Flight Paths (eBook)

Introduction

Where Do the Birds Go?

I used to think I knew a lot about bird migration.

After all, I’d studied zoology in college, collecting data on the behavior of robins and sparrows for class projects and volunteering to help survey bird populations and monitor nest boxes. After graduation I’d found work as a seasonal field assistant on ornithology research projects on the prairies of Saskatchewan and in the Australian outback. I’d even kept a “life list” for a time, documenting more than six hundred bird species that I’d observed on my travels. Eventually I ended up working for the American Ornithological Society (AOS), the world’s largest professional organization for scientists who study birds.

At AOS, a large chunk of my job was to publicize the research being published in its two venerable ornithological journals, which until recently were known as The Auk and The Condor (in 2021, these historic names were changed to Ornithology and Ornithological Applications). Instead of working in the field to help collect data, I spent my days at a desk reading cutting-edge migration research produced by others. I waded through scientific papers and exchanged emails with the researchers behind them as I translated their work into digestible blog posts, tweets, and press releases.

And for someone who thought I knew plenty about bird migration, I found myself being surprised an awful lot. Not just by what these scientists were learning—although that was fascinating, too—but by how they were learning it, the details in the sometimes-overlooked “methods” section of a scientific paper, where researchers spell out exactly how they produced their data. Despite my own background in ornithology, it was news to me that birds’ migration patterns could be studied with weather radar. Or by analyzing the hydrogen isotopes in their feathers. Or with tiny devices that used the movement of the sun to calculate location. How, I wondered, did we even figure out how to do any of this? Almost every branch of science, it seemed, had been co-opted in service of figuring out the answer to one question: just where it is that birds go when they disappear south over the horizon in autumn.

Humans’ curiosity about this goes back a very long time. Native American cultures seem to have figured out early that birds were flying away to distant locations when they vanished in the fall; Athabascan people in what is now Alaska, for example, have an old story about how Raven fell in love with a goose but had to part with her when fall arrived and she flew away over the ocean. European thinkers, however, took awhile to catch up.

Although some ancient Greek writers speculated that birds left for warmer locations, Aristotle threw things into confusion when he wrote his Historia Animalium in the fourth century BC. In it, he hypothesized that swallows hibernated in crevices in trees and that some winter and summer residents were in fact the same birds in different plumages—for example, that the common redstarts he saw in summer transmogrified into European robins when the seasons changed. Inspired by Aristotle, the Swedish priest Olaus Magnus suggested in the sixteenth century that perhaps swallows hibernated in the mud at the bottom of lakes and rivers, a misconception that persisted into the nineteenth century.

Perhaps the most outlandish idea, however, came from the English minister and educator Charles Morton. In the late seventeenth century, Morton, better remembered for writing a physics textbook that long remained in use at both Harvard and Yale, wrote a treatise in which he laid out his own fantastical theory of bird migration: they were simply flying to the moon. He estimated that if they could fly 125 miles per hour, it would take a flock of birds about two months to make the journey (although his approximation of Earth’s distance from the moon was short by about 25 percent). As ridiculous as this sounds today, Morton was writing at a time when it was popularly believed that other planets must be inhabited and no one realized that there was a crucial lack of oxygen in the space between them. Some of the things he intuited about the natural history of migration turned out to be more or less right. He speculated that birds may be spurred to move to new areas by changing weather and a lack of food, and he even noted that body fat might help sustain them on their journey.

Early naturalists could guess all they wanted, but the first truly concrete evidence of where birds disappeared to every year arrived in the form of an unfortunate stork shot outside a German village in 1822. When the hunter went to pick up his prize, he must have been astonished to see that it had a massive spear impaled clear through its neck, which it had apparently been carrying around with it for some time. A German newspaper eventually analyzed the wood in the spear and its iron tip and concluded that it must have originated somewhere in Africa. Dubbed the Pfeilstorch (German for “arrow stork”), the bird was taxidermized—spear and all—and is still on display in a natural history museum in Rostock, Germany.

Since it had last departed Germany, the Pfeilstorch had not hibernated, transformed into a different species, or gone to the moon. Instead, it had been to Africa. Birds, it seemed, were traveling between continents.

Before we go any further, I should probably talk about what exactly bird migration is and how it came to be. Migration is simply the seasonal movement of animals between regions. Birds can be “permanent residents” that opt out of migration entirely to spend their whole lives in one place, short- or medium-distance migrants that move anywhere from a few miles up or down a mountainside to a few hundred miles, or—like most of the birds in this book—long-distance migrants, whose journeys span entire continents. Birds make these treks to take advantage of shifting resources at different locations throughout the year, chasing booms in the availability of insects and other key foods and the right conditions to nest and raise babies. The urge to migrate when spring and fall arrive can have a range of complicated triggers including changes in weather and day length as well as genetic programming. However it happens, though, there’s a lovely German word that ornithologists use to describe this feeling that comes over birds: Zugunruhe, which literally means “movement restlessness.”

Scientists have come up with two competing theories to explain how long-distance migration might have originated. The “northern home” hypothesis supposes that migrants are descended from birds that evolved at northern latitudes and eventually started to push farther and farther southward in the winter in search of milder climates. The “southern home” hypothesis is, just as you’d expect, the opposite—the idea that migrants started out as tropical birds looking north for better breeding grounds.

Analyzing the evolutionary family tree of long-distance migrants in the Americas suggests that the northern-home scenario was probably more common, and that some birds that live in the tropics year-round today are in fact the descendants of northern-home migrants that eventually began sticking around on their wintering territories permanently. Either way, striking out in search of better habitats made the ancestors of today’s migrants more successful, they passed their wanderlust on to their offspring, and the evolution of long-distance migration was the result.

To make their epic voyages, birds rely on a range of navigation techniques. Genetic hard wiring seems to play a role, but migrating birds can also adjust on the fly (so to speak) by taking cues from the appearance of landmarks below and the orientation of the sun and stars above. Birds can even sense Earth’s magnetic fields, through an inscrutable mechanism that recent research suggests may have something to do with quantum physics (!).

In the spring of 2020, vast new numbers of people discovered the magic of migration for themselves. Bird-watching, it turned out, was the ideal hobby for pandemic lockdowns. Visits to websites listing local bird sanctuaries and downloads of bird ID apps soared during those months. Retailers couldn’t keep up with the increased demand for bird feeders and birdbaths, and people flocked (forgive the pun) to the Facebook groups of local bird-watching clubs.

The appeal was clear. You can watch birds almost anywhere, including, literally, your own backyard. The equipment needed to get started is minimal, and especially early on, when almost every species you learn to identify is new to you, it can provide a sense of novelty that’s otherwise sorely lacking when you’re stuck at home. And birds, after all, had never heard of coronavirus. Entering their world, if only for a little while, gave us a chance to forget what was going on in our own, and that first COVID wave in the United States happened to coincide with one of the most tantalizing phenomena in the natural world: spring migration. We might have been stuck at home, but birds of all shapes, sizes, and colors were traveling thousands of miles, returning from wintering grounds in Central and South America to their summer homes where they would find mates, build nests, and raise babies.

I was one of the many people paying closer-than-usual attention to the waves of migrants arriving that spring, enjoying seeing the cold and silent woods of my local birding patch filling anew with color and song. But like many, I was experiencing profound upheaval in both my personal and my professional life during those months. In need of a...