August 2021

Vol. 49, No. 4

About Books: We Used to Think…

Mark Lynch

A World on the Wing: The Global Odyssey of Migratory Birds.
Scott Weidensaul. 2021. New York, New York: W.W. Norton & Company.

“You know what bothers me about scientists?” my mother asked some years ago. “They always say we used to think, but now we know.” (p. 99)

We may finally be looking at migratory birds the way they should be viewed—not as residents of any one place, but of the whole. These are creatures whose entire life cycles must be understood if we are to have a prayer of preserving them against the onslaught they face at every moment, and at every step, of their migration journey. (p. 14)

Avian migration is one of the most spectacular natural wonders that we enjoy every year in New England. Every spring, summer, and fall we can bear witness to mass movements of birds as many thousands of land birds, water birds, and pelagic species pass through our forests, shores, and seas on their way to their breeding or wintering grounds. All you have to do is get yourself to some well-known migration hotspot, of which there are many in New England, and lift up your binoculars. I have seen movements of nighthawks and raptors even from my front walk in the city of Worcester. Though many of us have read details of some of the spectacular migratory flights of species such as Arctic Tern, how many of us have really deeply grasped how incredible migratory journeys are? For many birders, migration means a time to tick uncommon species that do not breed here, but how many of you have been awed by the incredible journey that even the common Red-eyed Vireo makes twice a year?

I have taught classes on migration for decades, but it wasn’t until I took a trip in the mid-1990s to Argentina that I got a deeper understanding of just how amazing these migratory journeys are. I was in Tierra del Fuego at the southern tip of Argentina in the city of Ushuaia. Many tour ships that visit Antarctica dock here. I took a boat trip far out into the Beagle Channel. Most people take it for the scenery and penguins, but I was there for all the other birds. There were new species—for me—of cormorants and shearwaters, and interesting regional specialties such as prions and diving petrels.

The small boat passed an outcropping of rocks, and there I spotted my lifer Snowy Sheathbill, a specialty of the Antipodes. The charge from that tick was short-lived when I noticed that right next to the sheathbill was a White-rumped Sandpiper. I was dumbstruck. I had just arrived in Tierra del Fuego after a long series of flights, which I found interminable. My rear end and legs were numb. But here was this small sandpiper that breeds in the northwest Arctic, with which I was well familiar from New England in migration. This tiny sandpiper had made the same trip I just had, but under its own steam, often without eating for days at a time, fighting wind and weather all along the way. It wasn’t just the extraordinary distance it had traveled; I started to think about how the sandpiper physically accomplished this extreme trip. I had plenty of food and drink and was lifted along with little physical effort on my part, and the distance seemed extreme. This sandpiper accomplished its extreme trip twice a year through the course of its life. How did it manage to do it? That one sighting changed how I thought about migration forever, and since then I have been in awe of avian migration. But I am a mere neophyte compared to Scott Weidensaul.

This has become a very personal crusade for me, as it has for many of the men and women who study and protect migratory birds; the idea of a world without epic migrations is simply too poor and melancholy to contemplate. As with many of them, migration has captured me all my life—an obsession that began in childhood and crystallized on a windy ridegetop in Pennsylvania, and which has led me from being an eager observer to an increasingly passionate participant; from recreational birder to someone in the trenches of migratory science. (p.15)

Quite a number of books have been written about avian migration, including large-format coffee-table books jammed with colorful pictures. Despite that, A World on the Wing is the book about migration you should read this year. Scott Weidensaul is a well-known writer and researcher specializing in birds and bird migration. He started out as an enthusiastic, serious birder and then became enamored of hawk migration. This eventually led him to a deep passion for all the different avian migrations. Today he participates in several migration studies as well as doing field reporting from around the globe where other ornithologists are working on getting a better handle on the parameters of how far birds travel and how they physically manage to travel that far. A World on the Wing is his report on what he has learned. This book is simultaneously mind-blowing, depressing, and hopeful. The book follows two parallel tracks. One is Weidensaul’s reporting from around the world from key migration spots. The other is summarizing some of the latest scientific research that is rewriting most of what we thought we knew about bird migration.

What has made a crucial difference in ornithological research is the miniaturization of a number of electronic monitoring devices such as geolocators. These allow researchers to attach tracking equipment on birds without interfering with their flight, which means that researchers can closely track individual birds over an entire course of their flight. With ringing (banding), scientists had to hope to retrieve the ringed bird; even if they did, they had little idea about where the bird went in between the ringing and retrieval locations. Now we know, and what has been learned is honestly mindboggling.

