For centuries people have marveled at the migratory abilities of birds, but new research is now putting numbers on those seasonal feats—for more than a hundred species at a time—using data contributed by thousands of amateur bird watchers.
In all, more than 2.3 million sightings were summarized to reveal migratory routes of 102 species in North America, in a paper being published August 1 in Ecology. The results provide a fascinating glimpse at an astonishing range of species: for instance, the tiny Calliope Hummingbird crosses the continent almost three times as fast as the Northern Shoveler, which outweighs it more than 300 times. They also highlight the immense scientific value to be gained from bird watchers’ sightings when they can be combined into a single large database.
But the new research is much more than a leaderboard of feathered sporting achievements. Its real value is its ability to move beyond one-off records to characterize the behavior of an entire population.
“Up to this point, migration theory has really only been examined at the individual level,” said Frank La Sorte, a research associate at the Cornell Lab of Ornithology and lead author of the new study. “But in the end, you want to conserve populations—you want to maintain their migration corridors, flyways, or stopover habitats. And that’s why there’s so much potential here.”
The new study plots the average route and speed of each species as it moves north and south each year. “Where the speeds slow down, the population is likely at a stopover location,” La Sorte said. “That’s a valuable patch of habitat where migrating birds can rest and refuel. With results from a study like ours, we can start to make very specific recommendations that we can be confident will impact a large component of the population.”
The researchers also tested basic aspects of migration theory that have proven nearly impossible to address until now. The typical method was to use expensive satellite tracking devices. But most birds are too small to carry these, forcing researchers to study larger species just a few individuals at a time, rather than learning about the behavior of a whole population.
With a single study, La Sorte’s team learned that larger-bodied birds tend to migrate more slowly than smaller species, confirming a long-held but untested idea. They also found that species migrating longer distances tend to travel faster. Birds also tend to migrate faster in spring than they do in fall.
La Sorte and his colleagues did no field work at all to arrive at their results; instead, they analyzed the sightings of thousands of bird watchers who contribute to an online checklist program called eBird, run by the Cornell Lab of Ornithology. Using some 2.3 million records from 2007–2011, the researchers calculated an average location for each species on each day of the year. (They excluded species that use multiple distinct migratory pathways.)
Though the resulting measurements of speed and direction are coarser than for individuals tracked by satellite, they represent major shifts by thousands or millions of birds that would be impossible for any one scientist or bird watcher to detect on their own. For example, they found that the population of Bobolinks moves across North America at about 43 miles per day; Black-billed Cuckoos are similarly fast; whereas Rock Wrens (the slowest of all) shift their distribution at a comparatively glacial 5 miles per day.
The numbers don’t indicate a species’ top flight speed, La Sorte noted. Rather, they are an integrated value that incorporates variation in flight speed and time spent resting at refueling sites across all the individuals in the population.
The continental scale of the analysis revealed another intriguing pattern: many species make looping migrations that take them on a more easterly route in fall than in spring. Some species, such as the Blackpoll Warbler and the Black-billed Cuckoo, leave land altogether—launching themselves out over the Atlantic Ocean for their southward migration. The finding suggests that environmental factors such as wind patterns may have played a role in the evolution of migratory routes. They also yield important insights for conservation planners striving to maintain habitat for migrants.
“In some ways this study is just opening up potential for a really innovative field of study,” La Sorte said, noting that the team plans to investigate the continent’s major migratory flyways next. “It really represents a new perspective on avian migration.”
La Sorte’s coauthors on the study include Daniel Fink, Wesley Hochachka, and Steve Kelling of the Cornell Lab, and John DeLong of the University of Nebraska. The study was funded by the Leon Levy Foundation, Wolf Creek Foundation, and National Science Foundation.
Photo: Northern Shoveler by Thomas Dunkerton