll right.

CHRISAnd I'll take my answer off the air. Thanks.

REHMThanks for calling, Chris.

GREENBERGWell, the first part with respect to the chestnuts is the answer is no, it really had no effect. In part because the chestnuts didn't disappear until probably 40 or 50 years after the passenger pigeons were gone or at least into their decline. So there really isn't any relationship between that. The birds probably fed on chestnuts, but the literature suggests they were a minor part of the food source to begin with. The other question is more complex. There certainly are events that occur that don't show up until later. I mean you think about all the different trophic levels in an ecosystem and how complex. So something happening in point A might take a while to manifest itself, which would further suggest that might be the case for example with the bearing beetle that I mentioned earlier.

REHMAnd from Twitter, a question, "Were the pigeons destructive or disruptive? What is the reason they were so greatly targeted for hunting?"

GREENBERGThe principle reason they were targeted is they were so abundant. They were easy to get at, as I said. I mean they weren't in remote areas. They were in the Midwest or even in Pennsylvania. And they congregated in masses. So short-tailed shrew might be the most abundant mammal in the eastern woods, but they're scattered. They're nocturnal. They live underground. You wouldn't get them, but the pigeons were diurnal, the pigeons, again, aggregated in large numbers. So it was very easy to go after them.

REHMAll right. And to Bruce, in Baltimore City. Hi there, Bruce.

BRUCEHi. First of all, I love your show. You're one of the more civilized radio talk show hosts. But anyway, I can understand conservation and I love birds. I live in a city, but I have a lot of mature trees, so I have sparrows, finches, robins. I love them. But then maybe I'm a little bit ignorant. I don't know that much about -- is that a particular species or subdivision of a species of pigeon? Because what I'm wondering about is, like I said, to a certain extent maybe they're a different species of pigeon, but when you see them in rural areas or suburban areas or even park-like settings they're beautiful.

BRUCEBut sometimes in urban areas or industrial areas, particularly small crowded urban areas they can be a nuisance. You know the damage they can do. And not that I have anything against that particular species, but maybe it's the contemporary species that causing the problems. I don't know. I'm not that knowledgeable.

REHMAll right. Okay. Thanks for calling. Might they draw rats, for example, Joel?

GREENBERGWell, passenger pigeons did not live in urban areas. Rock pigeons from Europe nested on cliffs. And so what's the cliff? It's the skyscraper. They're domestic. So really, again, as I said, to compare rock pigeons with passenger pigeons would be like comparing your kitty with a tiger. They're profoundly different animals.

REHMAll right. And to Los Angeles, Calif. Hi there, Alice.

ALICEHi. I just wanted to mention that I know you were talking about many of the activists for the passenger pigeons were people that hunted them maybe earlier in life or at the same time. Here in California, that kind of preservation and activism is really going on inside the hunting communities. There's Quail Forever. There are many groups for the grouse and other birds. And they're some of the people trying to preserve the environment. So I just wanted to give a little bit of a shout out for the hunters out there who are aware and who are trying to protect the birds themselves in many cases.

REHMAll right, Alice. Thanks for calling. Go ahead, Joel.

GREENBERGThe kind of hunting that wiped out the pigeons is different than what we have now. I mean that was uncontrolled market hunting with some recreational hunting. And so today, in part because of the passenger pigeon, because of the game laws that were installed in the early 20th century, hunting now is regulated. And so in this country nothing like the passenger pigeon could be repeated. It is being done in other parts of the world. So absolutely, recreational hunting really wasn't regulated hunting like we have today. We have to see that as different from the kind of hunting that occurred.

GREENBERGAnd surely it's true, no question, that hunters were one of the three principle groups that worked hard to get game laws installed.

REHMAll right. Let's got to Ann, in St. Louis, Mo. Hi, there.

ANNHi. I'm a great fan of alternative energy and it brings jobs and it helps the environment, global warming. But the wind farms, which I was totally in favor of, seem to be doing what the hunters did to the passenger pigeons they're doing to other bird species with birds running into the areas and being killed. And then I was sort of shocked when President Obama -- I think he decreed that wind farms, at least out West, have a 30 year okay to not worry about killing eagles, which tend to run into the wind veins.1

REHMAll right. Let's see what Joel's comment is. Joel?

