A Global Account of Bird Migration

December 26, 2021

A review by Marcel Gahbauer

A World on the Wing: The Global Odyssey of Migratory Birds, by Scott Weidensaul

W. W. Norton & Company, 2021

385 pages, hardcover

ABA Sales–Buteo Books 15114

It is difficult to imagine any birder who doesn’t get excited by migration. But many may primarily appreciate it for the pulses of birds that pass by each spring and fall. While A World on the Wing celebrates these spectacles, it also delves deeper into their underlying biology.

Author Scott Weidensaul is no stranger to writing about migration. In 2000, he was a Pulitzer Prize finalist for his book, Living on the Wind: Across the Hemisphere with Migratory Birds. Two decades later he has revisited the topic from a more global perspective and has integrated a compelling narrative on the urgent need for conservation.

Weidensaul’s fascination with birds and migration extends back to childhood. He fondly recalls the annual thrill of “Big Goose Day” in eastern Pennsylvania, when the year’s first large flocks of northbound Canada Geese passed by overhead. Although he did not pursue biology as a career, he notes that ornithology has traditionally welcomed dedicated amateurs, and this is reflected in the personal experiences that make A World on the Wing a particularly relatable and accessible window into the science of migration. Through his firsthand involvement with projects ranging from banding hummingbirds to geotagging Blackpoll Warblers and tracking Snowy Owls with GPS transmitters, Weidensaul provides readers with an insider’s perspective.

As his earlier nomination suggests, Weidensaul is an engaging writer. Right from the first sentence of the prologue, he transports the reader into the realities of Alaskan field work. At a time when many of us have had our travel substantially restricted over the past year, it’s easy to be a bit envious of some of the exotic destinations and migratory spectacles that Weidensaul describes. However, he tempers that by also relating some of his less glamorous or outright harrowing experiences, including sleep deprivation, unnervingly close encounters with bears, and a police interrogation.

A World on the Wing is divided into ten enigmatically named main chapters, plus a substantial prologue and epilogue. Some chapters focus on the author’s time spent with researchers and conservationists around the world, such as “Spoonies” (Spoon-billed Sandpipers and other shorebirds in China), “Hangover” (Kirtland’s Warblers in the Bahamas), and “To Hide from God” (anti-poaching efforts in Cyprus). Others integrate a variety of shorter anecdotes around central themes, whether groups of birds (for example, shorebirds in “Quantum Leap” and seabirds in “Off the Shelf”) or phenomena such as climate change (“Tearing up the Calendar”) and technological advances in research (“Big Data, Big Trouble”).

Through much of A World on the Wing, the reader sees migration through Weidensaul’s eyes. But the author also provides a voice to lifelong researchers such as Theunis Piersma and Pete Bloom, who have witnessed dramatic changes over the course of their careers and are justifiably concerned about what the future holds for the species they study. At other times Weidensaul encourages us to consider the perspectives of the birds themselves—for example, what must a juvenile Bar-tailed Godwit be thinking as it heads south across the Pacific for the first time, continuing night and day under mounting exhaustion until it lands in a completely foreign environment in the southern hemisphere?

The narrative flows well, in part because Weidensaul makes judicious use of footnotes to direct readers to additional details, often in the primary scientific literature. Experienced birders will recognize many of the species that Weidensaul highlights, but no background familiarity with them is needed to appreciate the book. His evocative descriptions of species are a pleasure to read, regardless of how well one knows a bird, such as his portrayal of a frigatebird as a seabird that has been stretched “well past any common-sense proportions,” adding that its skeleton weighs less than its feathers. He also helps readers relate to some of the incredible physiological feats of migration by comparing them to human abilities. In short, we don’t stack up well: Imagine staying awake (let alone exercising without pause) for days on end, and this immediately after doubling your weight just before setting out! As Weidensaul notes, some of what migrants do reads like science fiction, yet is absolutely real.

A World on the Wing is well-researched and well-written, and there is relatively little to nitpick. The first couple of chapters are quite heavy on shorebirds, and there is some repetition in the description of their physiology. Some of the chapters were adapted from articles previously published in Bird Watcher's Digest, Living Bird, or Audubon magazine, resulting in a bit of redundancy of coverage. Otherwise, the only disappointments of note concern the 16 pages of photos in the middle of the book. It would have been helpful to have had them cross-referenced with the text. And although the choice of monochrome images is understandable from a production standpoint, they don’t do justice to the subject matter.

However, these are minor quibbles. A World on the Wing is notable because rather than just describing migration as static elements of a bird’s biology, Weidensaul emphasizes that many of these incredible journeys are increasingly in jeopardy from human-caused alterations to the environment. The extent of change in some areas is drastic, and it’s remarkable that certain birds have been able to persist at all; some populations may well be at the limits of their ability to do so. For example, 50,000 dams have been built along the main stem and tributaries of the Yangtze River in China, reducing by 97% the amount of silt reaching the Yellow Sea. This material is critical for supporting the prey for migratory shorebirds, and there simply is no longer “somewhere else” for these birds to go.

