How Do Tired Birds Choose Where to Stop During Migration?By Jennie Miller
January 7, 2015
From the sky, the Eastern U.S. resembles a colorful patchwork quilt, with swaths of forest sewn amidst squares of agricultural fields and cities. But fly over the West and you’ll encounter wrinkles of mountains bordering wide, flat ribbons of urban sprawl. From the comfort of an airplane, land use differences seem merely aesthetic. But a recent study published in Global Ecology and Biogeography has revealed that for migrating birds, these patterns translate into distinct preferences for where to stop for breakfast.
Fatigued and famished from a night of flying, a migrating bird navigates vast tracts of terrain in search of a site with ample food, suitable shelter, and few competitors or predators. These stopover sites play a crucial role in a bird’s survival by serving as pit stops where it can rest and refuel for a few hours or days before moving onward. In fact, stopover sites are so critical for bird recovery that during migration birds actually spend more time on the ground than in the air.
Recognizing the importance of stopovers for the conservation of migrants, researchers at the Cornell Lab began characterizing the sites chosen by migrating birds in North America. “We wanted to know how their distributions changed,” explains Frank La Sorte, a research scientist at the Cornell Lab and lead author of the study, “so we took snapshots across time in different land uses.”
Using eBird checklists, the team examined stopover sites in North America for 738 bird species over 10 years. They tallied how many species were found in each month and the number of different species migrating through each region. And to understand where the birds were flocking, the team kept track of the type of land use where species were seen, comparing intact, agricultural, and urban areas.
When La Sorte’s team averaged trends across the country, results were as expected: more migrating birds rested in areas with vegetation than in fields or cities. The team then divided the country and found that the western U.S. similarly followed nationwide trends. But when they examined the East, a surprising difference emerged: birds did not avoid human development. Instead, migrating birds were seen resting in agriculture and urban stopovers nearly as often as in vegetated sites.
So why are Eastern migrants more cosmopolitan? “Migrating birds are navigating different patterns of land use in the eastern and western U.S.,” says La Sorte. He points out that long-term, opportunistic urban development in the East has created a patchy landscape mixed with vegetation, fields, and homes. Here, he explains, birds don’t need to seek out habitat because it’s always fairly close.
But in some parts of the West, human growth has been more rapid and organized, segregating paved cities from lush foliage. “Migrating birds need places rich with resources,” La Sorte explains. So Western migrants head for the woods where food and shelter are more readily available.
The team’s findings add a new piece to the puzzle of how birds make decisions during migration. Their work also builds on recent collaborations by the Cornell Lab and The Nature Conservancy to use eBird migration trends to assist farmers in California’s Central Valley in optimizing stopover sites for migrating shorebirds. These efforts demonstrate that with a new perspective through citizen science, we are strengthening our ability to understand the world from a bird’s eye view.