When birds set out for a long journey, they don't need roads and they certainly don't need road maps. They learn the route from others or intuit it from their DNA, an urge to point their bodies one way at a certain time of year and stop flying a few thousand miles later. To understand these journeys better, researchers mapped the most efficient routes through the world's winds. The highways that emerged weren't the shortest paths—but they did strikingly match the behavior of real bird species. At the Max Planck Institute for Ornithology in Germany, Bart Kranstauber and his colleagues wondered whether migration routes have evolved to fit wind patterns. If certain routes take more energy to fly, shouldn't birds be less likely to survive those journeys? And if wind patterns are consistent from year to year, won't species evolve to follow the easier migratory paths? The scientists gathered 21 years' worth of global wind data. These patterns do seem to be consistent over the long term, they write. To represent possible start and end points for migrations, the scientists chose 102 spots in the northern hemisphere and 65 in the southern hemisphere. They eliminated the least likely pairs of points (such as routes that were more east-west than north-south). This left them with 2,065 location pairs. To keep things simple, sort of, the researchers decided the birds in their calculations would be flapping (not gliding) at a constant airspeed of 10 meters per second. The calculated routes would be continuous, even though in the real world some birds rest and refuel along the way. Next, they began modeling possible routes. Given the headwinds, tailwinds and crosswinds, what was the fastest way for a bird to travel between each pair of points? They asked this question for all 21 years and all 2,065 location pairs. But wind patterns depend on the season, and on what elevation you're flying at. So within each year, they tested 3 different starting months for the north-south journeys (August, September and October) and 3 more for the return trips (March, April and May). They also plotted each journey at 4 different elevations. This gave them more than a million routes in total. When all these routes were overlaid, some patterns jumped out. The brightest routes below are the most popular ones from the model:
"I was quite astonished," Kranstauber says. The model produced clear "flyways," like highways for birds. Given an altitude and a time of year, certain routes were consistently better. Only very rarely were these routes the geographically shortest paths. Instead, the optimal routes were about 14 percent longer than the shortest routes. But they would take birds only three-quarters of the time to fly, thanks to the wind. This model is simple compared to all the factors a real bird faces: How heavy is the animal? How much fuel can it carry on its body? Yet the flyways that the scientists uncovered were a surprising match to real migration routes. For example, the model predicted that some birds should fly south over the Atlantic Ocean directly toward the coast of South America. (You can see that straight vertical route in the left column of the image above.) This matches the migration of the blackpoll warbler, the authors write, as well as the Hudsonian godwit, which breeds just below the Arctic Circle before flying all the way to the southern coasts of South America. In migrations between Europe and Africa, the model says southbound flights should veer farther to the east than their return trips. This clockwise loop matches the migration of the common cuckoo. In Asia, the predicted bird highways that cut through the Philippines and Malaysia also echo the migrations of real birds. This model seems to be a decent match for the evolutionary pressures that real bird migrations face. But Kranstauber wants to make it better. For species whose migrations don't match the map yet, he says, the model might be improved by adding specific information about those birds' flight speeds or where they stop for food. Researchers could also compare the predictions with data from GPS tracking to find out what's really happening. Evolution has likely put migrating birds into the fast lane, even if we haven't discovered all of their highways yet. Images: from Kranstauber et al.
Kranstauber B, Weinzierl R, Wikelski M, & Safi K (2015). Global aerial flyways allow efficient travelling. Ecology letters PMID: 26477348
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