A naturalist traces the astounding flyways of migratory birds

DAVE DAVIES, HOST:

This is FRESH AIR. I'm Dave Davies, in for Terry Gross, who's off this week. Did you know that when some migratory birds prepare for flights that can take them thousands of miles, their intestines and digestive organs shrink while their heart, lung and leg muscles can double in size? That's just one of the amazing facts you can learn from our guest, Scott Weidensaul. He's spent decades studying migratory birds, reporting on and writing about them and doing fieldwork and tracking and conservation efforts. The scale of bird migration is staggering, involving billions of birds, and the diversity of the species' mating, nesting and flying habits is awe inspiring. Weidensaul's latest book, "A World On The Wing: The Global Odyssey Of Migratory Birds," is now out in paperback.

Weidensaul writes about what he calls this majestic global pageant and about the threats the animals face. One study found that since 1970, roughly 30% of North American birds have disappeared, more than 3 billion of them. Scott Weidensaul has written 30 previous books, and his articles have appeared in Audubon, National Wildlife and other publications. He co-founded Project SNOWstorm, which tracks and studies snowy owls and is a founder of the Critical Connections project, which is tracking the migration of birds that breed on national park lands in Alaska. I spoke to him in March of last year, when "A World On The Wing" was first published.

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DAVIES: Scott Weidensaul, welcome back to FRESH AIR. You know, you've been doing this for so many decades, but you write in this book that your fascination with birds really goes back to a pivotal moment when you were 12 years old. You want to share this with us?

SCOTT WEIDENSAUL: Sure, Dave. I grew up in the mountains of eastern Pennsylvania. That's where I've lived almost my entire life. And not all that far from where I grew up is a place called Hawk Mountain Sanctuary, which is the world's oldest sanctuary for birds of prey. It was established in the 1930s. And it's this high, windswept ridge along the Appalachian Ridge-and-Valley system, and along which every fall, tens of thousands of hawks and eagles and falcons and vultures migrate south. And I had been campaigning pretty hard as a kid to - for my parents to take me to Hawk Mountain. I was a nature nut pretty much right out of the womb. And as you say, you know, when I was 12 years old on what turned out by dumb luck to be just one of the most - one of the best migration days of the fall, there was a strong northwest wind blowing. The sky was full of migrating raptors.

And, you know, it's rare that you can point to a single day in your life and say, this day changed my life. But that day, actually, I got hooked on raptors because birds of prey are cool. You know, they're majestic, and they're awe-inspiring. And for a 12-year-old to be sitting on these rocks a thousand feet above the valley, watching red-tailed hawks and sharp-shinned hawks and golden eagles passing by was intoxicating. And ever since that day, I've - migration has had its hooks in me, raptors have had its hooks in me, and it's taken me all over the world. And that passion has never diminished at all.

DAVIES: One other thing I want to - that you describe in the book that I want you to share with us is - particularly those of us, you know, on the East Coast or in the mid-Atlantic corridor, that we don't know it, but there are times at night when overhead there is this massive nighttime migration happening. What's the scale of this?

WEIDENSAUL: Yeah, and it's happening everywhere. It's a - this is a global phenomenon. Most birds migrate after dark, including almost all of the birds that are normally active at daytime. You know, the night sky is cooler, and the air is calmer. There are fewer predators. And yes, I mean, over the megalopolis of eastern North America every spring and fall, you have hundreds of millions to billions of migratory birds passing through the darkness over - you know, over the roofs of people who have no idea that - if you could strip away the night sky and make this visible, it would be probably the largest, greatest natural spectacle on the planet. Also, if you have a pair of binoculars or a telescope pointed at the full moon, especially early in the evening, right after the moon has risen in April or May or September and October, and you can actually see birds flying across the disc of the moon. In fact, back in the 1940s and '50s, before radar became common, that's how scientists studied nocturnal bird migration, what they called moon watching.