For other birds, hunting remains a major threat, especially in places where people have learned that migrants congregate in large numbers. In northeastern India, a mind-boggling 140,000 Amur Falcons were killed in a week and a half as recently as 2012. In parts of the Mediterranean, songbirds are the main target. The situation is particularly dire in Cyprus, where acacia stands have been planted and irrigated specifically to concentrate migrants, which are then caught by the gruesome technique of “liming”—coating branches in a strong adhesive to which birds get hopelessly stuck. As Weidensaul notes, songbirds are a traditional part of the diet in the region, but not every tradition is worth preserving.

Probably the most pervasive threat though is climate change. Almost all species will be affected to some extent, but birds reliant on coastal habitats are among the most vulnerable, especially those that nest in such areas and that could have their populations decimated by surging tides long before their habitat is fully submerged. Weidensaul also points out some of the less obvious ways in which climate change can pose threats. For example, lemming abundance in Eurasia has declined substantially as the Arctic has warmed, with a corresponding effect on the number of Snowy Owls, which depend heavily on them as prey; we can anticipate the same will happen in North America.

Complex though climate change is, at least there is increasingly broad awareness of the threats it poses. But as Weidensaul points out early on, ecology is an “almost perversely complicated subject,” and he illustrates how easy it is for us to misinterpret what we are seeing. For example, if you live in a large city, or have birded in one during migration, you've probably been impressed by the number of birds in some urban parks. Chances are, you may have even found it reassuring that numbers are so high despite the context. But in reality, birds are being attracted to the lights of cities from up to 300 kilometers away. They concentrate there while surrounding natural areas are passed over, even though those would likely serve the migrants better and expose them to less risk of common urban threats such as building collisions and cat predation. Mitigating these risks and conserving natural habitat in cities may therefore be even more important than we have realized.

It’s all too easy to unwittingly contribute to habitat loss. Buying strawberries in mid-winter seems innocuous enough, right? Not so, unfortunately. North America’s increasing demand for year-round production has resulted in large swaths of northern California being devoted to strawberry farms, which, it turns out, are largely sterile for wildlife. It is further worth noting though that anecdotes like this do not come across as preachy, but rather as efforts to raise awareness about the complexities of our influence on the environment at a global scale.

All of these concerns add up to a gloomy picture, and roughly midway through the book, Weidensaul notes that “we are on the cusp of catastrophe.” But while it can be tempting to despair that many of the world’s birds are doomed by some combination of habitat loss, overhunting, climate change, and other threats, A World on the Wing also offers glimpses of hope. There are still opportunities to be blown away by the sheer number of migrants—Weidensaul describes how the Tadoussac Bird Observatory in Quebec documented a single-day passage of over 700,000 warblers in late May 2018, a checklist he described as a “gift from a lusher, richer past.”

Moreover, he profiles some conservation successes, including how the insecticide monocrotophos was banned soon after it was found to be responsible for mass mortality of Swainson’s Hawks in Argentina, and signs of growth in the populations of rare species such as Kirtland’s Warbler and the critically endangered Spoon-billed Sandpiper, after years of dedicated conservation efforts. However, recovery can be fragile. The Amur Falcons that were so recently the victims of mass harvest are now instead being protected and promoted as a star attraction. Unfortunately, there and elsewhere, concern lingers that if tourism falters for any reason (political instability, economic barriers, or as we have recently seen, pandemic travel restrictions), the tide could quickly reverse if hunting or habitat exploitation again become more profitable.

Despite all that we have learned about migration, Weidensaul maintains that we still are only scratching the surface. Our capacity for research has grown dramatically in recent years, though, and we are on the cusp of having the ability to track the movements of nearly all species. In the third chapter, entitled “We Used to Think,” he describes how tools such as satellite telemetry and the Motus network have already yielded important discoveries. For example, we are learning about key stopover sites that can be critically valuable in guiding conservation priorities, such as the remarkably few locations Whimbrels routinely use, despite their migratory routes spanning thousands of kilometers.

Weidensaul emphasizes the need to be open to reconsidering our conservation strategies. Species such as Red-eyed Vireo and Wood Thrush have become widely recognized as “forest-interior specialists,” with the corollary that fragmentation and edge habitat are undesirable. But the reality is more complicated. Recent tracking studies have shown that juveniles of these species favor dense thickets along edges. The explanation is surprisingly simple: Such habitat provides excellent cover for inexperienced individuals and supports a much higher density of food sources (insects, berries) that allow birds to efficiently fuel up in advance of migration, a part of the life cycle that is increasingly recognized as crucial to survival, and by extension, to population trends. In other words, edge habitat ironically provides vital habitat to some of the very species it was thought to threaten.

A World on the Wing is a compelling celebration of the wonders of migration. Weidensaul observes with respect to the precision of Swainson’s Thrush migration, “Science be damned; the results are simply, breathtakingly cool.” Appropriately, he ends the book with a passage of unabashed reverence for the routine yet still impressive achievements of individual migrants, and admiration for the way in which their travels highlight the interconnectedness of the world. Weidensaul concludes with a subtle reminder to us all about our responsibilities: “May it always be so.”


Marcel Gahbauer lives in Ottawa, Ontario, where he is a biologist with the Canadian Wildlife Service and serves as a member of the Committee on the Status of Endangered Wildlife in Canada. Marcel cofounded the nonprofit Migration Research Foundation in 2002 and contributes to banding programs in Canada, Mexico, Belize, and Costa Rica.