Many birds may be small, but they're great natural athletes. Every year, birds literally cross mountains, deserts, and seas to reach their favorite breeding and feeding grounds. Arctic terns — each weighing about a fifth of a pound — migrate halfway around the world, flying to Greenland in the spring and Antarctica in the winter. And despite the idiom, birds rarely go "as the crow flies," in a direct line from Point A to Point B. So then, how do they choose their routes?

Answering this question would shed light on what's likely a driving force in birds' evolution. Migration can be perilous business, which means those migratory routes that have survived to this day likely have major benefits for those that take them, even if they might seem circuitous at first glance. Bird biologists have already figured out a few factors that make certain, longer routes better for birds, including avoiding the open ocean, and having safe rest stops along the way. Now, a new study offers evidence that birds consider annual wind patterns too. Together, these factors make certain paths appealing to birds of many kinds, creating the world's great migratory flyways.

To check the effect of wind on migratory routes, biologists from several institutes in Germany created a mathematical model of more than a million potential migration routes, all around the world. They added 21 years' worth of worldwide wind data into the model. They then compared how long it would take a flapping bird to fly between pairs of points around the world if the bird took the shortest route, versus the wind-optimized route. Wind optimization shaves a median of 26 percent off the time it takes to migrate between places, the researchers found. That 26 percent translates to more time for feeding and breeding for the bird — big boons, from an evolutionary point of view.

Of course, a mathematical model isn't exactly the same as observing real species' migration routes. But some of the model-derived routes did coincide with known bird paths. For example, the model found that the winds favor a southward migration over the Atlantic Ocean, into South America, and, indeed, blackpoll warblers and Hudsonian godwits take such a route. The model also found that, for birds that fly between central Africa and Europe, it's best to curve eastward on the way north, but westward on the way south. Common cuckoos do just that.

Of course, it's important to remember that a bird doesn't actually plan its migration. It follows instinct, or what it learned from its parents. Nor is a bird able to predict what winds will blow over its itinerary. Over generations, however, evolution can whittle down the birds to those that take optimal paths, leaving even the most bird-brained prone to taking smart routes.

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