Geolocator tracking has revealed that red-necked phalaropes from the Shetland Islands cross the North Atlantic and Central America to winter off the northwestern coast of South America, staging up in the Bay of Fundy along the way. While the wintering area of the eastern Canadian Arctic population remains a mystery, it seems likely that they, too, follow a similar route to the Pacific. (p. 265)

In older books on migration, Arctic Terns are typically cited as the champion long distance migrators, flying yearly trips of 22,000–28,000 miles each year. New research has shown that Arctic Terns can fly 37,000–51,000 miles per year. “Any seabird biologist will admit, especially after a beer or two, that no one really has a clue what the limits of tern migration might be.” (p. 11)

And Arctic Terns aren’t the only species to make unbelievably long yearly flights.

In 2006, scientists using geolocators announced they had successfully tracked 19 sooty shearwaters from their breeding colonies in New Zealand. Even a “local” feeding run during the breeding season, when parents forage for squid and fish to bring back to their nest burrows for their chicks, carried these plump, dark gray birds from New Zealand down into the frigid sub-Antarctic waters thousands of miles away, and back. Once the chicks fledged, however, they and the adults all headed north, crossing the equator to reach “winter” feeding grounds in the boreal summer off Japan, Alaska, or California. By following wind and ocean currents in looping curlicues across the Pacific, the birds (in the words of the researchers) enjoyed “an endless summer.” It’s a helluva road trip, since the routes taken by some shearwaters exceed 46,000 miles a year. (p. 10)

An important concept emphasized in A World on the Wing is the importance of stopover sites for migrants. These are areas in between breeding locations and the wintering areas, where birds stop to feed and rest. Here in New England, birding hotspots such as Plum Island, Monomoy, and Nauset Beach are local examples of these stopover sites for shorebirds and terns. Inland, there are many locations where, in spring and fall, large numbers of north or southbound migratory songbirds stop for a day or more before they continue on. Many of these sites are being degraded or lost completely due to habitat loss or pollution. When these stopover spots disappeared, it was always assumed that the birds would simply just go to similar habitat elsewhere. Contemporary research, cited in Weidensaul’s book, shows that this might not be true. Long-distance migrants, such as Whimbrels, are faithful to just four or five places where they stop at along the way. For migrants there is “strong connectivity” (p. 122) and fidelity to these sites. Whimbrels may migrate 18,000 miles a year but use only a specific approximate 500 acres of stopover sites en route. Even different populations of the same species may use a different set of stopover sites, and these may not overlap. The Whimbrels have been using their sites for hundreds of years. If these sites are disturbed or are lost to development, the birds that used these locations will likely disappear.

Stopover sites can range in size and quality; somewhat playfully, ornithologists have categorized them as fire escapes, convenience stores, and five-star hotels, though the importance to migrant birds is in dead earnest. Like a crowded truck stop serving as its own best advertisement, the premier stopover locations—those offering the richest bounty of food, abundant at just the right season, with safety from danger and plenty of elbow room—are crowded with migrants that have evolved to depend on these often widely scattered places. (p. 27)

Sometimes in A World on the Wing it can feel as if everything we thought we knew about migrants is wrong. We used to think we needed to save the tropical habitats in order to preserve populations of “our” breeding songbirds. Then we learned that we needed to save our temperate forests as well. Our forests were being chopped up into small parcels (forest fragmentation) due to development, and some species needed certain sized forests in order to breed and feed. Now it has been learned that right after fledging, many warblers are leaving the forest habitat to feed in different habitats.

To the scientists’ surprise, those supposed deep-woods specialists were moving into the polar opposite habitat—dense thickets and tangled undergrowth in old clear-cuts, field margins, abandoned farms, roadsides, and the like. The juveniles were gorging there on ripe blackberries, in brambles so dense it was hard to imagine even the most agile hawk squeezing itself through; they haunted poison ivy jungles, jumbles of wild grape vines, and sumac so thick one could barely see into them. (p. 104-5)

The reasons for this are not totally clear. Have breeding warblers always done this? Part of the answer may be in the way we are managing our forests. As forest program manager Ron Rohrbaugh tells Weidensaul,

The forest is no longer producing the food and the calories and the energy necessary for migratory birds to get what they need, because there’s no understory left—they stop in these forests, and it’s all just canopy. They fly down to the midstory and the understory, and there’s nothing there. The natural structure of the forest has been lost. (p. 106)

So, in order to “save” certain species of American warblers, we have to make sure tropical wintering habitats are intact, their breeding forest habitats are preserved, AND brushy and brambly understory-like habitats are available for the postbreeding birds to feed in before they migrate south. The complexity of the challenge is sobering. Some species of warblers, such as the Cerulean and Golden-winged, have declined as much as 98% in recent years.