DAVIES: You've been all over the world tracking birds and writing about them. And one of the places that you describe for this book is a trip to a huge tidal mudflat on the Yellow Sea, which is the sea between China, I guess, and the Korean Peninsula. When you were there, there were a lot of birds. Why so many there?

WEIDENSAUL: Well, the mudflats of the Yellow Sea are the largest tidal mudflats on Earth. And it's interesting because when the tide goes out, I mean, it goes out. It goes out kilometers. So it's this enormous expanse of tidal mud, and it looks featureless to a human eye, but it's full of marine invertebrates. And it's the critical migratory stopover point in this massive hemispheric migration network called the East Asian-Australasian Flyway. So it's drawing in particularly shorebirds - sandpipers, plovers, you know, these birds that you see at marshes and along beaches. It's drawing them in from as far away as New Zealand and Australia across parts of Oceana into southeastern Asia. And these are birds that will fly, you know, 6,000 miles to get to the Yellow Sea and then try to rapidly build up their fat reserves there. I mean, they've just flown 6,000 miles nonstop from, say, New Zealand. They need to fly on to the Russian Far East or Siberia or even parts of Alaska. So they have another 4- or 5,000 miles to go.

So it is an absolutely critical stopover point for tens of millions of migratory birds, including about 11 or 12 million migratory shorebirds. The problem is that particularly China and South Korea have destroyed a huge percentage of the mudflats along the Yellow Sea. You know, China - probably 60% or 70% of the Yellow Sea mudflats in China have been, quote-unquote, "reclaimed." It's - you know, it's - basically, it's coastal destruction. They build seawalls. They pump in hundreds of millions of cubic meters of sediment and create dry land where you had this once-thriving coastal ecosystem. So the birds are getting squeezed into a smaller and smaller area there. And it's become one of the greatest conservation concerns for migratory birds in the world.

DAVIES: You were there with a lot of other conservationists from many countries. Just set the scene for us. What about the birds you see?

WEIDENSAUL: So when the tide goes out, you have these waves of birds coming across from farther inland at high tide roosts along shrimp ponds and other areas they can find habitat. And pouring by the tens of thousands down onto these tidal flats and wasting not a second. I mean, the moment they land, they immediately start feeding, their beaks going up and down like sewing machines into the mud, probing for worms and tiny mollusks and other marine invertebrates. And you get the sense of frenetic activity, the sense that they really do not have a moment to spare. And the fact of the matter is, they don't.

You know, they're pausing here in this enormous hemispheric, sometimes transhemispheric, migration. And they're on such a tight timetable. I mean, they've got to arrive, more than double their weight in less than two weeks, make another flight of, you know, another 4- or 5- or 6,000 miles up into the Arctic, arriving with enough fat stores in their body to get them through the first weeks in the Arctic because when they arrive, everything is still frozen. But they need that time to find - you know, set up a territory and find a mate and as soon as the ice and snow melt to build a nest and lay their eggs because they've got to get their chicks old enough to fly and out of there before winter comes back in in the Arctic where, you know, summer really lasts only a matter of, you know, a month, month and a half up there.

So, you know, their timetable is so tight. That's, again, why the Yellow Sea is so critically important because it's one of the few places in the world that provides the resources that they need that they can feed quickly enough and regain their weight fast enough to keep up with this relentless timetable that they're laboring under.

DAVIES: You know, there's been all this development which threatens this stopover for these migratory birds. But there's been some progress, in part because of a celebrity kind of species, the spoon-billed sandpiper. It's one of these birds that - I guess that - you know, it eats stuff along the shore but has this particular kind of interesting bill. It's become a darling of conservationists. And it's prompted measures by the Chinese government, right?

WEIDENSAUL: Yeah. The spoon-bill sandpiper is - it's only about six inches long, but it's got outsized charisma. And, yeah, it has this really funky bill. It looks like - it's this hugely spatula tip on the end of the bill. Nobody really knows why it evolved that. We assume it's for something to do with feeding. It's incredibly rare. There's probably only about 400 of them left in the world. They breed in the Russian Far East. They winter across Southeast Asia. Their numbers have plummeted because of, you know, coastal destruction in the Yellow Sea and also because of illegal hunting in places like Thailand and Myanmar, where they spend the winter.