GREENBERGYeah, no energy source is totally benign. And there are things that can be done to make wind farms less hazardous to birds. You don't want to place them on migration routes. It's my understanding that there's ways to build them where they're less likely to destroy birds or bats. I mean I don't think anyone's saying they should be banned altogether, but use great circumspection as to where you place them and how you operate them. And I think that has to be the goal to make them as benign as possible.

REHMJoel, I gather out of Martha's death and the extinction of the passenger pigeon, there did arise this awareness and move and perhaps even movement toward the environmental consciousness and the awareness that we as a people can be responsible for a species' continuation or demise. Is that the legacy of Martha, do you think?

GREENBERGThat is probably the principle legacy. The destruction of North American game spawned the country's first great environmental movement. There were state laws, but it's obvious that if you're going to protect migrating wildlife you need federal jurisdiction, you need federal government to be strongly involved. The very first law to do that was the Lacey Act. Congressman Lacey introduced it in 1900 and he said, "It's too late for the passenger pigeon, but there is still good work to be done." So the Lacey Act. And then in 1920 the Supreme Court approved the Migratory Bird Act. Those are still the foundations of how we regulate our biological heritage.

GREENBERGAnd out of those laws came the Endangered Species Act, which is so vitally important and needs all the support it can get at this point.

REHMAnd Ellen, in Columbia, Mo. writes, "Could you comment on the similarities and differences between the fate of the passenger pigeon and today's catastrophic decline in monarch butterfly numbers?"

GREENBERGI mean, as I understand, with the monarchs, some of it -- so the passenger pigeon was killed because of direct take. And my friend, David Blockstein, talks about the four horsemen of extinction. So there's pollution, there's habitat loss, there's direct take and there are alien species. And the monarch is dying out not because people are collecting them in mass. I think it's because of habitat loss. Some of the new strains of crops that perhaps are toxic to it. So there's probably two of those horsemen that are involved in these declines in monarch population. So the problems are still ongoing.

REHMAnd you're listening to "The Diane Rehm Show." Let's go to Janet, in Pikesville, Md. You're on the air.

JANETHi. I'm a novice birder and I've been thinking about the starting of the testing of these delivery drones in six states. And I think that that would have a bad effect on bird nesting. And I think that everybody who appreciates birds should think about this. These drones are supposedly to be kept under 400 feet, but that's the area where neighborhood birds and songbirds fly around.

REHMWell, that's an interesting point, Janet. Thanks for your call. What do you think, Joel? These delivery drones may be here sooner than we think. Might they be a risk to birds?

GREENBERGThere are always new threats emerging. To be honest I don't know. It would depend on how many of these things there would be.

REHMYes.

GREENBERGExactly how they flew. As I suggested earlier, you know, a technology can be better or it can be worse in terms of its impacts on nature. And I just hope that if this new technology does have the potential to harm birds, that those who are in charge of regulating them take that into account.

REHMSo you have a project devoted just to this subject today. Tell us about Project Passenger Pigeon.

GREENBERGAs I started doing research on my book, I learned that other people were thinking about the 2014 anniversary as well. There was a meeting in Chicago at the Peggy Notebaert Nature Museum, which drew people from various parts of the country. And out of that emerged Project Passenger Pigeon. The goal of it is to tell people about the bird and then to emphasize the lessons that are still relevant today. So we have a website. We have over 160 organizations. My book is a major part of it. We're making a documentary that should be available in the spring called, "From Billions to None: The Passenger Pigeons Flight to Extinction." There are going to be a plethora of articles in birding magazines, like Bird Watching, that's coming out.

GREENBERGIn environmental magazine of a broader nature, like Audubon. And then even in general publications, The New Yorker, Macleans in Canada. The Wall Street Journal's going to have a review as well. So we're trying to get the message out. Art is a powerful tool to move people.

REHMAbsolutely. And art, also in the form of music. We're going to go out with a song written by John Herald called, "Martha." It's about the very last passenger pigeon. Joel Greenberg, thank you so much for joining us and for your book, "A Feathered River Across the Sky."

GREENBERGThank you very much.

REHMAnd thanks for listening all. I'm Diane Rehm.