A Test of Endurance: Bird Migration

“The magnitude of birds’ migratory performances staggers our imagination, in terms of both physical exertion and feats of navigation, because they are vastly superior to anything we could, as individuals, accomplish.” [The Homing Instinct]

A Test of Endurance

Twice a year, millions of birds fly immense distances “to take advantage of shifting resources at different locations … chasing booms in the availability of insects and other key foods and the right conditions to nest and raise babies.” [1] While some birds only migrate a short distance, others undertake unimaginably long and treacherous flights. Tiny gray-cheeked thrushes, weighing just over an ounce, travel from northern Alaska and the Canadian subarctic to South America and back each year. “At least some of them cross the Gulf of Mexico in a 600-mile nonstop leap, while others may follow the long finger of Florida and then overfly the Caribbean.” [2] Bar-headed geese cross the Himalayas, the world’s highest mountain range.

Most birds migrate at night when there are fewer predators, the air is less turbulent, and the stars and the moon can assist with navigation.

An immense amount of effort goes into preparing for these long-distance tests of endurance:

“Godwits make a 7,200-mile nonstop flight each autumn from western Alaska to New Zealand, a journey that takes them eight or nine days of uninterrupted flight – the longest nonstop migration known … They accomplish this astounding feat by first larding themselves with thick layers of fat, feeding with manic energy on the rich tidal flats of the Alaskan Peninsula, eating marine worms and other invertebrates. They more than double their weight in about two weeks, so that a 1.5-pound godwit is carrying more than 10 ounces of fat under its skin and within its body cavity. So obese that they jiggle when they walk, the godwits then undergo a rapid reorganization of their internal anatomy. Digestive organs like their gizzard and intestines, which they no longer need, shrink and atrophy, while the pectoral muscles that power their long, slender wings double in mass, as does their heart muscle, and their lungs increase in capacity … The godwits time their departure from Alaska with the passage of autumn gales, when they get a boost from powerful tail winds speeding them along the first 500 to 1,000 miles of their journey across the Pacific … Once in Australasia, the godwits quickly regrow their digestive organs and spend the austral (or Southern Hemisphere) summer feeding normally, but as the days shorten, hormonal changes trigger another episode of binge feeding (known as hyperphagia) and explosive weight gain, followed by a similar but not as extreme atrophy of digestive organs.” [2]

The flight itself is not the only risk the birds face. Poaching and subsistence hunting continue to kill migrating birds in much of Asia, Africa, the Caribbean, northern South America, and parts of the Mediterranean. Wetlands are disappearing, while intensive agriculture and urban sprawl are changing the landscape. The dangers aren’t over once the chicks are hatched. Songbirds that nest in forests require edge scrub such as overgrown clearings where their young can safely feast on insects and other prey before starting off on their first migration.

Tracking the Birds

Early Europeans believed that birds flew to the moon when they migrated or hibernated in the mud at the bottom of lakes and rivers. Native American cultures were more realistic and understood that the birds were flying away to distant countries. Aided by technology, scientists have devoted a great deal of time and energy to working out birds’ flight paths and destinations. Two recent books [1, 2] outline in fascinating detail how scientists have tracked birds using sound, radio telemetry, radar, and eventually tiny miniature transmitters. Bill Cochran and a student followed a thrush equipped with a radio transmitter for 930 miles in a converted Chevy station wagon with a large antenna poking out of a hole in the roof. In order to track Swainson’s thrushes in Colombia, Ana González first had to assemble the Motus transmitters and put them on site, including erecting a “Motus tower on a hard-to-access mountaintop requiring an hour-long trek with a solar panel and two fifty-pound batteries on the back of a horse.” [1]

Even the minuscule weight of a tracking device places a burden on birds that already face such immense challenges, but researchers strongly believe in “the necessity of knowing every major aspect of a bird’s life in order to understand what it needs, to identify where the threats lie, and to take action to protect the species.” [2]

Staging areas and layover spots are of critical importance, allowing the birds to rest and refuel before continuing their journey. Certain locations are migratory hotspots. The Fraser River estuary in Vancouver plays host to 95% of the world’s western sandpipers each year (along with over 100,000 dunlins and large numbers of black-bellied plovers). Nearly three quarters of birds migrate through the central US coast of the Gulf of Mexico each year.

The areas that require protection aren’t always the most picturesque. “Along the Gulf Coast, banding has shown that the habitats most valuable to hungry migrants aren’t the lovely beach dunes and graceful pine forests we’ve protected in national seashores and parks – they are the greenbrier tangles, oak thickets, and deep swamps that few people bother to visit, and which have largely been ignore for preservation.” [2]

We’ve had a tendency to imagine large flocks of birds heading generally south or north. But different species or even sub-groups may have very specific migratory routes and destinations. For example, “Swainson’s thrushes from Marin County, California, migrate 1,600 miles to the Mexican state of Jalisco on the Pacific, while those from farther up the coast, near Vancouver, British Columbia, leapfrog over them and migrate clear to Central America.” [2] The birds may travel huge distances but actually use a very limited amount of space. Hope, a tagged whimbrel, migrated 18,000 miles a year but always wintered in the same small mangrove swamp known as Great Pond on St. Croix in the US Virgin Islands, always stopped along Boxtree Creek on the way north, and always nested in the same part of the MacKenzie Delta.

A Changing Climate

Protecting key bird areas is even more important in a changing climate. As Scott Weidensaul explains in A World on the Wing,

“Climate change is reshaping every single thing about migration. It is tearing up the calendar, altering the timetable on which birds must travel in order to find the food they need along their pathways, or accelerating the seasons in ways that increasingly leave them farther and farther behind during crucial periods like the nesting season. It is modifying the weather; not only are storms growing stronger, but continental winds are strengthening at some times and places and weakening at others, with unknown consequences for the many birds that depend on reliable tailwinds at critical steps in their migration – never mind the rising temperatures that alter when insects emerge, or simply make it too hot for a baby bird to survive. Climate change is reshaping the landscape … as regions dry out or become seasonally sodden, as shorter winters and longer, hotter summers (or changing wet/dry seasons in the tropics) mangle once-stable plant and animal communities. Climate change, we now know, is even altering the physical size and shape of many migratory birds, as their bodies shrink in response to the rising warmth.”

Sources: This article shares some of the information about bird migration more comprehensively discussed in A World on the Wing by Scott Weidensaul and Flight Paths: How a Passionate and Quirky Group of Pioneering Scientists Solved the Mystery of Bird Migration by Rebecca Heisman. Both books are well-written and extremely informative. Flight Paths focuses on North America and excels at story-telling, while A World on the Wing has a more global perspective with interesting sections on places such as China’s Yellow Sea and Nagaland. The Homing Instinct: Meaning and Mystery in Animal Migration by Bernd Heinrich covers a wider range of topics, exploring the nature of home and our tendency to head towards home for birds, animals (including humans), insects, and trees.

[1] Flight Paths: How a Passionate and Quirky Group of Pioneering Scientists Solved the Mystery of Bird Migration, Rebecca Heisman

[2] A World on the Wing, Scott Weidensaul

The Homing Instinct: Meaning and Mystery in Animal Migration, Bernd Heinrich

Photo credit: https://www.flickr.com/photos/apmckinlay/31819424703

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