But yeah, they've - because they have so much charisma, because they are just such a funky-looking, you know, dare I say it, really cute little shorebird, they have captured the world's attention. And they've served as kind of a conservation umbrella species for the other somewhat less charismatic but, in some cases, almost equally endangered shorebirds that use the Yellow Sea. And you're right, when I went over to the Yellow Sea to do this reporting in 2018, I thought this was going to be one of the grimmest stories that I was going to be telling in the book because you just had this, you know, juggernaut of coastal destruction going on in China, mostly being driven at the local level, you know, destroying tidal flats for industrial sites and shrimp farms and things of that sort.

But, you know, the Chinese government, being a totalitarian government, can do what totalitarian governments can do from time to time, which is make sweeping declarations. And just a few months before I went to China, the Chinese government announced an almost complete ban on further coastal destruction on these mudflats around the Yellow Sea. And they're doing it, I think, for a number of reasons. Part of it is certainly shorebird conservation. They'd received an awful lot of international pressure on that front, also, the recognition that millions of Chinese citizens depend on these mudflats for shellfish and other resources. And also, these coastal mudflats provide some resiliency in the face of climate change and rising sea levels.

But, you know, with a few exceptions that the Chinese government carved out for what they call, you know, projects of national importance, they banned all of this locally driven coastal destruction, which is huge. And I found myself in the midst of conservationists grappling with an unfamiliar emotion, which was hope. My friend Theunis Piersma, the Dutch scientist that I was spending a lot of time with, told me that when he first started working ten years earlier in the Yellow Sea, he thought his job was to document the extinction of these birds. And he said, you know, there would have been value in that. But he said - now, he said, I hope I live long enough to see a recovery. And he's about my age. So I hope we both get to live long enough to see that recovery.

DAVIES: You know, it really makes you reflect on the fact that these birds, you know, evolved in a world so completely different from the world that exists today, where all the lands that they rely on - their nesting grounds, their stopover points - are now controlled by, you know, countless private property owners under the control of, you know, dozens and dozens of sovereign states. It must - it just makes the challenge of conservation so daunting, doesn't it?

WEIDENSAUL: Absolutely. I mean, one could argue that migratory bird conservation is one of the most complex conservation problems in the world for all the reasons that you just outlined. It also - it's kind of astounding that migratory birds are doing as well as they're doing. And, of course, as you indicated in the introduction, you know, we've seen some tremendous declines in their numbers over the years. But there is a resiliency there in these birds that I find humbling and I find awe-inspiring, that they are still able, you know, to knit this enormous planet together through their migrations despite everything that we keep throwing at them.

DAVIES: We need to take a break here. Let me reintroduce you. We are speaking with Scott Weidensaul. His new book is "A World On The Wing: The Global Odyssey Of Migratory Birds." We'll continue our conversation in just a moment. This is FRESH AIR.

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DAVIES: This is FRESH AIR. And we're speaking with Scott Weidensaul, who has spent decades writing about and doing fieldwork on migratory birds. His new book is "A World On The Wing: The Global Odyssey Of Migratory Birds."

I want to talk about what the bodies of migratory birds - what changes they go through in their life cycle. I mean, you write about this bird called godwit, which make a 7,000-mile, nonstop journey from West Alaska to New Zealand, right (laughter)? Let's talk about some of the physical adaptations that occur in these birds. I mean, one thing is it's literally nonstop for 7,000 miles. What do they do about sleep?