We all know that warblers such as Blackpolls put on lots of subcutaneous brown fat by power feeding before they launch themselves in a wide arc over the Atlantic in fall. It is this fat that powers their long overseas flight, right? Well, yes, but that is not the whole picture. Think about Boston Marathon runners. They may have the muscles to run those 26 miles, but they need to stay well hydrated throughout the course of the run or their bodies will shut down. That is why so many bystanders hand out water to the runners along the length of the course. All vertebrate bodies lose metabolic water just by breathing and excreting, let alone through extreme exertion. Now, think about birds migrating over 1,000 miles or more, sometimes with no stops for food, rest, or water. How do they accomplish this? That brown fat contains very little water. Weidensaul cites the research of Alex Gerson at the University of Massachusetts, who has found that long distance migrants like the Swainson’s Thrushes he studies do something quite extraordinary.

He’s found that by cannibalizing its muscles and organs in addition to burning fat, a bird can constantly adjust its production of metabolic water to keep up with the loss from breathing and excretion. In the process, a thrush weighing a shade more than an ounce can extend its flight range by almost 30 percent, to more than 2,000 miles, beyond what it could fly on fat alone—a critical cushion for birds that, like the Swainson’s thrush, make long overwater crossings. (p. 71)

Actually, referring to the flight as a marathon does the birds a considerable disservice. Theunis has noted that an elite human athlete, performing at a maximum exertion—a male Tour de France cyclist in mid-race is a good example, he says—is operating at about five times his base metabolism. That seems to be the upper limit for sustained exercise by even the fittest, most highly trained human. A shorebird, on the other hand, is working at a rate eight or nine times its base metabolism, and is doing so for days at a stretch without food or water or rest. (p. 40)

I am quoting just a bit of the research described in A World on the Wing. But before I move on, I have to mention one finding that made me slack-jawed thinking about its possibility. One of the classic mysteries of migration is how birds find their way. We now know that some species can partially read constellations in the night sky to guide them. Other species may use their ability to perceive polarized light. Pigeons were found to have tiny bits of magnetite in their heads, and it was postulated that by using these minimagnets they could somehow follow the earth’s magnetic fields to navigate. It was a great idea. Well, it turns out that the pigeon magnetite does no such thing. It is not used to guide them in migration. What is being looked at is something so weird, that Einstein had problems dealing with the concept: “It now appears that birds visualize the earth’s magnetic field through a form of quantum entanglement, which is just as bizarre as it sounds.” (p. 8)

If you don’t know about quantum entanglement—what Einstein dubbed “spooky action at a distance”—rest assured that Weidensaul gives a fairly good explanation of it in the book. Be forewarned, quantum theory is tough to understand because it appears to go against everything we have been taught about how the world works. But don’t be intimidated if the concepts of quantum theory and quantum physics leave you blank faced. You are not alone. This quote—attributed to the physicist Richard Feynman but probably apocryphal—says it all: “Anyone who claims to understand quantum theory does not understand quantum theory.” And birds are using that principle to navigate? Mind: blown.

Now, if all A World on the Wing contained was Weidensaul’s synopsis of some of the latest research into avian migration, it would be a great book to read. Weidensaul spends time in the field with these scientists, and he knows the material well enough to clearly explain it to the reader. But this material is only half the book. The other half of the book covers Weidensaul’s “reports from the field” as he travels around the world to see these critical stopover locations for migration and to talk with the scientists doing critical research there. In each case, there is a real danger that these locations could be lost forever, but in each case Weidensaul manages to find some hope.

The book opens with a chapter that is both hopeful and scary as hell. Weidensaul and a few other hardy banders are in Denali National Park banding thrushes, when a grizzly bear appears. The bear is far, far too close for comfort. This humorous and frightening section is a perfect introduction to the crazy life of researchers who study migration. In the following chapter, we find Weidensaul and a cadre of researchers censusing shorebirds on the vast Yellow Sea flats in Jiangsu Province, China. Every year thousands of shorebirds use this stopover as a critical feeding area as they continue south along the East Asian-Australasian Flyway. This includes the extremely rare but charismatic Spoon-billed Sandpiper. This beloved and much sought-after species has declined dramatically in the last decades. In the late ‘70s, Soviet researchers estimated that 2,000–2,800 pairs of these sandpipers were left breeding in Siberia. Now, hopeful estimates put their numbers at 300–600. In areas where past censuses had found 65 pairs, there are now eight.

Other critically-endangered species also use these Yellow Sea flats:

Nordmann’s greenshank, with barely 1,000 individuals left in the world, is only slightly further from extinction, and many of the other shorebirds that use the Yellow Sea—red knots and great knots, black-tailed and bar-tailed godwits, curlew and Terek sandpipers—are declining at up to 24 percent in some years. (p. 33)

Part of the reason for the shorebird’s decline has been the rampant reclamation of the flats for development by the Chinese government. Over the past 50 years, it has been estimated that two-thirds of China’s coastal wetland has been destroyed this way. And it is not just the flats. At high tide, shorebirds used to fly into pools just a short distance inland to rest and feed even more. Many of these pools have been reclaimed as fish and shrimp farms, and many of them are also now covered with solar farms, making many of these spots unusable by the shorebirds. But all is not quite lost. Recently China indicated that the wholesale destruction of this globally important habitat is on hold for now. But of course there is a catch: “On the other hand, the ban leaves a loophole for projects in keeping with ‘national economy and people’s livelihoods,’ and no one could say exactly what that might mean.” (p. 51)

