Unlike waterfowl or shorebirds that are heavily dependent on wetlands and coastlines, scientists have not observed strong fidelity to specific stopover sites among songbirds. Migrating along a broad front across the continent, these forest-dwelling songbirds often stop wherever they can, making migration a perilous venture.

“Songbirds are often naive about the places that they’re stopping over; they don’t necessarily know where the resources are or where the dangers are,” said Jeff Buler, professor of wildlife ecology in UD’s College of Agriculture and Natural Resources. “When they’re in these unfamiliar places, they may be more susceptible to things like predation. Throughout the annual cycle, the migration period is a time when these birds experience the greatest mortality.”

Using weather radar data —  a technology prized for its utility in comprehensively tracking the nighttime movements of bird populations — the research team found that birds concentrate in certain regions and along certain pathways during migration. 

In the Eastern U.S., songbirds tend to loosely travel along the Mississippi River or along the Appalachian Mountains. Within these pathways, the study found that the large expanse of croplands in the mid-Western U.S. act as an ecological barrier, with birds congregating in high densities along the edges. This unexpected discovery is the first recorded evidence of a large man-made barrier to bird migration akin to natural barriers like large water bodies or deserts.

“The prairie biome is now dominated by croplands, especially along the Corn Belt that extends into Illinois, Indiana and Ohio. We found that this is acting as an ecological barrier similar to the way that the Sahara Desert in Africa is a barrier to European and African bird migration,” Buler said. “The Corn Belt is a habitat desert for these forest birds and they concentrate in forests along the southern edge of that biome before and after making the trek across.”

Researchers also detected high-density concentrations of birds in forest patches embedded within landscapes that otherwise have very little forest. That means that in developed suburban or urban areas, or in agricultural regions like the Corn Belt, even small patches of forest that dot the landscape provide refuge for significantly more birds than previously understood. 

“One way we think about it is that these little forest patches are wicking up all the birds in a larger landscape,” Buler said. “There are huge numbers of birds that are moving across the U.S. but when it comes to landing and finding stopover habitats, they have to search and settle for the closest habitat in these landscapes where there are sometimes very few options.” 

Underscoring the conservation value of these stopover sites, the study indicates that the protection of forested land is necessary, particularly in landscapes dominated by agriculture, to create a network of suitable habitats through which songbirds can travel. As many bird populations face rapid declines, stopover site preservation could help ease the burden of an otherwise vulnerable migration journey and help sustain healthy bird populations.