WEIDENSAUL: Well, yes. So this is a bird called the bar-tailed godwit. And it has the longest nonstop migration of any land bird that we know of. This is a bird that can't rest on the water. So - you know, (laughter) it's - and it's going to cross the widest part of the Pacific Ocean. So if it stops flying over the course of the up to 11 days that it takes it to make that flight, if it stops flapping for any length of time, it's going to end up in the drink. So yeah, it's got a whole host of physiological challenges it's got to overcome, including, as you say, sleep. You know, it's got to get some sleep.

It turns out that migratory birds use an approach called unihemispheric sleep, where they put one half of their brain to sleep for a couple of seconds at a time, back and forth and back and forth and back and forth - almost continuously. Now, a few mammals can do this. Dolphins, for example, and other marine mammals which have to consciously breathe, they can undergo unihemispheric sleep as well because, otherwise, they would drown. But with migratory birds, this seems to be the typical approach. And they can - you know, they can keep their brain functioning, at least during migratory season.

When these birds are in migratory condition, they can do things that they can't do at other times of the year. For example, if you take a migratory songbird and experimentally prevent it from even undergoing unihemispheric sleep, where part of its brain gets a break every couple of seconds, not only will it not suffer sleep deprivation effects, it will - its reaction time will actually get better, but only during migratory season when it's in migratory condition. A lot of the physical adaptations that migratory birds have evolved, a human physiologist would love to know how to replicate in humans. For example, migratory birds can lay on a tremendous amount of muscle tissue without exercise.

DAVIES: (Laughter).

WEIDENSAUL: Would I like to get ripped and buff without exercise? Yes. Sign me up for that pill, please. And you mentioned the bar-tilled godwits and their digestive system. I mean, this is a bird that makes the 7,000-mile flight across the Pacific. So before it leaves Alaska in late August and September, it undergoes a process called hyperphagia, which is just - basically just means binge-feeding. And they just eat and eat and eat and eat and eat. They double their weight. They're 50% fat by the time they take off. They are squishy when you pick them up. They jiggle like water balloons, they've got so much fat under their skin.

At the same time, their heart muscle and their flight muscles increase by 50% in mass, and then very rapidly, after they have eaten as much as they possibly can and can't put any more fat on, their digestive organs shrink dramatically. Their stomach, their intestines - to a lesser extent, their liver - shrink dramatically 'cause they - basically, they jettison their digestive system before they make this flight. They fly anywhere from 7 to 11 days nonstop across the Pacific Ocean, land in New Zealand or Australia, regrow their guts and spend the austral summer down there feeding again and then undergo the same thing before they fly from there up to the Yellow Sea, land, regrow their guts, feed dramatically, you know, increase dramatically in weight and then fly the last 4,000 miles or so back to Alaska. And they do this again and again. This is a bird that can live for 25 or 30 years...

DAVIES: Wow.

WEIDENSAUL: ...Flying 18,000 miles a year. By the time the bar-tilled godwit dies, it's flown the equivalent of the distance from here to the moon and most of the way back again.

DAVIES: How do they stay hydrated?

WEIDENSAUL: Well (laughter), interesting, interesting adaptation there as well. They're able to burn their muscle tissue and their organ tissue. So, I mean, a bird's primary migratory fuel is fat, which is incredibly energy-rich. We mammals have a hard time burning fat, so most of our energy comes from carbohydrates. But birds are five times as efficient at burning fat as mammals are. The fat doesn't produce a lot of metabolic water, so dehydration is a real problem for birds. So what they are able to do is catabolize the tissue in their organs and their muscles and basically drink from this reservoir of metabolic water that they have in their bodies. And so they're kind of sipping from their organs as they're making these flights.

DAVIES: And oxygen, right? I mean, they fly at pretty high altitudes - right? - where air pressure is thinner. How do they have the oxygen to keep flapping those wings for 11 days?