Weidensaul visits Cyprus, where every year thousands of migrants were taken illegally by netting or, horrifically, using limed branches. The bird slaughter here is considered to be one of the three worst killing sites in the Mediterranean. These songbirds are trapped only to be cooked for a dish called ambelopoulia, in which the whole bird, head attached, is cooked in oil and salted. The scope of this slaughter is truly shocking. For instance, up to 300,000 Ortolan Buntings were trapped every year until recently. It is now illegal to trap migrants in Cyprus, but that doesn’t mean the killing has stopped. Weidensaul travels out on the back roads of Cyprus with a group of citizens determined to catch these illegal trappers. It is a process that is risky and has the real possibility of deadly violence.

In another chapter, Weidensaul travels to the Naga Hills of northeast India. This is a location where every year at least 50,000 Amur Falcons gather at one time in migration to feed on dragonflies. Until recently, the poor villagers would kill thousands of the falcons to cook and then sell them elsewhere to earn some much-needed cash. Locals were encouraged by Indian conservationists to stop the slaughter with the promise of attracting ecotourism, and the slaughter stopped. The catch is that the Naga Hills is a very difficult place to reach, and you have to travel over some of the worst roads in India. It is also the site of a long guerilla war. The villagers are wonderful folks, now waiting to put up some birders, but so far the conditions on the ground have prevented many birders from coming. What is the solution?

Weidensaul takes a pelagic trip out of Cape Hatteras with Brian Patterson. This leads to a lively discussion about the worldwide travels of pelagic species. Last year a Tahiti Petrel was seen on this particular pelagic. That species is known only from the waters around its namesake island in the mid-Pacific. How did it get to the offshore mid-Atlantic? Did it cross over Panama? Did it wing its way south and cross at the Straits of Magellan? Shearwaters, albatrosses, and petrels seem to rewrite the book on bird movements every year. Pelagic birds as a group seem to look at all of the earth’s waters as their home. “In some cases, we’re not sure exactly which hemisphere they inhabit; to a surprising degree, we don’t even know what species are out there, still unknown.” (p. 248)

Hardcore listers and pelagic enthusiasts will enjoy Weidensaul’s discussion of the current mess of pelagic taxonomy, a result of recent in-depth genetic studies. For example, the Band-rumped Storm-Petrel nests in the eastern Atlantic Ocean along a 4,500-mile arc of islands. Seabird geneticists have found no evidence of gene flow among any of these colonies, and the birds are utterly faithful to their breeding island. Does this mean each population is a species? Even on one island, it has been found that that there can be two distinct populations of the same species. They just nest at different times of the year. Of course, visually separating these populations in the field is another challenge entirely.

In A World on the Wing, Weidensaul conveys a global sense of how extraordinary migrating birds are, and how their existence is being threatened at every turn by human activity. But thankfully, it is not all bad news. For example, Weidensaul includes a vivid description of the day when Ian Davies and friends witnessed one of the greatest spectacles of land bird migration in recent history. This took place on May 28, 2018, along the Côte-Nord on the north shore of the St. Lawrence River in Quebec. Because of particular weather patterns there was a once in a lifetime movement of landbirds. Davies was unprepared for what he witnessed:

The songbirds, almost exclusively warblers, were often flying by at a rate of 20 per second, more than 1,000 a minute. A single binocular view might encompass several hundreds to several thousand songbirds.” (p. 152–3)

At their peak, there were 50 birds passing per second, 72,000 in a half an hour for hours! It was estimated by Davies that totals included 28,900 Blackburnian Warblers, 72,000 Yellow-rumps, and 14,000 Canada Warblers. But don’t think all is well because so many birds were seen that one time. Weidensaul cautions that what is astounding today, would have been commonplace just a few decades ago.

A World on the Wing is an important book about bird migration. It should be mentioned that there is a generous section of color photographs included. But it is the text that will grab your attention. Weidensaul is an award-winning writer and reporter. He knows how to convey complex ideas to general audiences. He also excels at capturing the sense of place as he ping-pongs around the globe. Finally, he is unabashedly passionate about his subject. That makes this an exciting, thoughtful, sobering, and even thrilling book. After you read A World on the Wing, you will never look at the spectacle of spring and fall migration in New England without a feeling of awe.

Reverence for an endurance and tenacity I cannot match nor fully comprehend, but which leaves me breathless when I am confronted with it.” (p. 347)

To listen to my conversation with Scott Weidensaul about this book, use this link:


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