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There are almost 30 species of shorebirds that breed in the Canadian Arctic, and all are strongly migratory. Surely the longest of their migrations must count among the most impressive feats in the natural world. Red Knots, for instance, are only nine inches long. And yet, every year, they fly some 9,000 miles from their summertime Arctic nesting territories to their South American vacation hideaways—and then another 9,000 miles back again.  

Unfortunately, shorebird population are hurting across the globe. In North America alone, shorebird populations have plummeted by 70 percent since 1973, and among those, birds that breed in the Arctic are especially threatened, . But a workable solution is hard to come by because the birds face a multitude of threats as they make their way across the Western Hemisphere. Munro writes:

Although the trend is clear, the underlying causes are not. That’s because shorebirds travel thousands of kilometres a year, and encounter so many threats along the way that it is hard to decipher which are the most damaging. Evidence suggests that rapidly changing climate conditions in the Arctic are taking a toll, but that is just one of many offenders. Other culprits include coastal development, hunting in the Caribbean and agricultural shifts in North America. The challenge is to identify the most serious problems and then develop plans to help shorebirds to bounce back. 

“It’s inherently complicated — these birds travel the globe, so it could be anything, anywhere, along the way,” says ecologist Paul Smith, a research scientist at Canada’s National Wildlife Research Centre in Ottawa.

Part of the problem is that, when birds migrate, they aren't just winging it; their trips are especially timed to sudden bursts of food resources along the way. The aforementioned Red Knot takes its regular pit stop in southern New Jersey to feast on the eggs of horseshoe crabs as they gather in the surf to mate. Western Sandpipers land in British Columbia's Fraser River estuary to  (more commonly known as biofilms) with their feathery tongues, right as the mudflats reach maximum slime. And most species lay their eggs so that they hatch concurrently with peak insect, which provides ample food for hungry chicks.

These tightly synchronized global patterns determine each species' breeding success for the year. Over the scale of evolutionary time, a missed breeding year here or there doesn't mean much. But today, shorebirds face extraordinary pressure at each juncture of their migrations thanks to rapid environmental changes caused by people. 

, Munro takes stock of these pressures as scientists race to understand them in time to help the birds avoid extinction. Red Knots no longer have enough food to refuel for the second leg of their northward journey because people have overharvested horseshoe crabs, leading to a shortage of their energy-rich eggs. (The Rufa Red Knot subspecies is listed as threatened under the Endangered Species Act.) For Western Sandpipers, from which they scrape together a living on biofilms. A warming Arctic has put  than hatching chicks, causing malnutrition and breeding failures. And the expansion of cornfields in middle America has caused goose populations to explode and encroach upon the smaller, more finicky shorebirds during nesting season.

Munro's reporting touches on all these pressures facing the birds and more. . Her findings are not clear-cut, and they can't be. But she does show the complex connections between seemingly disparate human-caused changes to the environment. And, by describing a problem with no easy solution, she presents a challenge to us all. Because ensuring a future with shorebirds in it means restoring habitat and protecting wildlife throughout the Americas, not only where the birds start and end their thousand-mile journeys.