WEIDENSAUL: Yeah. You can feel sorry for somebody who's, you know, laboring up the side of Mount Everest or K2 in the Himalayas, where they're - you know, they're, you know, dragging one foot in front of another. They've spent weeks acclimating to this higher and higher altitude. And as they get near the summit of 28,000 feet, they look up, and there's bar-headed geese or ruddy shelducks or cranes flying effortlessly overhead. And the reason birds can do this is, well, you know, any off-the-shelf bird has a dramatically more efficient respiratory system than a human. Our respiration system is what's known as tidal. We draw air into our lungs, and it goes back out the same way, like a tide going in and out of a marsh. So we really - we extract relatively little oxygen for each breath that we take.

Birds, on the other hand, have a system of air sacs that run through their body and connect with their lungs. And it actually takes them four respirations for each breath to move all the way through this system of lungs and air sacs. So they're able to extract 90% of the oxygen from every breath they take. On top of that, the birds that migrate over the Himalayas, like bar-headed geese that winter in southern India and will just simply fly in one nonstop flight over the Himalayas, have even more efficient lungs. They have very efficient hemoglobin in their blood that's much more efficient at pulling oxygen from their blood. They are somehow able to avoid the effects of things like pulmonary edema, which kills a lot of human climbers. We don't really know how they do that, but they're - yes, they're able to function at oxygen levels equivalent of about 40,000 feet. I mean, a human at 40,000 feet without supplemental oxygen would die. These birds are not only able to survive it, but they're able to exercise it at high levels while they're doing that.

DAVIES: Scott Weidensaul's book, "A World On The Wing: The Global Odyssey Of Migratory Birds," is now out in paperback. We'll hear more about the amazing world of migratory birds after this short break. I'm Dave Davies, and this is FRESH AIR.

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DAVIES: This is FRESH AIR. I'm Dave Davies in for Terry Gross. We're listening to my interview recorded last March with Scott Weidensaul, who spent decades writing about migratory birds and also doing fieldwork to track their movements and help preserve their habitats. His book "A World On The Wing: The Global Odyssey Of Migratory Birds" is now out in paperback.

You know, digital technology has changed everything in our lives and certainly has affected bird research. You know, for a long time, naturalists would capture birds and put little bands on their legs - right? - and then hope to catch them somewhere later on and glean information. How has digital technology affected our ability to track and study these birds?

WEIDENSAUL: Oh, it's absolutely revolutionized it. And bird banding is still an important tool for studying bird migration. It's cheap. It's easy. It's safe for the birds. But you're right, miniaturization of technology has now allowed us to track even the smallest birds across immensities of distance, and it's been one of the most intriguing and exciting things that I've been involved in. I've had a chance to work with some of this groundbreaking technology like light sensitive geolocators, which are little data loggers that go on birds. We put it on the bird and release the bird. And it goes on its migration to Central America or South America, comes back the next year. You recapture it and take the data logger off and download the data and find out where it's been.

You know, I study - with a group of colleagues, I study snowy owls. And because that's a bigger bird that can handle a little bit more robust piece of technology, we have a GPS GSM transmitter about the size of a matchbox that weighs about an ounce or so, sits in the middle of the bird's back. It has a solar panel, so it's continually recharging. And it has two technologies in there that most people are carrying around in their pocket right now. It communicates with the GPS satellites overhead, so it's continually recording incredibly precise three-dimensional GPS locations. And then a couple of times a week, it dials up through a cellphone modem and sends us the data through the cellphone network. And so I like to joke that I get text messages from snowy owls.

DAVIES: (Laughter).

WEIDENSAUL: My friends are demonstrably cooler than your friends are.

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WEIDENSAUL: But it gives us an opportunity to see, sometimes in almost a second-by-second basis, where these birds are going. And we've got little accelerometers inside these - some of these transmitters so that we can actually see almost each wing beat that the bird takes as it's migrating.

DAVIES: And then there are times that with smaller birds, that - I mean, you will have to capture them again, set up nets and - that you do in various places. But I'm wondering what it's like when you get one and then you get this information and download and then open up a computer screen. What do you learn? What do you see?

