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Analysis: Big Data Reveals Big Picture of Migration

By Gustave Axelson
Blackpoll Warblers follow a clockwise loop in fall and spring migration. Photo by tfells via Birdshare
Blackpoll Warblers follow a clockwise loop in fall and spring. Photo of Blackpoll Warbler by tfells via BIrdshare.

From the Spring 2017 issue of Living Bird magazine. Subscribe now.

Frank La Sorte is a macroecologist, that is, a scientist who investigates ecological questions at scales that span landscapes, even continents. A research ecologist in the Infor­mation Science program at the Cornell Lab of Ornithology, La Sorte combines radar images of massive clouds of birds with on-the-ground eBird observations to analyze how entire bird populations move across vast distances. In doing so, he has discovered some interesting pat­terns in bird migration.

Long-Distance Migrants Fly Faster

In 2013, La Sorte published an analysis of migration speeds for populations of 102 migratory species. He found that species that travel the longest distances tend to migrate faster. For example, pop­ulations of Black-billed Cuckoo, a long-distance migrant, move five times faster (about 50 miles per day) than populations of Field Sparrows, a short-distance migrant.

“Migration is an inherently danger­ous activity, the period containing the highest mortality,” La Sorte says. “Birds in migration want to minimize time, energy expenditure, and risk.”

La Sorte, F. A., et al. 2013. Population-level scaling of avian migration speed with body size and migration distance for powered fliers. Ecology 94:1839-1847.

Eastern Birds Migrate in a Clockwise Loop

In further re­search, La Sorte and his colleagues discovered that many eastern North American bird species mi­grate in looping, clockwise routes. The driving factor appears to be wind conditions. Migrating along eastern routes in autumn—even out over open ocean off the East Coast—provides birds a path with less severe headwinds. In spring, these same species follow longer routes farther west across the Gulf of Mexico in order to capitalize on strong tailwinds.

La Sorte, F. A., et al. 2016. Convergence of broad-scale migration strategies in terrestrial birds. Proceedings of the Royal Society B: Biological Sciences 283:20152588.

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Spring Migrants Follow a Green Wave

Springtime green-up has emerged as another factor that affects bird migration in La Sorte’s research. He analyzed migration patterns for more than 50 species and found that birds tend to follow the flush of new plant growth in spring—the so-called “green wave” that begins in March.

La Sorte’s study found that insec­tivorous species in the East, such as Black-throated Blue Warblers, showed a very tight correlation with areas showing green-up, which also correlates with insect hatches. In the West, insect-eating migrants such as Lazuli Buntings followed the topography of green river valleys and mountain foothills where insects emerged first.

La Sorte, Frank A., D. Fink, W. M. Hochachka, J. P. DeLong, and S. Kelling. 2014. Spring phenology of ecological productivity contributes to the use of looped migration strategies by birds. Proceedings of the Royal Society B. 281:20140984.

Some Birds May Migrate Faster in the Future

The latest research from La Sorte examines how weather conditions in the future may impact migratory bird popula­tions. While climate change could mean a greater frequency of severe storms, the prevailing westerly winds at mid-latitudes in the East are also projected to decrease in strength by 20 percent during the autumn. That’s good news for fall migrants along the Atlantic seaboard, as it means that crosswinds will diminish and species such as Blackpoll Warblers and Pectoral Sandpipers will be less likely to be blown off course.

“Climate change research often fo­cuses on warming temperatures, but for long-distance migratory birds, changes in wind speed and direction may have greater significance,” La Sorte says. “Our findings suggest weaker crosswinds may increase survivorship during a limited but critical portion of these species’ migration journey.”

La Sorte stresses that these findings do not address other implications of climate change for migratory birds, such as impacts to breeding and wintering habitats.

La Sorte, F. A., and D. Fink. 2017. Projected changes in prevailing winds for transatlantic migratory birds under global warming. Journal of Animal Ecology 86:273-284.

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