How do millions of tiny songbirds that often weigh less than a standard one-ounce letter migrate thousands of miles each spring and fall, sometimes in a single overnight journey? It's a question that has bedeviled bird researchers, and the answer seems to be, well, blowing in the wind.
"Most of what we’ve known about migration routes comes from ducks and geese," said Frank La Sorte, a Cornell Lab of Ornithology research associate and lead author of a new paper published in the Journal of Biogeography on atmospheric conditions and seasonal dynamics in bird pathways. The long-held assumption about smaller land birds was that they, too, took the same rather direct air highways as the waterfowl. But those assumptions were difficult to turn into facts, thanks to the fact that songbirds migrate at night, making data about their movements difficult to track. (Waterfowl routes are typically garnered with standard leg-band tracking and hunting records.)
To get a better understanding of land bird migration pathways, LaSorte and his colleagues analyzed eight years of data collected through the Cornell Lab of Ornithology's eBird project -- from 2004 through 2012. The project lets bird-watchers log into the system the types of birds they have seen as well as their geographical location. From those years of data, patterns emerged about which birds were seen where during migration times, and computer models helped them compare the migration routes with prevailing wind patterns during the night for a given season.
What they learned after crunching all of this bird data was that some 93 land bird species fell into three rough migration pathways: western, central, and eastern, with some overlap of routes possible, and that these pathways were much more spread out than the relatively stright lines taken by waterfowl.
It also became clear that many more land bird species than previously thought used different routes for their spring and fall migrations. Changing their flight plans with the season allows the small birds to take advantage of a strong tailwinds in the spring and less challenging currents in the fall. This helps them conserve energy and calories on the long journey. For them, the best route between two points is certainly not a straight line.
Why this matters beyond its being a neat fact about small birds is that the more scientists learn about the migration patterns of these tiny nighttime travelers, the better they can plan their conservation efforts along those routes.