WEIDENSAUL: It's absolutely magical. I mean, I'm thinking in particular here about a gray-cheeked thrush that breeds in Denali National Park in Alaska where my colleagues and I have been studying bird migration for a number of years. I mean, this is a gray-cheeked thrush that we've caught repeatedly over the course of the last five or six years, and we've been able to track several of its migration routes. And the last time we caught it, we had tagged this bird with what's known as a pinpoint GPS tag where it records at preset intervals, you know, a single, incredibly precise GPS coordinate.

And, yeah, when you download this and you see this unspool on your computer screen, these long lines of vivid green, you know, from central Alaska across northern Canada and down through the American Midwest and then, you know, across the Gulf of Mexico into northern South America and then deep into the jungles of the Orinoco River Basin where this particular bird wintered - and it's a glimpse into the life of this bird at a really extraordinary level of detail. And to see this - you know, a bird that I have held in my hands on multiple occasions, that - you know, God, these things weigh barely more than air. There's hardly anything to them.

And to think that this bird is going from one of the most remote tundra wildernesses in North America to one of the last, you know, truly remote, truly pristine jungle wildernesses deep in the heart of South America and passing through backyards - I mean, literally, you know, this bird stopped off at a place in Indiana, a little housing development called Feather Cove of all things and was hanging out in a woodlot behind these subdivision tract houses, taking or taking a break during its southbound migration. To see the way it connects these distant parts of the world in such detail is - that's probably the single best thing about doing what I do.

DAVIES: You know, there's all this amazing technology which allows you to attach these tiny, lightweight trackers to birds, and this yields a lot of new information. But then there's the matter of, how do you aggregate this information and discern patterns and really get meaningful data? And a lot of this is really interesting, too. Who's putting all this together?

WEIDENSAUL: Right. Well, big data has really opened up a whole world of both understanding bird migration and giving us avenues to better conserve migratory birds, to understand where the problems are and where some of the solutions are. And certainly, some of that is tracking data. I mean, we band 1.2 million birds in North America every year, and just aggregating that data over the years has led to a lot of discoveries. But the advance in computing power in recent years has allowed us to see things that we would never have been able to see before.

For example, you know, the 148 Doppler radar stations that the National Weather Service operates across the lower 48, in addition to tracking precipitation and things that are of interest to meteorologists, also allow us to track birds at night. You can see not just that there are large numbers of birds in the night sky migrating, but the new dual polarization weather radar that rolled out a couple of years ago can actually allow an ornithologist to distinguish small birds from medium birds, medium birds from large birds, can actually tell the beak end of a bird from the tail end of the bird. You can calculate how many birds per cubic meter of airspace are aloft up there.

And because we have archived weather radar images going back to the beginning of the Doppler radar era in the 1980s, because we have so much computing power now to take these, you know, millions and millions and millions and millions and millions of radar images, we can actually look back in time and calculate how the number of migratory birds has changed over time. And so that's one reason that we know - is that since 1970, we've lost about a third of North America's birds. About - almost 3 billion birds have disappeared from North America compared to the 1970s. And, you know, and that's not a guesstimate. That's based on solid radar data.

DAVIES: There's also something called eBird, which, I gather, taps the information of avid birders around the world, right? How is this integrated?

WEIDENSAUL: Right. So eBird is a project of the Cornell Lab of Ornithology in New York. And if I'm going out birding, I have eBird on my phone. It's an app on my phone. And at the end of my birding trip, I just simply tally in all the individuals of all the species of birds I saw on that trip. I put some geolocation information in there, and I send that off to eBird. And every year, about 100 million checklists like that come in. It has rapidly turned into the largest wildlife database in the world. And birders do it because it's fun. Birders do it because it's a great way to keep records. But this aggregation of tremendous amounts of observational data by people who know what they're looking at - because most birders are really good at what they do - has been an absolute godsend for understanding bird migration and distribution and abundance and also understanding where we can get the most bang for the buck for our conservation efforts.

You know, for example, using eBird data, scientists in California realized that they could combine eBird data with high-resolution imagery from NASA showing surface water conditions in the Central Valley of California, which is an incredibly important migratory chokepoint for waterfowl and shorebirds and a lot of other species moving along the Pacific Flyway. And they could figure out the times when the most numbers of birds were passing through the - especially shorebirds and waterfowl - were passing through the Central Valley. They could pay rice farmers to flood their rice fields to precise steps for precise periods of time in precise areas and create tens of thousands of acres of ideal habitat for these birds at exactly the right time. And they could do it for a fraction of the price of buying that land and turning it into bird habitat on a permanent basis because some years, there's enough rain that you don't really need to do that at all.

DAVIES: We need to take another break here. Let me reintroduce you. We are speaking with Scott Weidensaul. His new book is "A World On The Wing: The Global Odyssey Of Migratory Birds." He'll be back to talk more in just a moment. This is FRESH AIR.

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DAVIES: This is FRESH AIR, and we're speaking with Scott Weidensaul, who has spent much of his life writing about migratory birds and doing fieldwork to track their movements and help preserve habitats. His new book is "A World On The Wing: The Global Odyssey Of Migratory Birds."

Now that we have all of this new data and this ability to integrate it and the eBird system collating all this information from birders, I wonder if it's changed any of our basic understanding about migration. Are there big questions that have been answered or new questions posed by all of this analysis?

WEIDENSAUL: Oh, absolutely. And in fact, the one area that probably has the greatest implications for migratory bird conservation here in North America is our understanding of how urban lights are changing bird migration. You know, think about it. Birds evolved to fly through dark night skies, navigating in part by the light from stars. And so we have these enormous cities, especially along the I-95 corridor in the Northeast, that are - that's just pouring an enormous amount of waste light into the night sky. And something that both eBird and radar studies have shown is that especially in autumn migration, when most of the birds in flight are young birds on their first migration, they are drawn like moths to a flame into these urban areas. They're pulled in by this light. They can become almost trapped by light.

For example, in New York City every fall when the tribute in lights, you know, the two big beams of light coming up from ground zero memorializing the 9/11 attacks, that almost always coincides with the peak of the fall migration. And these birds will get trapped into those beams of light. But what that means is we know that there's a solution to that, and that's simply turning the lights out. And increasing numbers of cities, including Philadelphia starting this spring, there's an effort to have skyscraper owners turn out their lights, to dim lights, across the city during just that couple of week period during the peak of the spring and fall migration.

DAVIES: How much of a problem are wind farms?

WEIDENSAUL: It depends. You know, the inherent conflict with wind development is that migratory birds and wind turbines need to be in the same places for the same reason - because they need winds. It really comes down to location, location, location. There are some places that are inherently more dangerous for migratory birds and also, I will say, for migratory bats because in some respects, wind turbines are even more dangerous for migratory bats than they are for birds. But there are some places that are more dangerous than others. And, you know, this is the difficulty for bird conservationists because, you know, I don't like ripping the tops off mountains in West Virginia for coal, either. And I certainly don't like what fossil fuels are doing to the climate and the way, you know, the changing climate is endangering bird migration. So, you know, finding the happy medium here is proving to be a real tough nut for migratory bird conservation.

DAVIES: Right. Is there anything you can do to mitigate the harm of wind farms that are in critical locations?

WEIDENSAUL: There are. And in fact, one of the things that is being experimented with is equipping wind farms with artificial intelligence and radar capabilities so that when the IA (ph) and the radar detect large numbers of birds moving in, they shut down the wind farms. Or in some cases, actually, in places where there are large endangered birds, like California condors in North America and some species of old world vultures, you know, even just detecting a single large vulture moving into a wind farm will, you know, theoretically, be able to turn off the turbines in those areas.

DAVIES: I could imagine that it could get pretty distressing when you think about the way development and climate change are destroying habitat around the world. You could kind of lose hope. On the other hand, I mean, if we've lost 3 billion birds in North America in the last - you know, since 1970 - but 70% of them are still there. One might say, well, you know, nature is resilient; nature will find a way. Where are you on the scale of hope and despair?

WEIDENSAUL: Depends on the day (laughter). As a friend of mine says, there's no percentage in pessimism. I am, at my core, an optimistic person. And, you know, that study that came out two years ago that pointed out that we've lost 3 billion migratory birds - or 3 billion birds in North America, there's actually, in some respects, a road map for restoration in that study.

For example, one group of birds that they found had actually increased dramatically over the last 40 years are waterfowl and water birds - marsh birds, like herons and egrets and ibises - and the reason for that is not a mystery. Starting in the 1980s, we as a society poured a tremendous amount of money and political will into restoring and protecting wetland habitats. And, you know, the reason for that is because ducks and geese have a very powerful constituency behind them. Waterfowl hunters are among the most engaged conservationists in the world, and they put their money where their mouth is. And state and federal agencies and private organizations, like Ducks Unlimited, poured millions of dollars into restoring wetland habitat, and it paid off for the birds.

And so for the groups of birds in North America that are in the worst shape today, like grassland birds - species like meadowlarks and bobolinks and upland sandpipers - that depend on natural grasslands, if we did the same thing for grasslands that we did for wetlands, we can bring those birds back. So, I mean, there are ways that we can turn this around. We just need to have the political will and the wherewithal to make it happen.

DAVIES: You know, you've spent so much of your life traveling over the world to report on birds and write about birds and participate in fieldwork. I wonder when you're not around them for a period of time, do you get withdrawal symptoms?

WEIDENSAUL: (Laughter) Well, the fortunate thing is, everywhere you go on this planet - with the exception of the central Antarctic ice cap - there are birds. So you're never far away from birds. So I don't - certainly, during the offseasons, when migration is not happening, I think a lot of birders get kind of twitchy because, you know, migration is the great spectacle that we as birders look forward to most every year. And, you know, we're speaking here at the end of March. I'm up in New England. The very first migrants are just starting to come back right now, and it's wonderful - you know, welcoming back old friends. At least they're old friends to me. They don't care about me at all.

DAVIES: (Laughter).

WEIDENSAUL: It is - this is very much a one-sided love affair here. But, I mean - and particularly after the kind of year that we've all just gone through, there is a tremendous solace in watching these natural rhythms play out again and again and to understand that, you know, despite everything that we've thrown at them, you know, these birds have taken it all and are still obeying these ancient rhythms and following these ancient flyways and coming back and doing the things that they were always meant to do.

DAVIES: Do you still go to Hawk Mountain?

WEIDENSAUL: Absolutely. Yeah, in fact, I just stepped down as the chair of the board at Hawk Mountain, one of the prouder experiences in my life for the last couple of years. I've remained involved with Hawk Mountain ever since I was a kid.

DAVIES: So despite - I mean, you've been to so many far-flung parts of the world. It still holds interest for you?

WEIDENSAUL: The Kittatinny Ridge on which Hawk Mountain sits is the anchor of my world. I wrote a book years ago about the Appalachians called "Mountains Of The Heart," and that is the mountain of my heart. That will always be the most important place in the world to me.

DAVIES: Well, Scott Weidensaul, thanks so much for coming back and speaking with us again.

WEIDENSAUL: Dave, it's always a pleasure. Thanks for having me.

DAVIES: Scott Weidensaul is a writer and a co-founder of Project SNOWstorm, which tracks and studies snowy owls, and of the Critical Connections project, which tracks the migration of birds that breed on national parklands in Alaska. I spoke with him last March about his book, "A World On The Wing: The Global Odyssey Of Migratory Birds," which is now out in paperback. By the way, April is a prime season for bird migration. And dozens of American cities are now participating in the Lights Out program, in which building owners are encouraged to dim their lights and reduce bird collisions. Coming up, Justin Chang reviews "Nitram," the new film about events leading up to the worst mass shooting in Australia's history. This is FRESH AIR.

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